Breakthrough in Generation of Polarized Electron Beams: Unveiling the World's First RF Electron Gun with GaAs Photocathode

Breakthrough in Generation of Polarized Electron Beams: Unveiling the World's First RF Electron Gun with GaAs Photocathode | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Breakthrough in Generation of Polarized Electron Beams: Unveiling the World's First RF Electron Gun with GaAs Photocathode Vladimir Litvinenko, Nikhil Bachhawat, Jean Brutus, Luca Cultrera, and 17 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6536191/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 02 Feb, 2026 Read the published version in Nature Physics → Version 1 posted You are reading this latest preprint version Abstract Polarized electron beams play critical role in fundamental physics research by providing additional observables and opening new channels of discoveries. This discovery potential is well-known and is successfully used in high-energy and nuclear physics research. Recently, more conventional branches of science, such as ultra-fast electron microcopy, started exploitation unique features of polarized electrons. Currently GaAs crystals illuminated by circular polarized IR lasers remain the best choice for generating polarized electrons. All existing polarized sources are an electrostatic electron guns providing extremely high vacuum (XHV) conditions for survival of GaAs photo-emissivity. But limits in accelerating voltage and its gradient limits both the quality and quantity of available beams from these guns. These are the reasons why accelerator community was and is attempting to extend this technology to the radio-frequency electron guns, which are capable of accelerating beams with significantly higher accelerating gradients and total accelerating voltage. Unfortunately, all previous attempts of operating GaAs photocathodes in RF guns were unsuccessful. In this paper, we report on successful operation of GaAs photocathode in superconducting RF gun, describe in detail the accelerator system, used techniques, evolution of the GaAs quantum efficiency, and parameters of the generated electron beam. Physical sciences/Physics/Techniques and instrumentation/Design, synthesis and processing Physical sciences/Physics/Particle physics/Experimental particle physics 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 I. Polarized electron guns Polarized electrons have been and continue to be important research tools in atomic physics and condensed matter research [1]. The opportunities presented by polarized electron beams in high-energy and nuclear physics led to the development of polarized electron sources in the 1970s – see review in [2]. After a brief period of using atomic sources for polarized electrons, a significant breakthrough occurred in 1974 with the discovery of GaAs photocathodes, which generate polarized electrons when illuminated by circularly polarized infrared photons [3-5]. After that, numerous GaAs sources were built and successfully used in high-energy and nuclear physics experiments [6-13]. In addition, novel techniques were developed for GaAs-based sources to achieve electron polarization approaching 85% and high QE [14-15]. Compared to other photocathodes, GaAs photocathodes exhibit extreme sensitivity to residual gas content and pressure, as well as to ion back-bombardment of the cathode surface [12,16]. The most important consequence of these requirements is that operational polarized GaAs sources [6-13,17-19] are low-voltage (50-320 KV), low-gradient (~ 5 MV/m) DC electrostatic electron guns. These guns provide an excellent vacuum level at 10⁻¹² torr level to ensure a long lifetime for the cathode's quantum efficiency (QE). Additionally, they all operate at relatively low accelerating voltages to minimize dark current emission, which could otherwise compromise the system’s vacuum, increase ion back-bombardment, and reduce the quantum efficiency (QE) lifetime. It is well known that generating high-quality electron beams requires a high accelerating gradient as well as a high accelerating voltage, i.e., sufficient beam energy at the gun exit [17-20]. In contrast with electrostatic guns, RF guns can operate with very high gradients and accelerating voltage measured in megavolts (MV). When combined with the negative affinity of GaAs photocathodes, this could enable the generation of electron beams with exceptionally low transverse emittances [12]. This is why the physics community has been seeking a method to operate GaAs photocathodes inside an RF gun as sources of polarized electrons for future