Development and Iodine Testing of a Novel C12A7 Electride Planar Hollow Cathode

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Becke, Nils Gerrit Kottke, Max Vaupel, Niccola Kutufa, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5738180/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Iodine exhibits promising characteristics as a propellant for electric propulsion (EP) systems. Its performance is comparable to xenon, yet it offers significant cost advantages. Moreover, it can be stored in a compact solid state and easily fed into the system through sublimation. Thruster operation has already been successfully demonstrated for the iodine fed Advanced Cusp Field Thruster, Hall Effect Thrusters and Radiofrequency Ion Thrusters (RIT), with the first in-space demonstration for an iodine RIT in early 2021. However, an iodine-fed plasma-bridged hot neutraliser is required to unlock the full systemic potential of an iodine EP subsystem at higher current levels. There exists no high-current iodine-fed neutraliser so far, as the corrosive nature of iodine and the potential for emitter material poisoning pose challenges. In the framework of the “IcoN” activity, a planar hollow cathode based on a C12A7 electride emitter has been developed and tested. The emitter has been manufactured by Fraunhofer IKTS, the neutraliser was manufactured and tested at Airbus Friedrichshafen. The initial testing was done with krypton and changed to iodine, as soon as a reliable cathode performance was achieved. After the first iodine tests were completed, the feed system was updated with a proportional valve to allow better control of the flow rate. In the final test series, two different emitter types and two planar cathode configurations have been tested with iodine. The longest achieved stable discharge was 26 minutes. While it was possible to develop a fully functional iodine feed system and a novel heaterless planar cathode based on C12A7 electride for krypton, no long-time stable iodine discharge was achievable in the frame of the project. Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 Figure 18 Figure 19 Figure 20 Figure 21 Figure 22 Figure 23 Figure 24 1 Introduction In the past ten years, several attempts have already been made at designing an iodine-compatible neutraliser. While this challenge is essentially solved for the extraction of small currents by using thermionic filament cathodes [ 1 ] or radio-frequency (RF) neutralisers [ 2 , 3 , 4 , 5 ], it is still open for mid- to large-scale thrusters. To extract currents in the ampere range for a reasonable energy consumption, only hot plasma bridged neutralisers offer a solution [ 6 ]. Hollow cathodes based on lanthanum hexaboride (LaB 6 ) [ 7 , 8 ], dispenser emitter [ 9 , 10 , 11 ] and lanthanated tungsten (WL20) [ 8 ] have been designed and tested with iodine. However, none of them was capable of reaching a continuous and stable plasma discharge. Dispenser emitter and WL20 seem to be incompatible due to their reaction with iodine [ 6 ], which leads to a fast depletion of the low work-function component. The compatibility of LaB 6 seems to be questionable [ 12 ] and no definitive conclusion can be drawn from the literature. A comparably new material is 12CaO·7Al2O3 (or short C12A7) electride [ 13 ], which has been tested as an emitter material in hollow and planar cathodes using iodine as a propellant [ 14 , 15 , 16 , 17 ]. Similarly, no definitive conclusion of the long-term compatibility of C12A7 electride and iodine has been published, although preliminary results look encouraging. For a comprehensive overview of iodine neutralisers, see Becke et al. [ 6 ]. The Laboratory for Enabling Technologies (LET) at Airbus in Friedrichshafen, Germany, has focused on the development of iodine-compatible electric propulsion (EP) technologies. Within the scope of the European Space Agency (ESA) fanded Iodine Compatible Neutraliser (IcoN) development activity, C12A7 electride as an emitter material for iodine-compatible cathodes has been examined. C12A7 is a ceramic compoand that contains an anion sublattice, which can be replaced with electrons, creating a so-called electride (C12A7:e- or C12A7:2e-). It has a work function of 2.4 to 2.96 eV, a melting point of up to 1410°C and a thermal conductivity of 1.7 to 2.3 W/(m×K) at higher temperatures [ 18 , 19 , 20 , 21 , 22 ]. The comparable low melting point is close to the temperature of 1340°C for the theoretical thermionic emission of 1 A from an emitter with a surface area of 10 mm², which can be calculated by using the Richardson-Dushman equation for a work function of 2.4 eV. The emission temperature being close to the melting point and the low thermal conductivity can easily lead to local overheating of the C12A7:e- insert, as it has been shown for C12A7:e- hollow cathodes tested with iodine. As the plasma in the hollow cathode and, therefore, its heating mechanism is usually located at the downstream end of the emitter, local hotspots of molten emitter material inside the cylindrical emitter have been described [ 16 ]. A different approach has been chosen by Drobny et al. [ 20 ]. They used planar C12A7:e- inserts and successfully demonstrated a plasma discharge based on krypton for nearly 1000 h. The advantage of the planar shape is the exposure of a larger area to the plasma and the cathode mount, allowing equally well the heating and conductive heat dissipation. Based on these results, the design of the IcoN cathode was based on C12A7:e- discs in a planar design. The greatest challenge next to the low melting point of the insert material is the compatibility of the hot materials with the iodine plasma. Therefore, the activity focused on solving these main issues by designing a planar C12A7:e- cathode from iodine-compatible materials. The development methodology is presented in the next section, followed by the description of the test facility, the preliminary development of the C12A7 electride neutraliser and the iodine feed system. Hereafter, the results of the iodine cathode tests are presented, as well as the post-test analysis. At the end, a conclusion and recommendation on further iodine neutraliser development based on the experiences from this activity are given. 2 Methodology The challenges of developing an iodine-compatible neutraliser are manifold and concern the selection of materials, the cathode geometry, the vacuum pumping system and the iodine feed system. The challenges of designing the cathode geometry are mostly based on the complexity of the molecular plasma discharge and the surface reactions. There have been fluid simulations on iodine plasma in a hollow cathode discharge [23], a global model for the discharge in an RF thruster [24, 25, 26, 27, 28], for a Hall effect thruster (HET) [29], a cusp field thruster (CFT) [30] and a helicon thruster [31]. There are no existing simulations on iodine plasma discharges in planar cathodes and similarly only very few experimental results in the literature [6]. Using a simulation to derive design decisions requires the knowledge of a few parameters, which can only be estimated prior to testing. Therefore, a simulation can be fitted to an existing cathode, however, modelling a completely new geometry comes with a high error. Therefore, an experimental approach was chosen. It is based on fast iterations and parameter studies, which allow to explore and evaluate design changes in a matter of days. These fast development cycles were aided by the access to on-site manufacturing, reducing the periods between the tests. However, thermal simulations were used to evaluate the required heating of the emitter to reach thermionic emission and the estimation of thermal losses. Each design change was first verified with krypton and eventually tested with iodine, if the performance was sufficient. Testing with noble gases requires a less complex setup and gives a baseline for each iteration, allowing the comparison between the changes. Although iodine is a molecular gas and, therefore, has different properties than krypton, this can be later taken into account by small adjustments, especially by changing the orifice diameter or propellant flow rate. Following this procedure is highly recommended for experimental designs without prior knowledge, as a lot of unforeseen problems only appeared while testing. This has led to many design iterations to solve these difficulties. The complete timeline of the IcoN project is shown in Fig. 1. At the beginning, an extensive literature study was conducted [6]. The following activities included the definition of the requirements, thermal simulations and the design of laboratory models. In the first year, the development was focused on a stable krypton discharge. After having achieved this step, the tests were continued with iodine. In the following, the iodine feed system was further improved and the cathode tests with iodine were finalised. 3 Development and Testing All tests referenced in this work were performed at the Laboratory for Enabling Technologies (LET) at Airbus Friedrichshafen. In this section, the test facility is first described, including the design of the iodine feed system. It is followed by the development and iterations of the cathode design based on preliminary krypton and iodine tests. The section is concluded by the cathode designs that were used in the final iodine tests. 3.1 Testing Setup The test chamber used for the cathode development and testing is a vacuum chamber with 40 l volume equipped for iodine cathode testing (see Fig. 2 ). The vacuum is produced by a two-stage pumping system, featuring a forestage pump and a turbomolecular pump to reach the 1×10 − 6 mbar range (cf. Table 2 ). To reduce the likelihood of iodine corrosion damage to the pumping system, nitrogen purging and a ceramic filter are installed. No cold trap was used, as both pump types were rated for iodine. However, the iodine condensation did damage the fore pump, which could no longer reach its final pressure after a few days of pumping iodine. We assume a damage to the sealing material, as the leakage through the pump increased. A temporary solution to this problem was the purging of the pump with high flows of nitrogen at regular intervals to remove iodine residues from the pump. However, a cold trap would be advised for more extensive test campaigns. Table 1 Laboratory power supplies and measurement systems used in the cathode tests. Model Manufacturer Type Range Usage MCA 3000–3000 FuG HV power supply 0 to 3 kV, 0 to 3 A Anode/keeper DP-PH DSC HV power supply 0 to 1 kV, 0 to 1.2 A Anode/keeper HMP4040 Rohde & Schwarz Power supply 0 to 32 V, 0 to 10 A Keeper/heater HM7044 Rohde & Schwarz Power supply 0 to 32 V, 0 to 3 A Valve/heater WR204 LeCroy Oscilloscope up to 2 GHz Cathode current CP031A LeCroy Current probe up to 100 MHz Cathode current ED 582 Brainbox Temperature meas. -200°C to 300°C Feed system temp. ED 549 Brainbox Thermocouple meas. 0°C to 2315°C Emitter temp. IDM 103N RS PRO Multimeter 0 to 1000 V Electrical inspection A graphite plate served as an anode and was placed 10 mm in front of the cathode, which was chosen to keep consistent results with earlier experiments. The keeper and anode are controlled by two HV-power supplies, which allow maximum currents of 1.2 A at the keeper and 3 A at the anode at a voltage of up to 1 kV (3 kV at the anode). The temperature inside the cathode is monitored by a thermocouple type C, which is mounted close to the emitter and a PT100 sensor at the base of the cathode. See Table 1 for more details. Table 2 Pumps, sensors and flow regulation devices used in the tests. Model Manufacturer Description Property Scrollvac 15 plus C Leybold Forestage pump max. pumping speed: 4 l/s (N2) Turbovac Mag 2200 Leybold Turbomolecular pump max. pumping speed: 2100 l/s (N2) Ionivac ITR200 Leybold Pressure sensor meas. range: 5×10 − 10 − 1000 mbar GE50A013501 MKS Krypton mass flow-controller 0 − 30 sccm Initially, the test setup was limited to a single feed line, resulting in the need to interrupt testing to switch between krypton and iodine tests. Since this procedure exposed the cathode to ambient atmosphere, with possible detrimental effects on the materials, the test setup was reworked. In a later iteration, the facility was equipped with a dual feed system for iodine and krypton. This allows testing the neutraliser with noble gases and iodine without venting the chamber and exposing the cathode to the humidity in the air. A similar approach has been chosen by Taillefer [ 9 , 10 ]. The krypton mass flow is controlled by a 30 sccm mass flow-controller and can be closed to prevent iodine backflow. Furthermore, an in-line filter was installed to protect the krypton feed system components from iodine. The iodine feed system (see Fig. 3 ) is located completely inside the vacuum chamber, allowing for a faster heating due to the missing convection losses and preventing an exposure of the laboratory personnel in the unlikely case of iodine leakage. The complete feed line is heated to 130°C to prevent iodine condensation. The iodine tank can be precisely heated to allow the adjustment of the pressure inside the feed system. The temperature is monitored at various locations along the feed line and adjusted by various heaters. In the first iteration of the iodine feed system, a pressure sensor was foreseen to determine the iodine pressure. However, different commercial-off-the-shelf pressure sensors were tested and failed due to corrosion. Moreover, the pressure regulation by using a simple on-off valve and the tank temperature proved to be unreliable. To address these issues, a feed block based on a proportional valve was developed. The valve was then calibrated with a pressure sensor and krypton mass flow, while the cathode was ignited. The tubes and the valve block were heated to 130°C during the test. The proportional valve was slowly closed, while the pressure in the upstream side of the feed block was measured. This way, it was possible to link an upstream pressure and valve position to the corresponding mass flow. The complete schematic of the vacuum facility can be seen in Fig. 4 . The valve setting in % plotted against the krypton mass flow rate in sccm for a given upstream pressure is shown in Fig. 5 . When testing with iodine, the upstream pressure is given by the temperature of the iodine tank. The comparable iodine flow rate inside the valve block at 130°C has been estimated from the krypton flow rate using the FLUIDAT database [ 32 ]. The results are shown in Table 3 . This can only be used as a first estimation, as other processes have not been included, especially the dissociation of the iodine molecule in the plasma. Table 3 Required iodine tank temperatures to reach flow rates from 5 to 20 sccm based on the test shown in Fig. 5 . Iodine flow rate Iodine pressure Iodine tank temperature in sccm in mbar in °C 5 10 67.2 10 13 71.4 15 17 75.9 20 20 78.7 To test the actual performance of the new feed block, it was tested with iodine and krypton using the pressure sensor of the vacuum chamber. The valve block was mounted to the iodine tank, which was heated to 85°C. Then the valve was slowly opened and closed again, while the vacuum chamber pressure was measured. The same process was repeated with krypton, but through a mass flow controller instead of the proportional valve. To compare the measured pressure, some corrections needed to be applied. The pressure was measured with a hot filament gauge controller. The gas correction factor for hot filament gauge controllers for krypton is 1.7 and for iodine 5.4 according to Summers [ 33 ]. Turbomolecular vacuum pumps are more efficient in pumping heavier gases. For light to medium-heavy atoms or molecules, the compression is proportional to the square root of the molecular mass [ 34 ]. Therefore, the square root of the molecular weight has been considered as well. The iodine vapor pressure at 25°C is about 40 Pa [ 35 ], which is way above the chamber pressure of 10 − 6 mbar. Consequently, the redeposition of iodine on the vacuum chamber walls has not been taken into account. The complete correction can be written as: $$\:\frac{{p}_{Kr}}{{p}_{{I}_{2}}}=\:\frac{5.4}{1.7}*\frac{\sqrt{83.798\:}}{\sqrt{253.809}}=0.18$$ and hence indicating that the measured iodine pressure corresponding to a comparable krypton flow rate needs to be multiplied by a factor of ~ 5,56. Figure 6 shows the results of the measurements. The left plot is the uncorrected measurement along the full valve range, the right plot is 50% of the corrected iodine valve range compared to a krypton flow of up to 30 sccm. It shows clearly that the new proportional valve is able to control the iodine mass flow comparably to the mass flow of the krypton mass flow controller. The exact range depends also on the iodine tank pressure, which has to be higher than the minimum pressure required by the cathode. Here, 85°C, so 28.6 mbar [ 35 ], were chosen. 