colliders [22-28]. Unfortunately, all previous attempts to operate an RF gun with GaAs photocathodes failed [29-34]. In the best case, the cathode's quantum efficiency (QE) survived only a few RF cycles [30]. There are three main challenges that cause most RF guns to operate only with very robust photocathodes, such as metal, Cs 2 Te, or CsK 2 Sb. The first challenge is the relatively poor vacuum in most normal-conducting (room temperature) high-gradient RF guns, where high power loss in the cavity’s walls leads to outgassing [32-33]. This problem made normal conducting RF guns incompatible with GaAs photocathodes. In contrast, properly designed superconducting RF (SRF) guns, operating at cryogenic temperatures (from 2K to 4.5K), can provide vacuum conditions suitable for long lifetime of GaAs photocathodes. The second challenge is to eliminate both multipacting—a resonant process where an electron avalanche develops on the cavity surface [35 and references therein]—and dark current, the uncontrolled cold emission of electrons [36-38]. Both effects significantly accelerate the degradation of the photocathode's quantum efficiency (QE). The third challenge is the significant decline in photocathode quantum efficiency at the cryogenic temperatures typical for SRF guns [39]. To address this, a specialized system known as the cathode stalk is required to thermally insulate the photocathode from the gun cavity walls. This system, equipped with an RF choke, must ensure adequate electrical and thermal conductivity to maintain the photocathode at the required temperature. Failure to do so may lead to the evaporation of volatile materials, such as cesium, from the photocathode surface, which could degrade the performance of the SRF cavity. II. SRF electron gun at Brookhaven National Laboratory All these challenges were overcome in May 2024 when we successfully demonstrated the sustained operation of two GaAs photocathodes in our SRF gun, shown in Figure 1. The gun was constructed as part of the SRF accelerator for a Coherent Electron Cooling (CeC) experiment at the Relativistic Heavy Ion Collider (RHIC) [40-43] and was brought into operation ten years ago. The successful operation with GaAs photocathodes was achieved after years of operation, resolving numerous challenges, implementing system improvements, and developing dedicated operating modes. After finding solution how to overcome multipacting problems [35], developing methods of conditioning out dark current [44], the gun started demonstrating reliable months-long operation with CsK 2 Sb photocathodes [45] at 1.25 MV accelerating voltage and generating world-brightest CW electron beams [20]. The months-long lifetime of the CsK₂Sb photocathodes, which are sensitive to all three effects mentioned above, was a very encouraging indicator that the CeC SRF gun could sustain operation with the more demanding GaAs counterparts. However, several remaining challenges still posed a risk to a successful test. First, the cathode transfer ultra-high vacuum (UHV) system was designed for operating with CsK₂Sb cathodes that is typically insufficient for more sensitive GaAs. Furthermore, unlike typical DC polarized guns, where GaAs photocathodes are prepared in a system connected to the gun, our SRF gun is located in the RHIC tunnel—a radiation-protected area with restricted access, available only once every two weeks for a limited time. Our photocathodes are prepared at the laboratory located few kilometers away from the gun - see the corresponding description in the Methods. They are then transferred into a portable vacuum chamber, known as the photocathode’s “garage,” which is sealed with a vacuum valve. The sealed chamber is transported to the RHIC tunnel, where it is connected to the SRF gun’s cathode transfer system load-lock chamber. After baking the load-lock chamber, the valve can be opened, allowing the photocathode to be transferred into the cathode stalk using three vacuum manipulator arms—one vertical and two horizontal—with a total transfer length of approximately two and a half meters. These operations demand an extremely high vacuum. Achieving the required vacuum level involved multiple upgrades to both the cathode deposition system and the portable vacuum chamber. Additionally, the entire cathode transfer system’s vacuum setup had to be overhauled, including the installation of two 1,500 liter-per-second non-evaporable getter (NEG) pumps. We reached an acceptable—though not ideal—level of vacuum for GaAs cathodes. To verify this, we conducted a partial GaAs cathode transfer and observed a fivefold decrease in its quantum efficiency (QE). Although this degradation was significant, the remaining QE was sufficient for our proof-of-principle test. Second, our SRF gun was designed to operate with CsK 2 Sb photocathodes, which are deposited on the polished front surface of the molybdenum (Mo) puck with two side groves for the puck transfer in the system and the inner stub that is used to conduct the electric current and the heat induced in the cathode to the end-effector and to the water-cooled stalk – see Figure 2. Excellent heat and electrical current transfer to the stalk are critically important for proper operation of the SRF gun and the photocathodes. Overheating of the cathode puck will result in evaporation volatile photocathode material, which would result in increase of dark current or even arcing and complete failure of the gun. In addition, deposition of the photocathode material inside the SRF gun will result in multipacting, which would kill the QE of the photocathode. Furthermore, poor current transfer could result in the arcing, vacuum bursts and destruction of the photocathode. In contrast with CsK 2 Sb photocathodes, the GaAs photocathode uses a cesiated GaAs crystal substrate, which should be connected to the cathode puck both electrically and thermally. We modified two Mo pucks as shown in Figure 3 (a) to house thin (100 to 300 microns) GaAs crystals. The crystals were connected to the body of the puck using Indium and were held in place by molybdenum cups that were crewed onto the pucks. The cups have openings with 5 mm diameter used to activate the GaAs (see methods for details). Third, we took every possible measure to ensure good thermal contact between the GaAs substrate and the Mo puck. We also conducted simulations to assess the heat generated in the substrate and its resulting temperature rise. The simulation predicted 5 W of RF losses in the GaAs substrate when operating the gun at its nominal voltage of 1.25 MV. Our standard SRF gun turn-on routine, used with CsK₂Sb photocathodes, involves a rapid initial voltage jump to approximately 200 kV (to prevent vacuum excursions caused by multipacting at low voltages) followed by a fast voltage ramp to 1 MV. However, previous failures with GaAs cathodes in RF guns introduced enough uncertainty that we chose not to attempt immediate operation at high voltages. Instead, we developed operation modes at lower voltages, starting as low as 52 kV, while maintaining excellent vacuum conditions—details of these developments are provided in the corresponding Methods section. We began our GaAs photocathode test at 52 kV, where estimated heat losses in the crystal were below 1 mW, and then gradually increased the operating voltage. Fourth, one of SRF gun important advantages is that it provides excellent vacuum. Its body operated at temperature of 4K serving as a very powerful cryogenic vacuum pump that freezes all gases including hydrogen. The frozen gasses attach to the surface and can be released only when the temperature rises to the evaporation level. Situation is similar for cold-to-room-temperature transition in the front and the back of the SRF gun cavity. The transitions are cooled by helium gas evaporating by the SRF cavity. The helium gas propagates trough channels attaches to the transitions and gradually warms up to the room temperature. Temperature gradient results is distribution of frozen residual gas species, originating in warm section of accelerator, along the surface of the transition according to their temperatures of freezing. Variations of the helium pressure would result in variation of its flow, changes in the temperature profile and corresponding vacuum pressure bursts. Such bursts were observed in our SRF gun system and were acceptable for operation with CsK 2 Sb photocathodes but would be intolerable for the GaAs. This was the reason that our cryogenic group developed mode of operating the cryogenic system that prevented variations of the helium gas flow and eliminated the vacuum bursts. III. Operation of SRF gun with GaAs photocathodes Completion of improvements mentioned above and opening in the CeC operation schedule allowed us to demonstrate sustained operations of our SRF gun with GaAs photocathodes. Two GaAs photocathodes (which we will call number #1 and #2) with initial QE of 4.8% in green (532 nm) light were produced in March 2024. The vacuum suit with these cathodes was moved to RHIC tunnel and connected to the SRF gun cathode transfer system on April 2, 2024. After finishing the bake-out of the load-lock system on April 8, 2024, we performed our