3.2 Preliminary Cathode Configurations The initial planar cathode design was based on a hollow cathode, but instead of the tube, a planar disc was mounted on a rod close to the keeper orifice. A simplified design is shown in Fig. 7 . The base of the prototype cathode is made from stainless steel, as it is only subjected to temperatures in the range of 100 to 300°C. For lifetime testing, an alloy like Inconel or Hastelloy should be selected [ 6 ]. The main rod consists of two parts, one is hollow to allow the gas flow from the mount to the emitter. The gas leaves the first section of the tube through four small holes. The second section is solid and connected to the first section by a thread. The emitter is mounted to the second section by a metal clamp. The diameter of the planar emitter is 8 mm, which results in a surface of ~ 50 mm² and therefore in a maximum temperature of ~ 1300°C to emit 3 A, assuming a work function of 2.4 eV. The heater is placed parallel to the tube behind the emitter and connected by two wires. The rod is made out of graphite to guarantee the iodine compatibility at high temperatures. The clamp, the heater, and the keeper consist of refractory metals. Figure 7 b shows the front section of the tube with the heater and the emitter clamped to the front. As the refractory clamp proved to be unreliable and was expected to be incompatible with iodine, the emitter was attached with a high temperature glue to the graphite rod in later iterations (see Fig. 7 d). This method has already been described by Drobny et al. [ 21 ]. The keeper was made from graphite as well and electrically insulated using a ceramic disc of boron nitride (BN). The first laboratory models were only tested with krypton to achieve a design that is able to deliver a stable discharge. The required heating power of the emitter with the inbuild heater to reach emission temperatures was estimated in a thermal simulation. However, the ignition process was not very reliable, leading often to the emitter melting due to overheating. A picture of the molten emitter is shown in Fig. 7 c. Although the molten emitter could be reignited, which means that at least parts of the material must have kept their properties, the test did quickly end because of short-circuits due to material build-up. Another problem encountered was the ignition of the plasma to the heater instead of the emitter. As the planar emitter has a large surface area, the thermal losses to the environment need to be compensated by the heater to reach emission temperatures. This requires more heating power compared to a conventional hollow cathode, which also increases the temperature of the heater. During the test, the side of the cathode showed hot spots (see Fig. 8 c), which meant the plasma was not only confined to the front. The inspection of the cathode after the test showed parts of the heater insulation were molten (see Fig. 8 b). To quantify this observation, a thermal simulation with different thermal contact resistances between the heater and the cathode tube were conducted (see Fig. 8 a). The thermal contact between the heater and the insulation (case 2) and additionally the insulation and the cathode tube (case 3) were reduced in comparison to the case with a perfect contact (case 1). The results indicated that in these conditions the heater starts emitting more current than the emitter, destroying the heater through the following plasma discharge. While the heater is protected by a ceramic sleeve, there a still tiny gaps remaining that allowed the discharge between the keeper and the heater. To circumvent the heater ignition problem, the pressure at the emitter was increased by changing the flow path. In this design iteration, the gas was fed through a centre hole in the emitter (cf. Figure 10 a) and a second graphite disc (a “secondary orifice”) was mounted in front of the emitter by gluing the disc to the tube (cf. Figure 10 c). This change also reduced the thermal stress on the emitter, as the heat radiation losses were decreased, leading to a lower required power to operate. This was further improved by impregnating the surface of the tube with ceramic glue to reduce the emissivity. Although metal shielding would be the preferred option due to the low emissivity, this was not chosen because it would be damaged by the corrosive iodine environment. The distance between the emitter and the secondary orifice could be varied by adding spacer rings of BN (cf. Figure 10 b). BN was chosen because it has a similar coefficient of thermal expansion (CTE) to graphite [ 6 ] and an improved ignition was planned to be achieved by connecting the secondary orifice to the keeper. However, graphite deposits inside the BN ring short-circuited the keeper, prohibiting this ignition method. As a further change, a ceramic sleeve inside the keeper was introduced, so that only the front was exposed to the plasma. In this way, unwanted discharges to other parts of the cathode than the emitter can be prevented. The comparison between both cathode types is shown in Fig. 9 . The configuration shown in Fig. 7 is referred to as configuration A, the configuration with a centre hole in the emitter as configuration B (Fig. 10 ). The electron cost is given in W/A, the fraction of the total power consumed by cathode and anode divided by the anode current, which allows a correlation between different cathode types and discharge modes [ 6 ]. The lower the electron cost, the higher is the efficiency of the cathode. Compared to configuration A, the required power to operate the cathode in configuration B is greatly reduced. Furthermore, the flow rate has an influence on the electron cost as expected. However, the tests revealed that the design changes had a more significant impact on the performance. While no plasma simulation was conducted, the surface changes of the C12A7 electride emitter after the operation indicated that the complete emitter surface was hot enough to emit electrons. We assume that this cathode type working principle is a combination of both hollow cathode and planar cathode discharges. 3.3 Preliminary Iodine Testing Results The planar hollow cathode in configuration B was tested with iodine. The iodine tank was heated to 80°C, the iodine feed system was not yet equipped with a proportional valve (configuration shown in Fig. 3 a) and therefore no fast control of the pressure inside the cathode was possible. After the emitter had reached emission temperature, indicated by a thermionic current to the keeper, the iodine flow valve was opened. A successful plasma ignition for aroand 3 minutes was achieved shortly after opening the valve. A plot of the keeper voltage is shown in Fig. 11 a. The discharge was in plume mode during the complete test. While the discharge collapsed multiple times, it could also be reignited again. The test was ended by a keeper short-circuit. The disassembly of the cathode revealed that the glue had failed and the secondary orifice was in contact with the emitter and keeper (cf. Figure 12 ). The emitter was also covered with a blackish substance, see Fig. 12 b. Although the coating of the emitter was not identified through material analysis, we assumed because of the soft quality of the structures that the high-power operation in plume mode led to graphite sputtering and carbon redeposition. Another possible alternative for the deposition was molten C12A7, however it is hard and greenish after solidifying, see also Fig. 7 c, and therefore an unlikely candidate. A third possibility would be that the C12A7:e- material was modified through the exposure to the iodine plasma. To compare the iodine discharge in the unfavorable “plume mode” [ 36 ], a discharge in plume mode with krypton for the same cathode configuration is shown in Fig. 11 c. Except for the ignition discharges in the first 150 s and between 350 s and 450 s in Fig. 11 a, the voltage fluctuations have a similar amplitude. A test with the same configuration and a smaller orifice size (see Fig. 11 e) has indicated a stable krypton plasma discharge in the favorable “spot mode” [ 36 ], with half the mass flow but the same pressure as during the test shown in Fig. 11 c. This highlights the importance of the balance between pressure, mass flow and orifice size for a stable cathode discharge. With the orifice size being set by design before the start of the test, the pressure inside the cathode depends, among others, on the mass flow and the internal temperature. A self-sustained discharge can be adjusted by changing the pressure or the mass flow, which can be done for krypton in a matter of seconds by using a mass flow-controller to arrive at a favorable working point. In the test setup described in Fig. 3 a, the iodine mass flow is controlled indirectly by adjusting the temperature of the iodine tank, which directly correlates to the iodine vapor pressure. This results in a slow response of the mass flow rates and further provides difficulties to calibrate the setup by correlating the pressure to mass flow rates. 3.4 Final Cathode Configurations To prevent the problems that were foand after the iodine test, the design was further improved. The feed system was changed to include a proportional valve for iodine and a dual feed line for krypton with an on-off valve (see section 3.1). After applying these changes, heaterless ignition became possible by creating a pressure surge and applying a voltage above 300 V to the keeper. Later versions of the cathode were only ignited heaterless, because the power required to heat the emitter to emission temperature was exceptionally high due to the high thermal emissivity of the graphite keeper and tube. Additionally, the heaterless ignition resulted in a faster ignition overall. Heaterless ignition of C12A7 electride was also studied by Drobny et al. [ 37 ]. The remaining difficulties were addressed by a partial redesign of the cathode. To remove the necessity of the glue, the emitter is held in place by a screwable graphite cup (cf. Figure 13 ). This also allows more efficient testing, as small changes can be implemented without reglueing parts of the cathode. The difficulty of the carbon deposition was solved by avoiding direct contact between the iodine plasma and graphite in the vicinity of the emitter. This was achieved by having the secondary orifice and a spacer ring made from LaB 6 . Similarly, the keeper front plate is made from molybdenum. This configuration C is shown in Fig. 13 c. A second version of this cathode (config. D) has been tested as well, but without the secondary orifice (cf. Figure 13 b). Here, the emitter is directly placed in front of the keeper plate. Two different emitter types have been developed in cooperation with the Fraunhofer IKTS: C12A7 electride type 1 and type 2. Both are doped with different metals. Type 2 is optimised for a lower work function; however, it has a low melting point and low thermal conductivity. Type 1 has been optimised for a higher thermal conductivity, but it has a higher work function and contains a significant amount of metal, which might react with iodine during a longer exposure. The preliminary development tests have been conducted with type 1. The assembled cathode is shown in Fig. 14 . 3.4.1 Krypton Testing of the cathode in configuration D The performance of the cathode in configuration D with the C12A7 electride emitter type 1 has been tested for three different keeper orifice diameters (cf. Figure 15 ) with krypton (cf. Figure 14 c). Before each of the three tests, the vacuum chamber was pumped overnight, resulting in a pressure of ~ 1×10 − 6 mbar. The cathode was ignited heaterless by applying a voltage of 400 V to the keeper and anode, then opening the krypton feed valve, resulting in a pressure surge. The keeper current was reduced afterwards. After the cathode reached a thermal equilibrium that was measured at the mount interfacing the vacuum facility, the anode current was reduced for a fixed flow rate. If the cathode discharge changed to plume mode, the keeper current was increased up to a maximum of 1.2 A. The discharge mode was verified by monitoring the oscillations of the anode current. Generally, a larger keeper orifice resulted in an operation at a higher mass flow. However, also the smallest keeper orifice diameter required a high mass flow of 30 sccm to operate in spot mode. The larger keeper orifice also allowed a higher anode current. 4 Iodine Testing Results The final iodine tests were conducted using the cathode in configuration C and D. Before each test, the iodine tank was refilled and weighed. By weighing the tank again after the test, the mean iodine flow rate was estimated. The chamber was pumped overnight, resulting in a pressure of ~ 1×10 − 6 mbar at the start of each test. The cathode was first ignited with krypton and the performance was measured. Then, the iodine flow was gradually switched on by opening the proportional iodine valve and closing the krypton valve. If possible, the test was repeated with krypton after the iodine discharge to check if the performance changed significantly. 4.1 Tests with a C12A7 electride type A emitter Figure 16 shows the first test of the planar cathode with a C12A7 electride emitter in configuration C with a secondary orifice. Total power refers to the combined power of anode and keeper. As the iodine proportional valve was opened and the krypton supply was closed, the power required to operate the cathode increased, then stabilised. After about 6 minutes, the discharge became unstable and went on/off in rapid succession. The reignition with krypton was possible again, but the cathode operated only shortly at a power above 150 W and at high anode currents only. A second test with the same emitter type but in configuration D has been performed next (cf. Figure 17 ). The result is very similar to the previous test, as soon as the discharge was switched to iodine, it became highly unstable. However, here the reignition with krypton was no longer possible. Both discharges were clearly in plume mode, as it can be seen in Fig. 18 and by the fluctuations of the cathode power. 4.2 Tests with a C12A7 electride type B emitter In a third test, the C12A7 electride type B emitter was tested with krypton in configuration D (cf. Figure 19 ) and subsequently with iodine (cf. Figure 21 ). It is clearly visible from the plots that the emitter did not produce a stable plasma, even with krypton. However, the iodine discharge was more stable than with the type A emitter, although fluctuating by ± 5 W. The discharge was going off a few times, but it did directly reignite. Figure 20 shows that the colour of the discharge was changing during test 3, indicating the evaporation of the emitter material. This assumption was further strengthened by the blocking of the keeper orifice that finally ended the test. A fourth tests as a repetition of test three led to similar results, the total power discharge curve is therefore not shown here. Tests in configuration C have been attempted with krypton, but the secondary orifice was immediately blocked by evaporated material. 4.3 Conclusion on the iodine tests The tests with the emitter type A follow a similar trend: After switching from krypton to iodine, the discharge stays stable for a few minutes, before becoming very unstable. This result was observed in both cathode configurations, independent of the secondary orifice. The emitter type B was already unstable with krypton, the main difference was the required total power, which has more than doubled with iodine. The reason for the instabilities is still ander investigation. However, it was not possible so far to achieve a plasma discharge in spot mode. This shows, that while a plasma can be achieved for several minutes, the conditions are not stable. The performance values of the four tests presented are shown in Table 4 . Table 4 Performance of the cathode during the final tests for IcoN. The gas flow is given in sccm for krypton and in the mean iodine consumption during the test in mg/s by weighting the tank before and after the test. Test Emitter Gas Flow Keeper Anode Duration Comment Power in W Current in A Power in W Current in A in min 1 C12A7 Type A1 Kr 30 sccm 20.3 1.2 61 3 25 Before iodine exposure. I 2 mg/s 14.5 1.2 155 3 16 Iodine discharge. Kr 30 sccm 2.1 0.1 158 3 6 After iodine exposure. 2 C12A7 Type A2 Kr 20 sccm 26.9 0.8 64 1.5 20 Before iodine exposure. I 1.9 mg/s 118 0.5 21 3 26 Unstable iodine discharge. Kr - - - - - - No reignition with krypton possible. 3 C12A7 Type B1 Kr 30 sccm 5 0.5 120 3 9.5 Before iodine exposure. I 1.9 mg/s 8 0.5 150 3 22 Iodine discharge. Kr - - - - - - No reignition with krypton possible. 4 C12A7 Type B2 Kr 30 sccm 16 1.2 100 3 10 Before iodine exposure. I 3.7 mg/s 21.5 1.2 141 3 18 Iodine discharge. Kr - - - - - - No reignition with krypton possible. 5 Post-Test Analysis After each test, the cathode was removed from the testing chamber, disassembled and the state of all components was documented, with special focus on the emitter. The iodine tank was removed after each iodine test as well and weighed, to give an estimate over the total consumption, which gives a conservative estimation of the flow rate during the test (cf. Table 4 ). 5.1 Tests with a C12A7 electride type A emitter The state of the type A emitter before integration (a), after testing with krypton (b) and after both tests with iodine (c) and (d) is shown in Fig. 22 . While the emitter seems to be already damaged after the krypton test, it shows clear signs of surface degradation after the iodine test. There are some black residues visible in both cases, but after the exposure to iodine plasma, also a metallic layer is visible. This can be caused by the diffusion of metallic components to the surface of the emitter or partly reduction of the material. The more the low-work function electride surface is covered, the higher the work function, until the discharge collapses. After the disassembly of the cathode, also the keeper orifice plate has been visibly covered with a black layer, that seems at least partially containing iodine (cf. Figure 23 b). This could become further problematic during longer tests, as the heating of the cathode does not seem to prevent this layer, which might eventually reach the emitter from building up and short-circuiting the keeper. 5.2 Tests with a C12A7 electride type B emitter The state of the type B emitter before integration (a), after testing with krypton (b) and after both tests with iodine (c) and (d) is shown in Fig. 24 . After the tests with krypton indicated the melting of the emitter of this type, it was also expected for iodine, especially after the test was ended by a blockage of the keeper (see Fig. 23 c). The emitter did lose a significant amount of material, as it can be seen in (c) and (d). While it was apparently still functional and the ending of the test was caused in both cases by the blockage of the keeper orifice, it still shows that a significant damage to the emitter was induced in the short period. In this case we have to assume that the temperature at which the emitter is generating enough electrons to stabilise the discharge is above the melting point. This makes this emitter type not viable for the operation in hot thermionic plasma-bridge cathodes. 6 Conclusion This paper gives an overview of the development and testing of an iodine-compatible neutraliser at the LET at Airbus in Friedrichshafen that was conducted during the IcoN project. A planar cathode was designed and developed with a C12A7 electride emitter and operated with krypton as a propellant. After a prototype was designed that could be repeatably ignited into a stable discharge, further tests with C12A7 electride and iodine were performed. An iodine discharge could be ignited two times, each lasted for about ~ 150 s. The lessons learned were incorporated in an improved cathode design. The cathode type has the novel feature of combining both the properties of planar and cylindrical hollow cathodes. For the C12A7 electride, the large surface area given by the planar design is an important factor to reduce melting damages through local plasma discharge hotspots. By enclosing the planar emitter and adding a secondary orifice, the efficiency could be further improved. This design was then successfully tested with krypton in a parameter study. The pumping system was also updated, as the forestage pump did not reach its final pressure anymore after a few days of pumping iodine, presumably because the sealing was damaged. This was partially solved by increasing the nitrogen purging mass flow, however, a cold trap would be advised to use for extensive iodine testing. The further development of the feed system allowed a stable and precise control of the iodine mass flow, permitting repeatable and stable ignition and therefore the detailed analysis of the impact of the iodine discharge on two different types of C12A7 emitter materials. The longest discharge had a duration of 26 minutes. Some krypton discharges after the test failed, indicating the damaging of the emitter. The post-test analysis revealed that the surface of all emitters had changed and residues were visible, which have not yet been identified. One of the two tested C12A7 electride emitter types was not stable at emission temperatures and melted during the test. All emitters did require a significant higher power to operate after the exposure to iodine, indicating a possible poisoning, independent of the geometrical configuration. The damage through the short iodine discharge indicates that even if the melting and the surface modifications are reduced, the iodine discharge is unlikely to last for several handred hours. Considering the results achieved in this study, we have to conclude that an iodine-fueled planar cathode based on C12A7 electride is not possible to operate for a longer period ander the tested configurations and material modifications. A possible approach to increase the temperature stability is the further development of the emitter material through doping. Abbreviations RF Radio-Frequency BN Boron Nitride LaB 6 Lanthanum Hexaboride C12A7 12CaO·7Al2O3 WL20 Lanthanated Tungsten LET Laboratory for Enabling Technologies EP Electric Propulsion ESA European Space Agency IcoN Iodine compatible Neutraliser HET Hall effect thruster CFT Cusp Field Thruster HV High voltage MFC Mass flow controller CTE Coefficient of thermal expansion Declarations Author Contribution P.B. wrote the original draft, did the methodology, investigation, formal analysis, data curation, conceptualization and prepared the figures. N.G.K. and M.V. did the project administration, reviewing and editing of the first draft. N.K. supervised the IcoN project in which context this paper was written. K.W. did the reviewing and editing of the first draft, the validation and contributed ressources. M.T. and F.H. supervised the work. All authors reviewed the manuscript. Acknowledgments The project was fanded by the European Space Agency as activity “Iodine-compatible neutraliser for electric propulsion of CubeSats and small satellites” and “Iodine fluidic subsystem for the Iodine-compatible Neutraliser” ander the contract No 4000136710/21/NL/RA. Data Availability Data is provided within the manuscript or supplementary information files. References D. Rafalskyi, J. Martínez, L. Habl and E. Z. Rossi, „In-orbit demonstration of an iodine electric propulsion system,“ Nature, p. 411–415, 2021. P. Dietz, F. Becker, K. Keil, K. Holste and P. J. Klar, „Tests of an iodine fed RF-neutralizer,“ in 36th International Electric Propulsion Conference, University of Vienna, Austria, 2019. M. Tsay, J. Model, C. Barcroft, J. Frongillo, J. Zwahlen and C. Feng, „Integrated testing of iodine bit-3 rf ion propulsion system for 6u cubesat applications,“ in 35th International Electric Propulsion Conference, 2017. E. Kulu, Lunar Icecube @ nanosats database, 2023. E. Morton, NASA’s LunaH-map mission ends, validates science instrument performance, NASA, 2023. P. S. Becke, N. G. Kottke, M. Vaupel, N. Kutufa, M. Tajmar and F. G. Hey, „Review on the Current State of Iodine Compatible Neutralizers,“ Submitted to the Journal for Electric Propulsion, 2024. G. F. Benavides, H. Kamhawi, J. Mackey, T. Haag and G. Costa, „Iodine Hall-effect electric propulsion system research, development, and system durability demonstration,“ in 2018 Joint Propulsion Conference, Cincinnati, 2018. N. G. Kottke, M. Tajmar and F. G. Hey, „Hollow cathode testing of Y2O3, La2O3-doped tungsten and LaB6 emitters with krypton and iodine,“ Vacuum, Vol. 220, p. 112812, 2024. Z. R. Taillefer, J. J. Blandino and J. Szabo, „Characterization of a Barium Oxide Cathode Operating on Xenon and Iodine Propellants,“ Journal of Propulsion and Power, Vol. 36, p. 575–585, 2020. Zachary R. Taillefer, „Characterization of the Near Plume Region of Hexaboride and Barium Oxide Hollow Cathodes operating on Xenon and Iodine,“ 2018. S. J. Thompson, J. J. VanGermert, C. C. Farnell, C. C. Farnell, S. C. Farnell, T. J. Hensen, R. Ham, D. D. Williams, J. P. Chandler and J. D. Williams, „Development of an Iodine Compatible Hollow Cathode,“ in AIAA Propulsion and Energy 2019 Forum, Indianapolis, 2019. N. G. Kottke, F. G. Hey and M. Tajmar, „Iodine hollow cathode development and testing with alternative emitters,“ in 37th International Electric Propulsion Conference, Massachusetts Inst. of Technology, Cambridge, MA, 2022. N. G. Kottke, M. Vaupel, K. Waetzig, J. Schilm, W. Konrad, M. Tajmar and F. G. Hey, „Investigation of C12A7 electride as a thermionic neutralizer,“ in SPACE PROPULSION 2020+1 Conference, Online, 2020. L. P. Rand and J. D. Williams, „Instant start electride hollow cathode,“ structure, Vol. 9, p. 11, 2013. L. P. Rand, „A CALCIUM ALUMINATE ELECTRIDE HOLLOW CATHODE,“ 2014. Z. Hua, P. Wang, Z. Ning, Z. Ye and Z. Xu, „Early experimental investigation of the C12A7 hollow cathode fed on iodine,“ Plasma Science and Technology, 2022. M. Reitemeyer, D. Zschaetzsch, P. J. Klar, A. Post, J. F. Plaza and J. Toledo, „C12A7:2e- neutralizer operation with alternative propellants,“ in 37th International Electric Propulsion Conference, Massachusetts Inst. of Technology, Cambridge, MA, 2022. S. W. Kim and H. Hosono, „Synthesis and properties of 12CaO*7Al2O3 electride: review of single crystal and thin film growth,“ Philosophical Magazine, Vol. 92, p. 2596–2628, 2012. Y. Huang, X. Wang, G. Cui, P. Wang and D. Cai, „Effect of Working Current on C12A7 Hollow Cathode,“ Aerospace, Vol. 10, p. 339, 2023. C. Drobny, K. Wätzig, A. Rost and M. Tajmar, „Endurance operation of a heaterless C12A7 electride plasma cathode and post operation material analysis,“ Acta Astronautica, Vol. 214, p. 231–239, 2024. C. Drobny, „Development of a Low-Current Plasma-Based Cathode using the Emitter Material C12A7 Electride for Space Applications,“ 2023. K. Waetzig and J. Schilm, „Electronic, mechanical, and thermal properties of [Ca24Al28O64] 4+(4e−) electride ceramic.,“ Ceramic Engineering & Science, pp. 165-172, May 2021. R. Liu, Wei, Y. Li, W. Wang, G. Zhang and H. Tang, „Low current iodine-fed hollow cathode discharge: insights from fluid model,“ Plasma Sources Sci. Technol., p. 33, 2024. F. Marmuse, Iodine plasmas : experimental and numerical studies. Application to electric propulsion., Sorbonne Université, 2020. T. Lafleur, L. Habl, E. Zorzoli Rossi and D. Rafalskyi, „Development and validation of an iodine plasma model for gridded ion thrusters,“ Plasma Sources Sci. Technol., p. 31, 2022. D. Levko and L. Raja, „Fluid modeling of inductively coupled iodine plasma for electric propulsion conditions,“ Journal of Applied Physics, p. 130, 2021. A. Saifutdinova, A. Makushev and F. Gatiyatullin, „Simulation of the Plasma Parameters Dynamics in Iodine in an Electric Rocket Engine based on ICP Discharge,“ High Energy Chem, No. 58, pp. 215-224, 2024. B. Esteves, Investigation of iodine plasmas for space propulsion, Institut Polytechnique de Paris, 2022. F. M. Bianchi, A. E. Vinci and L. Garrigues, „A global model of an iodine-fed Hall thruster,“ in 38th International Electric Propulsion Conference, Toulouse, France, 2024. X. Niu, X. Li, H. Liu and e. al., „Fluid simulation of ionization process in iodine cusped field thruster,“ Eur. Phys. J. D, No. 73, p. 169, 2019. N. Souhair, M. Magarotto, S. Dalle Fabbriche, R. Andriulli, S. Andrews, F. Ponti and D. Pavarin, „Simulation and modelling of an iodine fed Helicon Plasma,“ in 37th International Electric Propulsion Conference, Cambridge, MA, 2022. Bronkhorst, „FLUIDAT ON THE NET,“ [Online]. Available: www.fluidat.com. [Zugriff am 15 10 2024]. R. L. Summers, „Nasa technical note tn d-5285,“ National Aeronautics and Space Administration, Washington, 1969. D. W. Umrath, Fandamentals of Vacuum Technology, Cologne: Leybold GmbH, 2016. M. O. McLinden, E. W. Lemmon and D. G. Friend, Thermo-physical Properties of Fluid Systems, P. J. Linstrom and W. G. Mallard, Ed., Gaithersburg, MD: National Institute of Standards and Technology, 2024. D. M. Goebel and I. Katz, Fandamentals of Electric Propulasion, Hoboken, NJ: John Wiley & Sons, Inc, 2008. C. Drobny and M. Tajmar, „Concerning the ignition of a C12A7 electride plasma-based cathode,“ Journal of Electric Propulsion, Vol. 3, p. 2, 2024. N. G. Kottke, M. Vaupel, M. Tajmar, W. Konrad, N. Saks and F. G. Hey, „Comparison of the thermionic emission properties of LaB6 and