planned “survival test” by transferring the cathode #1 out of the vacuum suit to the vertical transfer arm, lowered to the tip of the long horizontal transfer arm and back to the suit. The cathode survived but its QE dropped to estimated value ~ 1% - the accuracy was limited by imperfect QE measurement system incorporated into transfer system. The cathode was held in the load-load chamber awaiting completion of the preparation for the actual tests. All necessary SRF gun operating modes were initially tested using a blank Mo puck. On May 15, 2024, Cathode #1 was transferred into the gun. Time allocated for this experiment was limited to twenty days, after which we must return to regular CeC program. Measurements taken before the transfer showed that the #1 cathode's QE had dropped to approximately 0.5%, a twofold decrease over 37 days. Following this measurement, the cathode was first transferred vertically using a manual system. After positioning it onto the end-effector of the long horizontal arm, we used a computer-controlled motor system (referred to as the “tractor”) to slowly move the cathode into its final position. This slow movement was crucial for minimizing vacuum spikes caused by friction (see Fig. 4(a)). The horizontal transfer process took about one hour. Initial measurements, conducted at a very low bunch charge to avoid saturation, revealed a degradation of the GaAs photocathode quantum efficiency (QE) to 0.04% during the transfer. This clearly demonstrated that our cathode transfer system, originally designed for other photocathodes, does not meet the stringent requirements for GaAs. This finding suggests that future polarized SRF guns should be designed with a more reliable—and likely simpler—cathode transfer system, possibly combined with a local GaAs cathode activation system. The primary goal of our experiment was to demonstrate the operation of an SRF gun with a GaAs photocathode. We gradually increased the operating voltage to 1 MV while carefully monitoring vacuum conditions, dark current, and liquid helium consumption. Throughout this process, we observed no QE degradation. Instead, the QE increased to 0.08% after one day of CW operation and to 0.093% after two days. We attributed this improvement to the electron beam effectively cleaning contaminants from the cathode surface that had been introduced during the transfer. While operating at 500 kV, we performed a QE map scan of the cathode and found that the QE varied by a factor of two, with a local maximum reaching approximately 0.1% (see Fig. 5(a)). We had sufficient laser power to saturate the cathode and observed a nearly linear relationship between the maximum extracted charge and the gun's accelerating voltage (see Fig. 5(c)). For laser spot with 5 mm diameter the slope of this curve was 1.2 nC/MV, and when operating at voltages above 1 MV, we successfully generated bunch charges exceeding 1 nC per bunch. Our initial successes were followed by a series of technical system failures, which caused brief but measurable vacuum spikes. These spikes led to a gradual decrease in QE to 0.033%, 0.025%, and 0.021% over three days. The turning on of the SRF gun to initial voltage of 52 kV did not generate any vacuum spikes and after illuminating the photocathode with our green laser we observed first electrons bunches with charge ~ 100 picocoulombs (pC). To demonstrate that our success with GaAs photocathode #1 was not merely due to luck, we replaced it with cathode #2 on May 22, 2024. During the transfer, the integrated pressure in the manipulator measured at 8.5×10⁻⁷ torr-seconds, and cathode #2 had an initial QE of 0.1%. We used this cathode to test operation at higher voltages (see Fig. 6(a)), along with tracking QE values, QE map evolution, and measuring electron beam parameters. Details of the photocathode’s QE evolution can be found in the corresponding section of the Methods. Our analysis showed that the QE lifetime is approximately 54 hours for the first photocathode and around 231 hours for the second. Following 1.4 MV quench event, we resumed operation at our standard voltage of 1.25 MV. We used the remaining five days of the test period to investigate some of the unexpected behaviors we had observed during the two weeks of operation: (a) significant non-uniformity and various structural patterns in the transverse beam profiles and (b) large measured transverse beam emittances, when compared with typical values observed in previous SRF gun operations [20]. We found that while QE non-uniformity contributed to uneven transverse beam density, the primary cause of