C12A7,“ in 36th International Electric Propulsion Conference, University of Vienna, Vienna, Austria, 2019. Additional Declarations No competing interests reported. Supplementary Files BECKETestingData.xlsx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5738180","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":400836021,"identity":"db1bafd4-4fdb-4a82-8dc9-a4519918cb99","order_by":0,"name":"Philipp S. Becke","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA9klEQVRIiWNgGAWjYDACCcbGAwlA2oCBgZmBoYKBh0GCsJYGJC1niNLCwHCAAaaFsQ0igheYSzc3HHi44w6DOXvzY4OP8+xkzGf3GH9gqLGJxqXFcs7BhgOJZ54xWPYcM06cuS2ZR+bOGTMJhmNpuQ04tBjcSARqaTsMZOQwH+bdxswjIZFjxsDYcJgILfffMB/+O6cepMX4A3FabvAwJwNVgrQYSODTYjkjEewXHsueNGPDnmPHeSRkjpVJJODxi7lE+sOHP3fckTNnP/xY4kdNtb2EdPPmDx9qbHA7DEQAY5MHVTgBh3JkLXiUjIJRMApGwYgHADMIX2dWp3CrAAAAAElFTkSuQmCC","orcid":"","institution":"Airbus","correspondingAuthor":true,"prefix":"","firstName":"Philipp","middleName":"S.","lastName":"Becke","suffix":""},{"id":400836022,"identity":"6bc1e3f2-530e-4814-9c8d-819a7eb49498","order_by":1,"name":"Nils Gerrit Kottke","email":"","orcid":"","institution":"Airbus","correspondingAuthor":false,"prefix":"","firstName":"Nils","middleName":"Gerrit","lastName":"Kottke","suffix":""},{"id":400836023,"identity":"a5d35368-c976-4283-a2fb-3b3e11867152","order_by":2,"name":"Max Vaupel","email":"","orcid":"","institution":"Airbus","correspondingAuthor":false,"prefix":"","firstName":"Max","middleName":"","lastName":"Vaupel","suffix":""},{"id":400836024,"identity":"677d4c1f-cb72-4847-8692-1f14cee4ff55","order_by":3,"name":"Niccola Kutufa","email":"","orcid":"","institution":"ESTEC, ESA","correspondingAuthor":false,"prefix":"","firstName":"Niccola","middleName":"","lastName":"Kutufa","suffix":""},{"id":400836025,"identity":"d08b618e-f6f3-4d6b-869c-1896d10a123a","order_by":4,"name":"Katja Wätzig","email":"","orcid":"","institution":"Fraunhofer IKTS","correspondingAuthor":false,"prefix":"","firstName":"Katja","middleName":"","lastName":"Wätzig","suffix":""},{"id":400836026,"identity":"e7783bca-52fd-43ea-ae33-89297f46d564","order_by":5,"name":"Martin Tajmar","email":"","orcid":"","institution":"Technische Universität Dresden","correspondingAuthor":false,"prefix":"","firstName":"Martin","middleName":"","lastName":"Tajmar","suffix":""},{"id":400836027,"identity":"a6e68657-c6cf-46c1-99f1-c856d5e1b91b","order_by":6,"name":"Franz Georg Hey","email":"","orcid":"","institution":"Airbus","correspondingAuthor":false,"prefix":"","firstName":"Franz","middleName":"Georg","lastName":"Hey","suffix":""}],"badges":[],"createdAt":"2024-12-31 00:23:06","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5738180/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5738180/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":73697512,"identity":"9a02ea23-f95d-4b1e-be3f-686079272772","added_by":"auto","created_at":"2025-01-13 16:33:05","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":876326,"visible":true,"origin":"","legend":"\u003cp\u003eTimeline of the IcoN project. In the first year, a literature study was conducted and the laboratory model was designed based on thermal simulations. It was then tested with krypton and later with iodine. After a redevelopment of the feed system, further iodine tests did conclude the project.\u003c/p\u003e","description":"","filename":"Figure1.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/7416b4cffb5315d9b6aa8582.png"},{"id":73695780,"identity":"80d4c40b-6ada-42e5-bc67-381dbd7ebf3b","added_by":"auto","created_at":"2025-01-13 16:17:06","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":21435504,"visible":true,"origin":"","legend":"\u003cp\u003eVacuum facility at the LET at Airbus Friedrichshafen used for the iodine neutraliser development. The turbomolecular pump is mounted directly on the vacuum chamber (a), which is connected through a filter to the forestage pump (b), the krypton feed system is next to the chamber (c). Also, some of the power supplies are visible (d).\u003c/p\u003e","description":"","filename":"Figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/0eab221b64d3f2a43faf54c7.png"},{"id":73696679,"identity":"ded5a76e-79ab-4150-ac68-63ba63ef8d40","added_by":"auto","created_at":"2025-01-13 16:25:05","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":817327,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic of the iodine feed system in the first version (a) and second version (b). The first version included only a simple on-off valve, while the second version allowed a precise mass-flow control with the proportional valve. Moreover, a dual feed system enabled switching between krypton and iodine without venting the chamber.\u003c/p\u003e","description":"","filename":"Figure3.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/9164f320c65f0f3b3abbb5f6.png"},{"id":73695763,"identity":"3441cb93-77e3-4ff0-bdae-ac97b8199c03","added_by":"auto","created_at":"2025-01-13 16:17:05","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":548447,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic of the vacuum facility, including the krypton feed system, the feed block inside the vacuum chamber (see Fig. 3), the cathode and anode, and the pumping system.\u003c/p\u003e","description":"","filename":"Figure4.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/096583b0cf6724175c15b5c6.png"},{"id":73695767,"identity":"35a39497-4726-458c-ace2-4fe4d2d0ebc3","added_by":"auto","created_at":"2025-01-13 16:17:05","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":2198268,"visible":true,"origin":"","legend":"\u003cp\u003eCalibration of the iodine feed system including a proportional valve with krypton, while the cathode was operating. The estimated corresponding iodine flow rate is shown in red on the right side of the plot. The fit has been calculated as a function of the inlet pressure.\u003c/p\u003e","description":"","filename":"Figure5.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/bc31cfd54f00874daea0bf40.png"},{"id":73695774,"identity":"d0dc4906-3054-42c5-b5f5-0c1a1c3ad945","added_by":"auto","created_at":"2025-01-13 16:17:05","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":2225646,"visible":true,"origin":"","legend":"\u003cp\u003eUncorrected pressure plot of the iodine gas flow through the proportional valve (a) and comparison of the krypton mass flow controller (MFC) flow rate to the corrected iodine mass flow (b). The error bars show the standard deviation.\u003c/p\u003e","description":"","filename":"Figure6.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/035c54b8346d3b499f9bd9c7.png"},{"id":73698422,"identity":"3b319015-67bd-4efa-b84c-c374c2343326","added_by":"auto","created_at":"2025-01-13 16:41:05","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":14148263,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic of the configuration A of the planar C12A7:e- cathode (a). The emitter is shown in orange, the keeper in green, the heater in red, the central tube in blue and the cathode mount in yellow. Picture (b) shows the planar emitter clamped with a refractory clamp, picture (c) the clamped, molten C12A7:e- emitter and picture (d) the emitter glued to the graphite mount.\u003c/p\u003e","description":"","filename":"Figure7.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/3726c1e2d5b2ec238c74806b.png"},{"id":73695792,"identity":"740ca980-47e8-4820-b6ff-ca0a2e635ac6","added_by":"auto","created_at":"2025-01-13 16:17:06","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":6332899,"visible":true,"origin":"","legend":"\u003cp\u003eThermal simulation for different thermal contact resistances between the heater and the cathode tube, leading to the heater emitting a higher current than the emitter in case 3 (a). The front part of the cathode tube with the molten metal shielding (b) and the cathode showing a bright spot on the upper side next to the heater during operation (c).\u003c/p\u003e","description":"","filename":"Figure8.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/0610bc1b1cd38b30317e1fb0.png"},{"id":73695769,"identity":"615c98be-1a27-401c-9a32-7c3ffa9ede93","added_by":"auto","created_at":"2025-01-13 16:17:05","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":991267,"visible":true,"origin":"","legend":"\u003cp\u003eComparison between configuration A (cf. Fig. 7) and B (cf. Fig. 10). The cathode operated in triode mode and with krypton as a propellant. No error bars are given, as the data was hand-recorded.\u003c/p\u003e","description":"","filename":"Figure9.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/f64cb2b8d02b7915f5fed2fe.png"},{"id":73695775,"identity":"d1c69cc5-db74-453f-94e4-00f74b35cc7b","added_by":"auto","created_at":"2025-01-13 16:17:05","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":9225126,"visible":true,"origin":"","legend":"\u003cp\u003eAdapted design of the planar cathode with an orifice disk in front of the emitter (config. B).\u003c/p\u003e","description":"","filename":"Figure10.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/884db90a3a351c984c940f66.png"},{"id":73696693,"identity":"bfe58af2-6550-44d4-aecd-2b3cf2cccfdf","added_by":"auto","created_at":"2025-01-13 16:25:06","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":2927583,"visible":true,"origin":"","legend":"\u003cp\u003eKeeper voltage of the C12A7:e- planar cathode in configuration B during the first longer iodine discharge (a). Phases of instability alternate with stable discharges. Picture (b) shows the iodine discharge in plume mode. Keeper voltage during an unstable krypton discharge in plume mode with a flow rate of 20 sccm (c). Picture (d) shows the krypton discharge in plume mode. Krypton discharge in spot mode with a flow rate of 10 sccm (e). Picture (f) shows the krypton discharge in spot mode. The red area in the graphs indicates the standard deviation over 8 seconds, the blue line is the mean over 1 second.\u003c/p\u003e","description":"","filename":"Figure11.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/d37187776391b93dbc50776a.png"},{"id":73697514,"identity":"4525ae61-6b5c-4184-8cac-e1e41938c8aa","added_by":"auto","created_at":"2025-01-13 16:33:05","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":4403778,"visible":true,"origin":"","legend":"\u003cp\u003eState of the neutraliser after the iodine test. The secondary orifice and the ceramic spacer (a) were disconnected from the tube (c). The emitter was covered with a black material (b).\u003c/p\u003e","description":"","filename":"Figure12.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/ac74bac67358d578fa5163a6.png"},{"id":73696698,"identity":"f3a61aaf-ab49-4be2-aa58-07b06ea64fca","added_by":"auto","created_at":"2025-01-13 16:25:07","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":762381,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic of the second version of the planar C12A7:e- cathode (a). Close-up of the front section in configuration D with the C12A7:e- emitter exposed (b), and close-up of the front section with a secondary orifice (c) in configuration C. The screwable graphite mount that holds the emitter is colored in blue, the emitter in gray, LaB\u003csub\u003e6\u003c/sub\u003e in purple, molybdenum in orange, the keeper in green and stainless steel in yellow.\u003c/p\u003e","description":"","filename":"Figure13.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/b00cff4050406d66c507d39e.png"},{"id":73695779,"identity":"a3275560-2175-4cd6-8398-baaebf3cc989","added_by":"auto","created_at":"2025-01-13 16:17:06","extension":"png","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":1913225,"visible":true,"origin":"","legend":"\u003cp\u003eSecond version of the planar hot plasma bridged neutraliser without (a) and with keeper (b), and in a plasma discharge in spot mode with krypton (c). The graphite anode is visible on the left side of the cathode.\u003c/p\u003e","description":"","filename":"Figure14.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/569792b8d6373f1f5965ab46.png"},{"id":73695805,"identity":"e9e1919c-6723-4167-8d69-b7895650ae44","added_by":"auto","created_at":"2025-01-13 16:17:07","extension":"png","order_by":15,"title":"Figure 15","display":"","copyAsset":false,"role":"figure","size":1552158,"visible":true,"origin":"","legend":"\u003cp\u003eMapping of the spot mode of the cathode in configuration D with the C12A7 electride emitter type 1. The keeper orifice was changed by exchanging the molybdenum plate and the anode current and flow rate were both varied until spot mode was reached. The color of the dots represents the total power necessary to sustain the discharge. Small dots without larger dots represent the operation in plume mode.\u003c/p\u003e","description":"","filename":"Figure15.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/f3b9988ea4e3bfe94dbefc12.png"},{"id":73696692,"identity":"67019b25-7dd6-4edb-8af2-5a7c61ef759b","added_by":"auto","created_at":"2025-01-13 16:25:06","extension":"png","order_by":16,"title":"Figure 16","display":"","copyAsset":false,"role":"figure","size":1173950,"visible":true,"origin":"","legend":"\u003cp\u003eTotal power as function of time in test 1 with the planar cathode in configuration C with a secondary orifice and a C12A7 electride type A emitter. The gas used during the discharge is indicated above the plot. The red area indicates the standard deviation over 8 seconds, the blue line is the mean over 1 second.\u003c/p\u003e","description":"","filename":"Figure16.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/f91e1f2229e31fde2b4939d5.png"},{"id":73697516,"identity":"59ceb740-3097-4cf2-89e4-e1de92df034d","added_by":"auto","created_at":"2025-01-13 16:33:06","extension":"png","order_by":17,"title":"Figure 17","display":"","copyAsset":false,"role":"figure","size":1233137,"visible":true,"origin":"","legend":"\u003cp\u003eTotal power as function of time in test 2 with the planar cathode in configuration D without a secondary orifice and with a C12A7 electride type A emitter. The gas used during the discharge is indicated above the plot. The red area indicates the standard deviation over 8 seconds, the blue line is the mean over 1 second.\u003c/p\u003e","description":"","filename":"Figure17.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/8b7e5d260132ad209a7e1a8f.png"},{"id":73695795,"identity":"6522bb82-dc9a-4b30-a43b-8c1664009dc1","added_by":"auto","created_at":"2025-01-13 16:17:06","extension":"png","order_by":18,"title":"Figure 18","display":"","copyAsset":false,"role":"figure","size":1073421,"visible":true,"origin":"","legend":"\u003cp\u003ePicture of the iodine discharge during test 1 (a) and test 2 (b).\u003c/p\u003e","description":"","filename":"Figure18.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/061c0c93588aea1a73b60e0b.png"},{"id":73696709,"identity":"1a20f81b-bf0e-44fb-84e3-fbfecf90f46c","added_by":"auto","created_at":"2025-01-13 16:25:08","extension":"png","order_by":19,"title":"Figure 19","display":"","copyAsset":false,"role":"figure","size":1542020,"visible":true,"origin":"","legend":"\u003cp\u003eTotal power as function of time in test 3 with the planar cathode in configuration D without a secondary orifice and with a C12A7 electride type B emitter. The discharge was only with krypton. The red area indicates the standard deviation over 8 seconds, the blue line is the mean over 1 second.\u003c/p\u003e","description":"","filename":"Figure19.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/ac6f01a89949edb7b4f8763d.png"},{"id":73695800,"identity":"118fceab-7a1d-43f0-bf13-e246cbdfe1ed","added_by":"auto","created_at":"2025-01-13 16:17:07","extension":"png","order_by":20,"title":"Figure 20","display":"","copyAsset":false,"role":"figure","size":1083470,"visible":true,"origin":"","legend":"\u003cp\u003ePicture of the iodine discharge of test 3 at three different times during the test shown in Fig. 21.\u003c/p\u003e","description":"","filename":"Figure20.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/b830489890ea66a2e10b926f.png"},{"id":73696717,"identity":"d1fd9d01-aa9b-4594-8007-c5ed051f2b7f","added_by":"auto","created_at":"2025-01-13 16:25:09","extension":"png","order_by":21,"title":"Figure 21","display":"","copyAsset":false,"role":"figure","size":2494939,"visible":true,"origin":"","legend":"\u003cp\u003eTotal power as function of time in test 3 with the planar cathode in configuration D without a secondary orifice and a C12A7 electride type B emitter. The discharge was only with iodine. The red area indicates the standard deviation over 8 seconds, the blue line is the mean over 1 second.\u003c/p\u003e","description":"","filename":"Figure21.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/18c49493622ef718052f4f57.png"},{"id":73695817,"identity":"30e1616b-ea76-4b35-a265-66111a3025b8","added_by":"auto","created_at":"2025-01-13 16:17:08","extension":"png","order_by":22,"title":"Figure 22","display":"","copyAsset":false,"role":"figure","size":4958515,"visible":true,"origin":"","legend":"\u003cp\u003eC12A7 electride type A emitter before the test (a), after the test with krypton (b), the first emitter after the iodine test (c) and the second emitter after the iodine test (d).\u003c/p\u003e","description":"","filename":"Figure22.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/bb394d43a8cf2860b22a7247.png"},{"id":73696711,"identity":"27e542fd-8969-48f6-a70e-7e078dbba41a","added_by":"auto","created_at":"2025-01-13 16:25:08","extension":"png","order_by":23,"title":"Figure 23","display":"","copyAsset":false,"role":"figure","size":3381302,"visible":true,"origin":"","legend":"\u003cp\u003eKeeper front molybdenum plate before the first test (a) and after the first test (b). Keeper orifice blocked by molten material after test 3 (c).\u003c/p\u003e","description":"","filename":"Figure23.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/987f7af9d648ff5f2b4031d1.png"},{"id":73695824,"identity":"679a2319-4e82-48a5-8cc5-3008276d5e3d","added_by":"auto","created_at":"2025-01-13 16:17:08","extension":"png","order_by":24,"title":"Figure 24","display":"","copyAsset":false,"role":"figure","size":4370349,"visible":true,"origin":"","legend":"\u003cp\u003eC12A7 electride type B emitter before the test (a), after the test with krypton (b), the first emitter after the iodine test (c) and the second emitter after the iodine test (d).\u003c/p\u003e","description":"","filename":"Figure24.png","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/5532ef55ceba79214802ae93.png"},{"id":73698421,"identity":"9f7b7543-75f3-4ffa-b940-c1edc4b21e73","added_by":"auto","created_at":"2025-01-13 16:41:05","extension":"xlsx","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":85667,"visible":true,"origin":"","legend":"","description":"","filename":"BECKETestingData.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-5738180/v1/43c83084be2cfa120544e3a9.xlsx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Development and Iodine Testing of a Novel C12A7 Electride Planar Hollow Cathode","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eIn the past ten years, several attempts have already been made at designing an iodine-compatible neutraliser. While this challenge is essentially solved for the extraction of small currents by using thermionic filament cathodes [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e] or radio-frequency (RF) neutralisers [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], it is still open for mid- to large-scale thrusters. To extract currents in the ampere range for a reasonable energy consumption, only hot plasma bridged neutralisers offer a solution [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eHollow cathodes based on lanthanum hexaboride (LaB\u003csub\u003e6\u003c/sub\u003e) [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e], dispenser emitter [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] and lanthanated tungsten (WL20) [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e] have been designed and tested with iodine. However, none of them was capable of reaching a continuous and stable plasma discharge. Dispenser emitter and WL20 seem to be incompatible due to their reaction with iodine [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e], which leads to a fast depletion of the low work-function component. The compatibility of LaB\u003csub\u003e6\u003c/sub\u003e seems to be questionable [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] and no definitive conclusion can be drawn from the literature. A comparably new material is 12CaO\u0026middot;7Al2O3 (or short C12A7) electride [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], which has been tested as an emitter material in hollow and planar cathodes using iodine as a propellant [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Similarly, no definitive conclusion of the long-term compatibility of C12A7 electride and iodine has been published, although preliminary results look encouraging. For a comprehensive overview of iodine neutralisers, see Becke et al. [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe Laboratory for Enabling Technologies (LET) at Airbus in Friedrichshafen, Germany, has focused on the development of iodine-compatible electric propulsion (EP) technologies. Within the scope of the European Space Agency (ESA) fanded Iodine Compatible Neutraliser (IcoN) development activity, C12A7 electride as an emitter material for iodine-compatible cathodes has been examined.\u003c/p\u003e \u003cp\u003eC12A7 is a ceramic compoand that contains an anion sublattice, which can be replaced with electrons, creating a so-called electride (C12A7:e- or C12A7:2e-). It has a work function of 2.4 to 2.96 eV, a melting point of up to 1410\u0026deg;C and a thermal conductivity of 1.7 to 2.3 W/(m\u0026times;K) at higher temperatures [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. The comparable low melting point is close to the temperature of 1340\u0026deg;C for the theoretical thermionic emission of 1 A from an emitter with a surface area of 10 mm\u0026sup2;, which can be calculated by using the Richardson-Dushman equation for a work function of 2.4 eV. The emission temperature being close to the melting point and the low thermal conductivity can easily lead to local overheating of the C12A7:e- insert, as it has been shown for C12A7:e- hollow cathodes tested with iodine. As the plasma in the hollow cathode and, therefore, its heating mechanism is usually located at the downstream end of the emitter, local hotspots of molten emitter material inside the cylindrical emitter have been described [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. A different approach has been chosen by Drobny et al. [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. They used planar C12A7:e- inserts and successfully demonstrated a plasma discharge based on krypton for nearly 1000 h. The advantage of the planar shape is the exposure of a larger area to the plasma and the cathode mount, allowing equally well the heating and conductive heat dissipation.\u003c/p\u003e \u003cp\u003eBased on these results, the design of the IcoN cathode was based on C12A7:e- discs in a planar design. The greatest challenge next to the low melting point of the insert material is the compatibility of the hot materials with the iodine plasma. Therefore, the activity focused on solving these main issues by designing a planar C12A7:e- cathode from iodine-compatible materials.\u003c/p\u003e \u003cp\u003eThe development methodology is presented in the next section, followed by the description of the test facility, the preliminary development of the C12A7 electride neutraliser and the iodine feed system. Hereafter, the results of the iodine cathode tests are presented, as well as the post-test analysis. At the end, a conclusion and recommendation on further iodine neutraliser development based on the experiences from this activity are given.\u003c/p\u003e"},{"header":"2 Methodology","content":"\u003cp\u003eThe challenges of developing an iodine-compatible neutraliser are manifold and concern the selection of materials, the cathode geometry, the vacuum pumping system and the iodine feed system. The challenges of designing the cathode geometry are mostly based on the complexity of the molecular plasma discharge and the surface reactions. There have been fluid simulations on iodine plasma in a hollow cathode discharge [23], a global model for the discharge in an RF thruster [24, 25, 26, 27, 28], for a Hall effect thruster (HET) [29], a cusp field thruster (CFT) [30] and a helicon thruster [31]. There are no existing simulations on iodine plasma discharges in planar cathodes and similarly only very few experimental results in the literature [6]. Using a simulation to derive design decisions requires the knowledge of a few parameters, which can only be estimated prior to testing. Therefore, a simulation can be fitted to an existing cathode, however, modelling a completely new geometry comes with a high error.\u003c/p\u003e\n\u003cp\u003eTherefore, an experimental approach was chosen. It is based on fast iterations and parameter studies, which allow to explore and evaluate design changes in a matter of days. These fast development cycles were aided by the access to on-site manufacturing, reducing the periods between the tests. However, thermal simulations were used to evaluate the required heating of the emitter to reach thermionic emission and the estimation of thermal losses. Each design change was first verified with krypton and eventually tested with iodine, if the performance was sufficient. Testing with noble gases requires a less complex setup and gives a baseline for each iteration, allowing the comparison between the changes. Although iodine is a molecular gas and, therefore, has different properties than krypton, this can be later taken into account by small adjustments, especially by changing the orifice diameter or propellant flow rate. Following this procedure is highly recommended for experimental designs without prior knowledge, as a lot of unforeseen problems only appeared while testing. This has led to many design iterations to solve these difficulties.\u003c/p\u003e\n\u003cp\u003eThe complete timeline of the IcoN project is shown in Fig. 1. At the beginning, an extensive literature study was conducted [6]. The following activities included the definition of the requirements, thermal simulations and the design of laboratory models. In the first year, the development was focused on a stable krypton discharge. After having achieved this step, the tests were continued with iodine. In the following, the iodine feed system was further improved and the cathode tests with iodine were finalised.\u003c/p\u003e"},{"header":"3 Development and Testing","content":"\u003cp\u003eAll tests referenced in this work were performed at the Laboratory for Enabling Technologies (LET) at Airbus Friedrichshafen. In this section, the test facility is first described, including the design of the iodine feed system. It is followed by the development and iterations of the cathode design based on preliminary krypton and iodine tests. The section is concluded by the cathode designs that were used in the final iodine tests.\u003c/p\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Testing Setup\u003c/h2\u003e \u003cp\u003eThe test chamber used for the cathode development and testing is a vacuum chamber with 40 l volume equipped for iodine cathode testing (see Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe vacuum is produced by a two-stage pumping system, featuring a forestage pump and a turbomolecular pump to reach the 1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e mbar range (cf. Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). To reduce the likelihood of iodine corrosion damage to the pumping system, nitrogen purging and a ceramic filter are installed. No cold trap was used, as both pump types were rated for iodine. However, the iodine condensation did damage the fore pump, which could no longer reach its final pressure after a few days of pumping iodine. We assume a damage to the sealing material, as the leakage through the pump increased. A temporary solution to this problem was the purging of the pump with high flows of nitrogen at regular intervals to remove iodine residues from the pump. However, a cold trap would be advised for more extensive test campaigns.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eLaboratory power supplies and measurement systems used in the cathode tests.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eModel\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eManufacturer\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eType\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eRange\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eUsage\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMCA 3000\u0026ndash;3000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFuG\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eHV power supply\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0 to 3 kV, 0 to 3 A\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eAnode/keeper\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDP-PH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eDSC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eHV power supply\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0 to 1 kV, 0 to 1.2 A\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eAnode/keeper\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHMP4040\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eRohde \u0026amp; Schwarz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePower supply\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0 to 32 V, 0 to 10 A\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eKeeper/heater\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHM7044\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eRohde \u0026amp; Schwarz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePower supply\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0 to 32 V, 0 to 3 A\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eValve/heater\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eWR204\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLeCroy\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eOscilloscope\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eup to 2 GHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eCathode current\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCP031A\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLeCroy\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCurrent probe\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eup to 100 MHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eCathode current\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eED 582\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBrainbox\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTemperature meas.