the transverse emittance blow-up was the failure of the laser system to deliver flat-top 360 ps pulses. This issue was resolved after the completion of our tests (see Methods for details). IV. Possible future steps While we successfully demonstrated more than a threefold increase in accelerating voltage and achieved a reasonable — for a proof-of-principle — QE lifetime for two GaAs photocathodes, not all aspects of our experiment were without issues: · The vacuum level in the cathode transfer system was insufficient to preserve the QE of GaAs photocathodes. As a result, each cathode transfer led to a significant reduction in QE and introduced strong QE non-uniformity. · The laser system’s performance was inadequate for a proper evaluation of the generated electron beam quality. A six-month operation of the CeC SRF gun is planned for 2025, with a primary focus on demonstrating Coherent electron Cooling. The laser system has since been repaired and will now provide 360 ps flat-top pulses. While there is a possibility that the CeC SRF gun could be used for another GaAs photocathode test, improvements to the cathode transfer system's vacuum conditions are unlikely. Consequently, we can expect the initial QE to be around 0.1%. However, the repaired laser system will enable accurate evaluation of the beam quality generated from GaAs cathodes. The CeC SRF gun relies on liquid helium from the RHIC cryogenic system. RHIC will cease operations at the end of 2025 to enable construction of the future Electron-Ion Collider in the RHIC tunnel. This will eliminate the possibility of using the CeC SRF gun for future GaAs tests, leaving the Rosendorff SRF gun [47, 48] — which has struggled to operate even with robust CsK₂Sb photocathodes — as the only remaining operational SRF gun in 2025. Fortunately, a new quarter-wave (QW) SRF gun is currently under construction at Michigan State University for the LCLS II CW X-ray FEL [49]. This new SRF gun could potentially serve as the next testing platform for exploring the GaAs photocathode technology. The authors would like to acknowledge the continuous support from the administration of the Collider-Accelerator and Instrumentation Departments at BNL, as well as the RHIC and CeC operations groups. Special thanks go to William Weldon and Rudy Begay (Instrumentation Department, BNL), Ron Napoli and Kirk Sinclair (Collider-Accelerator Department, BNL), and Jyoti Biswas (Electron-Ion Collider, BNL) for their significant contributions to this project. This research was supported by the DE-SC0021426 award from the Office of Nuclear Physics and by Brookhaven Science Associates, LLC under Contract No. DE-SC0012704 with the U.S. Department of Energy. Methods GaAs photocathodes preparation GaAs Puck Design and Activation Procedure The GaAs puck was designed to maintain full compatibility with the existing infrastructure used for growing and transporting alkali antimonide photocathodes to the SRF gun. To enable the use of GaAs-based photocathodes while shielding the sharp edges of the sample from the gun’s internal electric fields, the puck consists of two molybdenum components. The main body of the puck includes one of the two grooves needed for sample manipulation and features a pocket on its top surface to hold a small 7×7 mm² GaAs sample. The second groove is provided by a cap that screws onto the top of the body. This cap, mechanically polished to a mirror finish, has a 5 mm diameter hole in its center, allowing the GaAs crystal surface to be exposed to Cs, O, and Te during activation, as well as to laser illumination for electron extraction (see Figure 3 in the main text). Before final assembly, both the body and cap are high-pressure rinsed to remove particulates and then sealed in a dedicated transporter within a cleanroom. The GaAs crystals are chemically etched to remove surface oxides, followed by rinsing with deionized water. The assembly of the GaAs sample into the puck also takes place in the cleanroom. A thin indium foil (30 µm thick) is inserted between the GaAs crystal and the puck body. During the heat cleaning process, this indium foil melts, improving thermal conductivity between the GaAs crystal and the puck. Preliminary Experiments on GaAs Activation and Lifetime Preliminary experiments were conducted to estimate the photocathode lifetime within our growth system and to identify the dominant mechanisms contributing to quantum efficiency (QE) degradation under the vacuum conditions of our activation chamber. These conditions range from a steady-state vacuum of ~1×10⁻¹¹ Torr to ~1×10⁻¹⁰ Torr during activation. A