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-200\u0026deg;C to 300\u0026deg;C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eFeed system temp.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eED 549\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBrainbox\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eThermocouple meas.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0\u0026deg;C to 2315\u0026deg;C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eEmitter temp.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIDM 103N\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eRS PRO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMultimeter\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0 to 1000 V\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eElectrical inspection\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eA graphite plate served as an anode and was placed 10 mm in front of the cathode, which was chosen to keep consistent results with earlier experiments. The keeper and anode are controlled by two HV-power supplies, which allow maximum currents of 1.2 A at the keeper and 3 A at the anode at a voltage of up to 1 kV (3 kV at the anode). The temperature inside the cathode is monitored by a thermocouple type C, which is mounted close to the emitter and a PT100 sensor at the base of the cathode. See Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e for more details.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003ePumps, sensors and flow regulation devices used in the tests.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eModel\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eManufacturer\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDescription\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eProperty\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eScrollvac 15 plus C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLeybold\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eForestage pump\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003emax. pumping speed: 4 l/s (N2)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTurbovac Mag 2200\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLeybold\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTurbomolecular pump\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003emax. pumping speed: 2100 l/s (N2)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIonivac ITR200\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLeybold\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePressure sensor\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003emeas. range: 5\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;10\u003c/sup\u003e \u0026minus; 1000 mbar\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGE50A013501\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMKS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eKrypton mass flow-controller\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0\u0026thinsp;\u0026minus;\u0026thinsp;30 sccm\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eInitially, the test setup was limited to a single feed line, resulting in the need to interrupt testing to switch between krypton and iodine tests. Since this procedure exposed the cathode to ambient atmosphere, with possible detrimental effects on the materials, the test setup was reworked. In a later iteration, the facility was equipped with a dual feed system for iodine and krypton. This allows testing the neutraliser with noble gases and iodine without venting the chamber and exposing the cathode to the humidity in the air. A similar approach has been chosen by Taillefer [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. The krypton mass flow is controlled by a 30 sccm mass flow-controller and can be closed to prevent iodine backflow. Furthermore, an in-line filter was installed to protect the krypton feed system components from iodine.\u003c/p\u003e \u003cp\u003eThe iodine feed system (see Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) is located completely inside the vacuum chamber, allowing for a faster heating due to the missing convection losses and preventing an exposure of the laboratory personnel in the unlikely case of iodine leakage. The complete feed line is heated to 130\u0026deg;C to prevent iodine condensation. The iodine tank can be precisely heated to allow the adjustment of the pressure inside the feed system. The temperature is monitored at various locations along the feed line and adjusted by various heaters. In the first iteration of the iodine feed system, a pressure sensor was foreseen to determine the iodine pressure. However, different commercial-off-the-shelf pressure sensors were tested and failed due to corrosion. Moreover, the pressure regulation by using a simple on-off valve and the tank temperature proved to be unreliable.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo address these issues, a feed block based on a proportional valve was developed. The valve was then calibrated with a pressure sensor and krypton mass flow, while the cathode was ignited. The tubes and the valve block were heated to 130\u0026deg;C during the test. The proportional valve was slowly closed, while the pressure in the upstream side of the feed block was measured. This way, it was possible to link an upstream pressure and valve position to the corresponding mass flow. The complete schematic of the vacuum facility can be seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eThe valve setting in % plotted against the krypton mass flow rate in sccm for a given upstream pressure is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. When testing with iodine, the upstream pressure is given by the temperature of the iodine tank. The comparable iodine flow rate inside the valve block at 130\u0026deg;C has been estimated from the krypton flow rate using the FLUIDAT database [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. The results are shown in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. This can only be used as a first estimation, as other processes have not been included, especially the dissociation of the iodine molecule in the plasma.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eRequired iodine tank temperatures to reach flow rates from 5 to 20 sccm based on the test shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIodine flow rate\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eIodine pressure\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eIodine tank temperature\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ein sccm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ein mbar\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003ein \u0026deg;C\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e67.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e71.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e75.9\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e78.7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eTo test the actual performance of the new feed block, it was tested with iodine and krypton using the pressure sensor of the vacuum chamber. The valve block was mounted to the iodine tank, which was heated to 85\u0026deg;C. Then the valve was slowly opened and closed again, while the vacuum chamber pressure was measured. The same process was repeated with krypton, but through a mass flow controller instead of the proportional valve. To compare the measured pressure, some corrections needed to be applied. The pressure was measured with a hot filament gauge controller. The gas correction factor for hot filament gauge controllers for krypton is 1.7 and for iodine 5.4 according to Summers [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. Turbomolecular vacuum pumps are more efficient in pumping heavier gases. For light to medium-heavy atoms or molecules, the compression is proportional to the square root of the molecular mass [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. Therefore, the square root of the molecular weight has been considered as well. The iodine vapor pressure at 25\u0026deg;C is about 40 Pa [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e], which is way above the chamber pressure of 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e mbar. Consequently, the redeposition of iodine on the vacuum chamber walls has not been taken into account. The complete correction can be written as:\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:\\frac{{p}_{Kr}}{{p}_{{I}_{2}}}=\\:\\frac{5.4}{1.7}*\\frac{\\sqrt{83.798\\:}}{\\sqrt{253.809}}=0.18$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eand hence indicating that the measured iodine pressure corresponding to a comparable krypton flow rate needs to be multiplied by a factor of ~\u0026thinsp;5,56.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the results of the measurements. The left plot is the uncorrected measurement along the full valve range, the right plot is 50% of the corrected iodine valve range compared to a krypton flow of up to 30 sccm. It shows clearly that the new proportional valve is able to control the iodine mass flow comparably to the mass flow of the krypton mass flow controller. The exact range depends also on the iodine tank pressure, which has to be higher than the minimum pressure required by the cathode. Here, 85\u0026deg;C, so 28.6 mbar [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e], were chosen.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Preliminary Cathode Configurations\u003c/h2\u003e \u003cp\u003eThe initial planar cathode design was based on a hollow cathode, but instead of the tube, a planar disc was mounted on a rod close to the keeper orifice. A simplified design is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. The base of the prototype cathode is made from stainless steel, as it is only subjected to temperatures in the range of 100 to 300\u0026deg;C. For lifetime testing, an alloy like Inconel or Hastelloy should be selected [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. The main rod consists of two parts, one is hollow to allow the gas flow from the mount to the emitter. The gas leaves the first section of the tube through four small holes.\u003c/p\u003e \u003cp\u003eThe second section is solid and connected to the first section by a thread. The emitter is mounted to the second section by a metal clamp. The diameter of the planar emitter is 8 mm, which results in a surface of ~\u0026thinsp;50 mm\u0026sup2; and therefore in a maximum temperature of ~\u0026thinsp;1300\u0026deg;C to emit 3 A, assuming a work function of 2.4 eV. The heater is placed parallel to the tube behind the emitter and connected by two wires. The rod is made out of graphite to guarantee the iodine compatibility at high temperatures. The clamp, the heater, and the keeper consist of refractory metals. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb shows the front section of the tube with the heater and the emitter clamped to the front. As the refractory clamp proved to be unreliable and was expected to be incompatible with iodine, the emitter was attached with a high temperature glue to the graphite rod in later iterations (see Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ed). This method has already been described by Drobny et al. [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. The keeper was made from graphite as well and electrically insulated using a ceramic disc of boron nitride (BN).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe first laboratory models were only tested with krypton to achieve a design that is able to deliver a stable discharge. The required heating power of the emitter with the inbuild heater to reach emission temperatures was estimated in a thermal simulation. However, the ignition process was not very reliable, leading often to the emitter melting due to overheating. A picture of the molten emitter is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec. Although the molten emitter could be reignited, which means that at least parts of the material must have kept their properties, the test did quickly end because of short-circuits due to material build-up.\u003c/p\u003e \u003cp\u003eAnother problem encountered was the ignition of the plasma to the heater instead of the emitter. As the planar emitter has a large surface area, the thermal losses to the environment need to be compensated by the heater to reach emission temperatures. This requires more heating power compared to a conventional hollow cathode, which also increases the temperature of the heater. During the test, the side of the cathode showed hot spots (see Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ec), which meant the plasma was not only confined to the front. The inspection of the cathode after the test showed parts of the heater insulation were molten (see Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb). To quantify this observation, a thermal simulation with different thermal contact resistances between the heater and the cathode tube were conducted (see Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea). The thermal contact between the heater and the insulation (case 2) and additionally the insulation and the cathode tube (case 3) were reduced in comparison to the case with a perfect contact (case 1). The results indicated that in these conditions the heater starts emitting more current than the emitter, destroying the heater through the following plasma discharge. While the heater is protected by a ceramic sleeve, there a still tiny gaps remaining that allowed the discharge between the keeper and the heater.\u003c/p\u003e \u003cp\u003eTo circumvent the heater ignition problem, the pressure at the emitter was increased by changing the flow path. In this design iteration, the gas was fed through a centre hole in the emitter (cf. Figure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003ea) and a second graphite disc (a \u0026ldquo;secondary orifice\u0026rdquo;) was mounted in front of the emitter by gluing the disc to the tube (cf. Figure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003ec). This change also reduced the thermal stress on the emitter, as the heat radiation losses were decreased, leading to a lower required power to operate. This was further improved by impregnating the surface of the tube with ceramic glue to reduce the emissivity. Although metal shielding would be the preferred option due to the low emissivity, this was not chosen because it would be damaged by the corrosive iodine environment. The distance between the emitter and the secondary orifice could be varied by adding spacer rings of BN (cf. Figure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003eb). BN was chosen because it has a similar coefficient of thermal expansion (CTE) to graphite [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e] and an improved ignition was planned to be achieved by connecting the secondary orifice to the keeper. However, graphite deposits inside the BN ring short-circuited the keeper, prohibiting this ignition method. As a further change, a ceramic sleeve inside the keeper was introduced, so that only the front was exposed to the plasma. In this way, unwanted discharges to other parts of the cathode than the emitter can be prevented.