GaAs sample with p-type doping of 1×10¹⁹ cm⁻³ was chemically etched with 35% HCl, rinsed with deionized water, and then transferred to the vacuum system. The sample was heat-cleaned at 580 °C for two hours to remove surface oxides and then twice activated to negative electron affinity (NEA) using Cs and oxygen. QE evolution was measured using a green laser (532 nm) at both 100% and 1% duty cycles. These experiments aimed to determine whether the observed QE lifetime was primarily affected by chemical poisoning or by ion back bombardment. Given that the photocathode bias was maintained at -50 V and the average extracted current was in the microampere range, ion back bombardment could not be ruled out as a significant factor. The results, shown in Figure 7, indicate that similar QE lifetimes were observed over time, suggesting that further investigation is required to isolate the dominant degradation mechanism. When the same QE data are plotted as a function of the extracted charge in Fig. 7 (b), it becomes evident that QE decay occurs much more rapidly—by more than a factor of 50—when the sample is operated with a 1% duty cycle compared to continuous (100%) illumination. This strongly suggests that under our experimental conditions, chemical poisoning is the dominant mechanism responsible for QE degradation, rather than ion back bombardment. Preliminary experiments were also performed using a modified activation procedure involving a tellurium (Te) interlayer between two Cs–O layers. The GaAs sample was initially activated by alternating exposures to Cs and oxygen. After reaching the maximum QE, a slight excess of Cs was deliberately deposited by continuing the Cs exposure, which resulted in a small decrease in QE, as shown in Figure 8. It was expected that the excess Cs would gradually react with residual oxygen in the vacuum chamber over the following hours, leading to a further increase in QE. This effect was indeed observed over the next two days, as QE measurements taken every 24 hours confirmed a gradual improvement (see Figure 9). Forty-eight hours after the initial Cs and oxygen activation, it was observed that further Cs deposition led to a decrease in the QE of the GaAs photocathode. At this point, tellurium was deposited onto the photocathode surface until no measurable photocurrent was detected under 532 nm laser illumination (see Figure 9). Subsequent Cs and oxygen exposure resulted in a renewed QE peak, reaching levels comparable to those achieved in the previous activation cycle. As before, the surface was slightly overexposed to Cs vapor, and over the following two days, the QE gradually recovered to its maximum value. All measurements described thus far were conducted with the photocathode inside the activation vacuum chamber. To evaluate the photocathode’s robustness against vacuum degradation—particularly from mechanical motion—we attempted to transfer the activated photocathode into the preparation chamber of the activation system. During the transfer, vacuum levels degraded rapidly to the 10⁻⁹ Torr range due to outgassing from the manipulator’s bellows. After the transfer was completed, the QE was measured again and found to have dropped to approximately 1.5% (see Figure 9). The QE of the GaAs photocathode also exhibited degradation with a decay time of approximately one hour when the magnetic manipulator was used to transfer the photocathode from the preparation chamber to the docking chamber, in preparation for insertion into the portable vacuum chamber (we call it a garage). This portable chamber is used to transport the activated photocathode to the SRF gun loading system. However, since the transfer typically takes only a few minutes, the resulting decrease in photocurrent was minimal and considered negligible. These measurements enabled us to determine the optimal maximum speed at which the linear manipulators can be operated to minimize QE loss due to the transient gas load released during the transfer process. Photocathodes lifetimes in the SRF gun For operation in the SRF gun, two GaAs photocathodes were prepared using the activation method involving a Te interlayer, as described previously. This technique produced photocathodes with very similar final QE values, each reaching approximately 4% following the intentional deposition of a slight excess of Cs at the end of the activation process (see Figure 10). For the operation in the SRF gun two GaAs photocathodes were prepared activating the surface with a Te interlayer as described in the previous paragraph. Both photocathodes were transferred from the activation