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe comparison between both cathode types is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. The configuration shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e is referred to as configuration A, the configuration with a centre hole in the emitter as configuration B (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e). The electron cost is given in W/A, the fraction of the total power consumed by cathode and anode divided by the anode current, which allows a correlation between different cathode types and discharge modes [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. The lower the electron cost, the higher is the efficiency of the cathode. Compared to configuration A, the required power to operate the cathode in configuration B is greatly reduced. Furthermore, the flow rate has an influence on the electron cost as expected. However, the tests revealed that the design changes had a more significant impact on the performance.\u003c/p\u003e \u003cp\u003eWhile no plasma simulation was conducted, the surface changes of the C12A7 electride emitter after the operation indicated that the complete emitter surface was hot enough to emit electrons. We assume that this cathode type working principle is a combination of both hollow cathode and planar cathode discharges.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Preliminary Iodine Testing Results\u003c/h2\u003e \u003cp\u003eThe planar hollow cathode in configuration B was tested with iodine. The iodine tank was heated to 80\u0026deg;C, the iodine feed system was not yet equipped with a proportional valve (configuration shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea) and therefore no fast control of the pressure inside the cathode was possible. After the emitter had reached emission temperature, indicated by a thermionic current to the keeper, the iodine flow valve was opened.\u003c/p\u003e \u003cp\u003eA successful plasma ignition for aroand 3 minutes was achieved shortly after opening the valve. A plot of the keeper voltage is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ea. The discharge was in plume mode during the complete test. While the discharge collapsed multiple times, it could also be reignited again. The test was ended by a keeper short-circuit. The disassembly of the cathode revealed that the glue had failed and the secondary orifice was in contact with the emitter and keeper (cf. Figure\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e). The emitter was also covered with a blackish substance, see Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003eb. Although the coating of the emitter was not identified through material analysis, we assumed because of the soft quality of the structures that the high-power operation in plume mode led to graphite sputtering and carbon redeposition. Another possible alternative for the deposition was molten C12A7, however it is hard and greenish after solidifying, see also Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec, and therefore an unlikely candidate. A third possibility would be that the C12A7:e- material was modified through the exposure to the iodine plasma.\u003c/p\u003e \u003cp\u003eTo compare the iodine discharge in the unfavorable \u0026ldquo;plume mode\u0026rdquo; [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e], a discharge in plume mode with krypton for the same cathode configuration is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ec. Except for the ignition discharges in the first 150 s and between 350 s and 450 s in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ea, the voltage fluctuations have a similar amplitude. A test with the same configuration and a smaller orifice size (see Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ee) has indicated a stable krypton plasma discharge in the favorable \u0026ldquo;spot mode\u0026rdquo; [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e], with half the mass flow but the same pressure as during the test shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ec. This highlights the importance of the balance between pressure, mass flow and orifice size for a stable cathode discharge. With the orifice size being set by design before the start of the test, the pressure inside the cathode depends, among others, on the mass flow and the internal temperature. A self-sustained discharge can be adjusted by changing the pressure or the mass flow, which can be done for krypton in a matter of seconds by using a mass flow-controller to arrive at a favorable working point. In the test setup described in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, the iodine mass flow is controlled indirectly by adjusting the temperature of the iodine tank, which directly correlates to the iodine vapor pressure. This results in a slow response of the mass flow rates and further provides difficulties to calibrate the setup by correlating the pressure to mass flow rates.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Final Cathode Configurations\u003c/h2\u003e \u003cp\u003eTo prevent the problems that were foand after the iodine test, the design was further improved. The feed system was changed to include a proportional valve for iodine and a dual feed line for krypton with an on-off valve (see section 3.1). After applying these changes, heaterless ignition became possible by creating a pressure surge and applying a voltage above 300 V to the keeper. Later versions of the cathode were only ignited heaterless, because the power required to heat the emitter to emission temperature was exceptionally high due to the high thermal emissivity of the graphite keeper and tube. Additionally, the heaterless ignition resulted in a faster ignition overall. Heaterless ignition of C12A7 electride was also studied by Drobny et al. [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe remaining difficulties were addressed by a partial redesign of the cathode. To remove the necessity of the glue, the emitter is held in place by a screwable graphite cup (cf. Figure\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e). This also allows more efficient testing, as small changes can be implemented without reglueing parts of the cathode. The difficulty of the carbon deposition was solved by avoiding direct contact between the iodine plasma and graphite in the vicinity of the emitter. This was achieved by having the secondary orifice and a spacer ring made from LaB\u003csub\u003e6\u003c/sub\u003e. Similarly, the keeper front plate is made from molybdenum. This configuration C is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003ec.\u003c/p\u003e \u003cp\u003eA second version of this cathode (config. D) has been tested as well, but without the secondary orifice (cf. Figure\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003eb). Here, the emitter is directly placed in front of the keeper plate.\u003c/p\u003e \u003cp\u003eTwo different emitter types have been developed in cooperation with the Fraunhofer IKTS: C12A7 electride type 1 and type 2. Both are doped with different metals. Type 2 is optimised for a lower work function; however, it has a low melting point and low thermal conductivity. Type 1 has been optimised for a higher thermal conductivity, but it has a higher work function and contains a significant amount of metal, which might react with iodine during a longer exposure. The preliminary development tests have been conducted with type 1. The assembled cathode is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section3\"\u003e \u003ch2\u003e3.4.1 Krypton Testing of the cathode in configuration D\u003c/h2\u003e \u003cp\u003eThe performance of the cathode in configuration D with the C12A7 electride emitter type 1 has been tested for three different keeper orifice diameters (cf. Figure\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e15\u003c/span\u003e) with krypton (cf. Figure\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003ec). Before each of the three tests, the vacuum chamber was pumped overnight, resulting in a pressure of ~\u0026thinsp;1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e mbar. The cathode was ignited heaterless by applying a voltage of 400 V to the keeper and anode, then opening the krypton feed valve, resulting in a pressure surge. The keeper current was reduced afterwards. After the cathode reached a thermal equilibrium that was measured at the mount interfacing the vacuum facility, the anode current was reduced for a fixed flow rate. If the cathode discharge changed to plume mode, the keeper current was increased up to a maximum of 1.2 A. The discharge mode was verified by monitoring the oscillations of the anode current.\u003c/p\u003e \u003cp\u003eGenerally, a larger keeper orifice resulted in an operation at a higher mass flow. However, also the smallest keeper orifice diameter required a high mass flow of 30 sccm to operate in spot mode. The larger keeper orifice also allowed a higher anode current.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"4 Iodine Testing Results","content":"\u003cp\u003eThe final iodine tests were conducted using the cathode in configuration C and D. Before each test, the iodine tank was refilled and weighed. By weighing the tank again after the test, the mean iodine flow rate was estimated. The chamber was pumped overnight, resulting in a pressure of ~\u0026thinsp;1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e mbar at the start of each test. The cathode was first ignited with krypton and the performance was measured. Then, the iodine flow was gradually switched on by opening the proportional iodine valve and closing the krypton valve. If possible, the test was repeated with krypton after the iodine discharge to check if the performance changed significantly.\u003c/p\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e4.1 Tests with a C12A7 electride type A emitter\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e16\u003c/span\u003e shows the first test of the planar cathode with a C12A7 electride emitter in configuration C with a secondary orifice. Total power refers to the combined power of anode and keeper. As the iodine proportional valve was opened and the krypton supply was closed, the power required to operate the cathode increased, then stabilised. After about 6 minutes, the discharge became unstable and went on/off in rapid succession. The reignition with krypton was possible again, but the cathode operated only shortly at a power above 150 W and at high anode currents only.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eA second test with the same emitter type but in configuration D has been performed next (cf. Figure\u0026nbsp;\u003cspan refid=\"Fig17\" class=\"InternalRef\"\u003e17\u003c/span\u003e). The result is very similar to the previous test, as soon as the discharge was switched to iodine, it became highly unstable. However, here the reignition with krypton was no longer possible.\u003c/p\u003e \u003cp\u003eBoth discharges were clearly in plume mode, as it can be seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig18\" class=\"InternalRef\"\u003e18\u003c/span\u003e and by the fluctuations of the cathode power.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e4.2 Tests with a C12A7 electride type B emitter\u003c/h2\u003e \u003cp\u003eIn a third test, the C12A7 electride type B emitter was tested with krypton in configuration D (cf. Figure\u0026nbsp;\u003cspan refid=\"Fig19\" class=\"InternalRef\"\u003e19\u003c/span\u003e) and subsequently with iodine (cf. Figure\u0026nbsp;\u003cspan refid=\"Fig21\" class=\"InternalRef\"\u003e21\u003c/span\u003e). It is clearly visible from the plots that the emitter did not produce a stable plasma, even with krypton. However, the iodine discharge was more stable than with the type A emitter, although fluctuating by \u0026plusmn;\u0026thinsp;5 W. The discharge was going off a few times, but it did directly reignite. Figure\u0026nbsp;\u003cspan refid=\"Fig20\" class=\"InternalRef\"\u003e20\u003c/span\u003e shows that the colour of the discharge was changing during test 3, indicating the evaporation of the emitter material. This assumption was further strengthened by the blocking of the keeper orifice that finally ended the test.\u003c/p\u003e \u003cp\u003eA fourth tests as a repetition of test three led to similar results, the total power discharge curve is therefore not shown here. Tests in configuration C have been attempted with krypton, but the secondary orifice was immediately blocked by evaporated material.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e4.3 Conclusion on the iodine tests\u003c/h2\u003e \u003cp\u003eThe tests with the emitter type A follow a similar trend: After switching from krypton to iodine, the discharge stays stable for a few minutes, before becoming very unstable. This result was observed in both cathode configurations, independent of the secondary orifice. The emitter type B was already unstable with krypton, the main difference was the required total power, which has more than doubled with iodine.\u003c/p\u003e \u003cp\u003eThe reason for the instabilities is still ander investigation. However, it was not possible so far to achieve a plasma discharge in spot mode. This shows, that while a plasma can be achieved for several minutes, the conditions are not stable. The performance values of the four tests presented are shown in Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003ePerformance of the cathode during the final tests for IcoN. The gas flow is given in sccm for krypton and in the mean iodine consumption during the test in mg/s by weighting the tank before and after the test.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"10\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTest\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eEmitter\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eGas\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eFlow\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c6\" namest=\"c5\"\u003e \u003cp\u003eKeeper\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c8\" namest=\"c7\"\u003e \u003cp\u003eAnode\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003eDuration\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c10\"\u003e \u003cp\u003eComment\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003ePower in W\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eCurrent in A\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003ePower in W\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eCurrent in A\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003ein min\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003eC12A7 Type A1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eKr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e30 sccm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e20.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e61\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003eBefore iodine exposure.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eI\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2 mg/s\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e14.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e155\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003eIodine discharge.