chamber to the preparation chamber and subsequently to the garage docking station in a carefully controlled manner, ensuring that vacuum levels remained below the 10⁻¹⁰ Torr range throughout. This procedure successfully preserved the QE of both cathodes, with no significant degradation observed after the transfer to the preparation system. Once in the garage chamber, QE measurements were performed using a collecting anode that could be positively biased. However, due to the geometric configuration—where the anode is positioned to the side of the cathodes—and the large incident angle of the laser beam, accurate QE estimation was not possible at this stage. Nevertheless, given the short transfer time, the consistently low vacuum levels (with the garage chamber maintained in the 10⁻¹¹ Torr range), and the controlled handling, we assumed that the QE of both photocathodes remained largely unaffected during the transfer. The ultra-high vacuum (UHV) garage chamber was then transported to the RHIC tunnel and connected to the load-lock of the SRF gun's cathode loading system. Immediately following the connection, photoemission from the photocathodes was confirmed, with photocurrent values comparable to those measured before transporting the garage. The load-lock system of the SRF gun was vacuum bake-out at approximately 150 °C for one week following connection. After the bake-out, one of the two photocathodes was transferred from the garage into the SRF gun’s loading chamber, where its QE was measured and found to have decreased to around 1%, with an estimated 1/e decay time of approximately 183 hours (see Figure 11(a)). The photocathode remained in the loading chamber, under vacuum at ~1×10⁻¹¹ Torr, for about 37 days as preparations for SRF gun beam operation progressed. Just before being loaded into the SRF gun, the QE of the first cathode was measured again and had further decreased to approximately 0.5%, corresponding to an estimated 1/e decay time of about 1,281 hours (see Figure 11(a)). Once in the SRF gun, the accelerating voltage was gradually ramped up. The first reliable QE measurement—free of image charge-induced saturation—showed a QE of about 0.08%. Over the following days, the photocathode was operated at varying accelerating voltages, with several additional QE measurements taken. A 1/e decay time of approximately 54 hours was estimated during this operational phase. As the QE of the first cathode continued to decline, it was removed and replaced with the second photocathode, which was then inserted into the gun. QE measurements for the second photocathode were conducted primarily while it was installed in the SRF gun, so its full degradation history is not as well documented as that of the first cathode. However, the initial QE in the gun was comparable to that of the first photocathode. As cavity operation became more stable and vacuum excursions during injector operation were less severe, a longer 1/e QE decay time was observed. The QE gradually decreased to 0.08% on May 23 during overnight operation at 410 kV, and further declined to 0.065%, 0.04%, and 0.031% on May 24, 27, and 28, respectively. Two unsuccessful attempts on May 25 and 27 to operate above 1 MV were followed by sustained SRF gun operation at 1.35 MV on May 29. However, an attempt to reach 1.4 MV resulted in a cavity quench, likely caused by GaAs cathode overheating and Cs contamination of the gun’s cavity. Notably, we observed that QE increased from 0.031% to 0.04% when the RF voltage was raised from 0.6 MV to 1.1 MV. We attribute such increase to a rise in the GaAs substrate temperature, which likely reduced the photocathode’s work function. This assumption aligns with the observed cavity quench at 1.4 MV, which corresponded to a 62% increase in RF losses within the GaAs crystal, when compared with operating at 1.1 MV. This decay time was estimated to be approximately 231 hours (see Figure 11(b)). The fourfold improvement in QE lifetime observed with the second photocathode was the result of enhanced SRF gun control systems and the avoidance of failures that occurred during the initial learning curve with the first cathode. On the final day of operation, a power dip triggered a gun trip, resulting in a vacuum spike that reached the 10⁻⁶ Torr range. This incident caused a sudden drop in QE by nearly an order of magnitude. With the QE reduced to the 10⁻⁴ range, the available laser power was insufficient to extract more than a few tens of picocoulombs. Consequently, the GaAs photocathode was removed and replaced with a standard alkali antimonide photocathode for the