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eKr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e30 sccm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e158\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003eAfter iodine exposure.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003eC12A7 Type A2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eKr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e20 sccm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e26.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e64\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003eBefore iodine exposure.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eI\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.9 mg/s\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e118\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003eUnstable iodine discharge.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eKr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003eNo reignition with krypton possible.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003eC12A7 Type B1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eKr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e30 sccm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e120\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e9.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003eBefore iodine exposure.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eI\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.9 mg/s\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e150\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003eIodine discharge.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eKr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003eNo reignition with krypton possible.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003eC12A7 Type B2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eKr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e30 sccm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003eBefore iodine exposure.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eI\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3.7 mg/s\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e21.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e141\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003eIodine discharge.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eKr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003eNo reignition with krypton possible.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"5 Post-Test Analysis","content":"\u003cp\u003eAfter each test, the cathode was removed from the testing chamber, disassembled and the state of all components was documented, with special focus on the emitter. The iodine tank was removed after each iodine test as well and weighed, to give an estimate over the total consumption, which gives a conservative estimation of the flow rate during the test (cf. Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e5.1 Tests with a C12A7 electride type A emitter\u003c/h2\u003e \u003cp\u003eThe state of the type A emitter before integration (a), after testing with krypton (b) and after both tests with iodine (c) and (d) is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig22\" class=\"InternalRef\"\u003e22\u003c/span\u003e. While the emitter seems to be already damaged after the krypton test, it shows clear signs of surface degradation after the iodine test. There are some black residues visible in both cases, but after the exposure to iodine plasma, also a metallic layer is visible. This can be caused by the diffusion of metallic components to the surface of the emitter or partly reduction of the material. The more the low-work function electride surface is covered, the higher the work function, until the discharge collapses.\u003c/p\u003e \u003cp\u003eAfter the disassembly of the cathode, also the keeper orifice plate has been visibly covered with a black layer, that seems at least partially containing iodine (cf. Figure\u0026nbsp;\u003cspan refid=\"Fig23\" class=\"InternalRef\"\u003e23\u003c/span\u003eb). This could become further problematic during longer tests, as the heating of the cathode does not seem to prevent this layer, which might eventually reach the emitter from building up and short-circuiting the keeper.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e5.2 Tests with a C12A7 electride type B emitter\u003c/h2\u003e \u003cp\u003eThe state of the type B emitter before integration (a), after testing with krypton (b) and after both tests with iodine (c) and (d) is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig24\" class=\"InternalRef\"\u003e24\u003c/span\u003e. After the tests with krypton indicated the melting of the emitter of this type, it was also expected for iodine, especially after the test was ended by a blockage of the keeper (see Fig.\u0026nbsp;\u003cspan refid=\"Fig23\" class=\"InternalRef\"\u003e23\u003c/span\u003ec). The emitter did lose a significant amount of material, as it can be seen in (c) and (d). While it was apparently still functional and the ending of the test was caused in both cases by the blockage of the keeper orifice, it still shows that a significant damage to the emitter was induced in the short period. In this case we have to assume that the temperature at which the emitter is generating enough electrons to stabilise the discharge is above the melting point. This makes this emitter type not viable for the operation in hot thermionic plasma-bridge cathodes.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"6 Conclusion","content":"\u003cp\u003eThis paper gives an overview of the development and testing of an iodine-compatible neutraliser at the LET at Airbus in Friedrichshafen that was conducted during the IcoN project. A planar cathode was designed and developed with a C12A7 electride emitter and operated with krypton as a propellant. After a prototype was designed that could be repeatably ignited into a stable discharge, further tests with C12A7 electride and iodine were performed. An iodine discharge could be ignited two times, each lasted for about\u0026thinsp;~\u0026thinsp;150 s.\u003c/p\u003e \u003cp\u003eThe lessons learned were incorporated in an improved cathode design. The cathode type has the novel feature of combining both the properties of planar and cylindrical hollow cathodes. For the C12A7 electride, the large surface area given by the planar design is an important factor to reduce melting damages through local plasma discharge hotspots. By enclosing the planar emitter and adding a secondary orifice, the efficiency could be further improved. This design was then successfully tested with krypton in a parameter study.\u003c/p\u003e \u003cp\u003eThe pumping system was also updated, as the forestage pump did not reach its final pressure anymore after a few days of pumping iodine, presumably because the sealing was damaged. This was partially solved by increasing the nitrogen purging mass flow, however, a cold trap would be advised to use for extensive iodine testing.\u003c/p\u003e \u003cp\u003eThe further development of the feed system allowed a stable and precise control of the iodine mass flow, permitting repeatable and stable ignition and therefore the detailed analysis of the impact of the iodine discharge on two different types of C12A7 emitter materials. The longest discharge had a duration of 26 minutes. Some krypton discharges after the test failed, indicating the damaging of the emitter.\u003c/p\u003e \u003cp\u003eThe post-test analysis revealed that the surface of all emitters had changed and residues were visible, which have not yet been identified. One of the two tested C12A7 electride emitter types was not stable at emission temperatures and melted during the test. All emitters did require a significant higher power to operate after the exposure to iodine, indicating a possible poisoning, independent of the geometrical configuration.\u003c/p\u003e \u003cp\u003eThe damage through the short iodine discharge indicates that even if the melting and the surface modifications are reduced, the iodine discharge is unlikely to last for several handred hours. Considering the results achieved in this study, we have to conclude that an iodine-fueled planar cathode based on C12A7 electride is not possible to operate for a longer period ander the tested configurations and material modifications. A possible approach to increase the temperature stability is the further development of the emitter material through doping.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003eRF\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Radio-Frequency\u003c/p\u003e\n\u003cp\u003eBN\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Boron Nitride\u003c/p\u003e\n\u003cp\u003eLaB\u003csub\u003e6\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/sub\u003eLanthanum Hexaboride\u003c/p\u003e\n\u003cp\u003eC12A7\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;12CaO\u0026middot;7Al2O3\u003c/p\u003e\n\u003cp\u003eWL20\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Lanthanated Tungsten\u003c/p\u003e\n\u003cp\u003eLET\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Laboratory for Enabling Technologies\u003c/p\u003e\n\u003cp\u003eEP\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Electric Propulsion\u003c/p\u003e\n\u003cp\u003eESA\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;European Space Agency\u003c/p\u003e\n\u003cp\u003eIcoN\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Iodine compatible Neutraliser\u003c/p\u003e\n\u003cp\u003eHET\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Hall effect thruster\u003c/p\u003e\n\u003cp\u003eCFT\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Cusp Field Thruster\u003c/p\u003e\n\u003cp\u003eHV\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;High voltage\u003c/p\u003e\n\u003cp\u003eMFC\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Mass flow controller\u003c/p\u003e\n\u003cp\u003eCTE \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Coefficient of thermal expansion\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eP.B. wrote the original draft, did the methodology, investigation, formal analysis, data curation, conceptualization and prepared the figures. N.G.K. and M.V. did the project administration, reviewing and editing of the first draft. N.K. supervised the IcoN project in which context this paper was written. K.W. did the reviewing and editing of the first draft, the validation and contributed ressources. M.T. and F.H. supervised the work. All authors reviewed the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgments\u003c/h2\u003e \u003cp\u003eThe project was fanded by the European Space Agency as activity \u0026ldquo;Iodine-compatible neutraliser for electric propulsion of CubeSats and small satellites\u0026rdquo; and \u0026ldquo;Iodine fluidic subsystem for the Iodine-compatible Neutraliser\u0026rdquo; ander the contract No 4000136710/21/NL/RA.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eData is provided within the manuscript or supplementary information files.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eD. Rafalskyi, J. Mart\u0026iacute;nez, L. Habl and E. Z. 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D, No. 73, p. 169, 2019. \u003c/li\u003e\n\u003cli\u003eN. Souhair, M. Magarotto, S. Dalle Fabbriche, R. Andriulli, S. Andrews, F. Ponti and D. Pavarin, \u0026bdquo;Simulation and modelling of an iodine fed Helicon Plasma,\u0026ldquo; in 37th International Electric Propulsion Conference, Cambridge, MA, 2022. \u003c/li\u003e\n\u003cli\u003eBronkhorst, \u0026bdquo;FLUIDAT ON THE NET,\u0026ldquo; [Online]. Available: www.fluidat.com. [Zugriff am 15 10 2024].\u003c/li\u003e\n\u003cli\u003eR. L. Summers, \u0026bdquo;Nasa technical note tn d-5285,\u0026ldquo; National Aeronautics and Space Administration, Washington, 1969.\u003c/li\u003e\n\u003cli\u003eD. W. Umrath, Fandamentals of Vacuum Technology, Cologne: Leybold GmbH, 2016. \u003c/li\u003e\n\u003cli\u003eM. O. McLinden, E. W. Lemmon and D. G. Friend, Thermo-physical Properties of Fluid Systems, P. J. Linstrom and W. G. Mallard, Ed., Gaithersburg, MD: National Institute of Standards and Technology, 2024. \u003c/li\u003e\n\u003cli\u003eD. M. Goebel and I. Katz, Fandamentals of Electric Propulasion, Hoboken, NJ: John Wiley \u0026amp; Sons, Inc, 2008. \u003c/li\u003e\n\u003cli\u003eC. Drobny and M. Tajmar, \u0026bdquo;Concerning the ignition of a C12A7 electride plasma-based cathode,\u0026ldquo; Journal of Electric Propulsion, Vol. 3, p. 2, 2024. \u003c/li\u003e\n\u003cli\u003eN. G. Kottke, M. Vaupel, M. Tajmar, W. Konrad, N. Saks and F. G. Hey, \u0026bdquo;Comparison of the thermionic emission properties of LaB6 and C12A7,\u0026ldquo; in 36th International Electric Propulsion Conference, University of Vienna, Vienna, Austria, 2019.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":false,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5738180/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5738180/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"Iodine exhibits promising characteristics as a propellant for electric propulsion (EP) systems. Its performance is comparable to xenon, yet it offers significant cost advantages. Moreover, it can be stored in a compact solid state and easily fed into the system through sublimation. Thruster operation has already been successfully demonstrated for the iodine fed Advanced Cusp Field Thruster, Hall Effect Thrusters and Radiofrequency Ion Thrusters (RIT), with the first in-space demonstration for an iodine RIT in early 2021. However, an iodine-fed plasma-bridged hot neutraliser is required to unlock the full systemic potential of an iodine EP subsystem at higher current levels. There exists no high-current iodine-fed neutraliser so far, as the corrosive nature of iodine and the potential for emitter material poisoning pose challenges. In the framework of the “IcoN” activity, a planar hollow cathode based on a C12A7 electride emitter has been developed and tested. The emitter has been manufactured by Fraunhofer IKTS, the neutraliser was manufactured and tested at Airbus Friedrichshafen. The initial testing was done with krypton and changed to iodine, as soon as a reliable cathode performance was achieved. After the first iodine tests were completed, the feed system was updated with a proportional valve to allow better control of the flow rate. In the final test series, two different emitter types and two planar cathode configurations have been tested with iodine. The longest achieved stable discharge was 26 minutes. While it was possible to develop a fully functional iodine feed system and a novel heaterless planar cathode based on C12A7 electride for krypton, no long-time stable iodine discharge was achievable in the frame of the project.","manuscriptTitle":"Development and Iodine Testing of a Novel C12A7 Electride Planar Hollow Cathode","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-01-13 16:17:00","doi":"10.21203/rs.3.rs-5738180/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"3a0ea9e8-ea31-452e-8e6e-0f78da569097","owner":[],"postedDate":"January 13th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-03-21T10:53:45+00:00","versionOfRecord":[],"versionCreatedAt":"2025-01-13 16:17:00","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5738180","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5738180","identity":"rs-5738180","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}