remainder of the run. Preparations of the SRF gun for operating with GaAs photocathodes Possible overheating of GaAs photocathodes, leading to contamination of the SRF gun cavity by cesium (Cs), posed a significant risk and required a cautious approach. To mitigate this, we developed a detailed procedure that began with operating the gun at the lowest possible voltage, followed by a gradual voltage increase while closely monitoring vacuum and radiation conditions in the gun area. Our SRF gun utilizes cathodes recessed by 10.5 mm from the cavity nose to provide the necessary focusing for the generated electron beams. This configuration reduces the peak electric field at the cathode surface by a factor of two compared to a non-recessed cathode (see Figure 12(a)). To evaluate potential heating, we first simulated temperature increases due to dielectric losses in the GaAs crystal at the SRF gun’s operating frequency of 113 MHz, as well as resistive losses in the molybdenum (Mo) pack (see Figure 12(b)). We employed p-doped GaAs crystals with thicknesses of 100 μm and 300 μm for cathodes #1 and #2, respectively. Since specific data on GaAs dielectric losses at the 113 MHz frequency of the SRF gun is not available, we extrapolated existing data to estimate<img data-fr-image-pasted="true" width="75" height="13" src="data:image/png;base64,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" alt="Title: tan delta almost equal to 10 to the power of negative 4 end exponent - Description: {"mathml":" tan δ ≈ 10 - 4 ","origin":"MathType for Microsoft Add-in"}" class="fr-fic fr-dii">. The resulting estimate predicted that GaAs dielectric losses of approximately 0.5 W for the 100 μm crystal and ~1 W for the 300 μm crystal dominate the overall losses. Simulations predicted temperature increases of approximately 14 K for 100 μm and 28 K for 300 μm GaAs crystals, raising serious concerns about potential overheating. This necessitated initiating gun operation at previously unexplored voltages below 100 kV. Based on our prior experience [35], there was a significant likelihood of multipacting activity in the SRF gun’s Fundamental Power Coupler (FPC) region at voltages below 200 kV—particularly in the presence of magnetic fields from the gun’s focusing solenoids. Any such multipacting would lead to a rise in vacuum pressure, which would inevitably degrade the quantum efficiency (QE) of the GaAs cathodes. To address this, we dedicated the week of May 8–14, 2025, to cleaning up potential multipacting zones and suppressing associated vacuum spikes in the FPC. This was accomplished by performing periodic bi-polar scans of the solenoid current, ramping it to the maximum values required for each specific SRF gun voltage (see Fig. 13(a)). In parallel, the RF group developed a new gun turn-on procedure featuring a flexible voltage target. As a result of these efforts, we successfully established multipacting-free operation of the SRF gun across a wide voltage range—from 52 kV to 1 MV—with FPC vacuum levels in the low 10⁻¹⁰ Torr range. As described in the main text, the SRF gun cavity transitions—from room temperature to cryogenic temperatures—are cooled by evaporated helium gas. This gas travels through a spiraling tube surrounding the transition vacuum chamber, moving from the cold end to the room-temperature end. The resulting temperature gradient causes residual gas components from the warmer regions to condense onto the vacuum chamber walls where their respective freezing points are reached. Any reduction in the helium gas flow leads to an increase in wall temperature and the release of previously frozen gases—a phenomenon illustrated in Fig. 13(b). While such gas bursts were tolerable during operation with CsK₂Sb photocathodes, they were unacceptable for use with GaAs. The RHIC cryogenics team refined the SRF gun cryo-system controls to eliminate fluctuations in helium flow, thereby preventing pressure spikes and ensuring stable vacuum conditions. With all preparations complete, we proceeded with testing GaAs photocathodes in the SRF gun. The initial test began at an SRF gun voltage of 52 kV, which ensured negligible power losses—on the order of milliwatts—and a sub-Kelvin temperature rise in the GaAs crystals. Measuring parameters of electron beam We employed well-established methods to measure key electron beam parameters from the SRF gun, including beam energy, bunch charge, and beam emittance. To measure the electron beam energy, we used a precise method described in [46], which is based on the principle that a solenoid rotates the plane of particle motion by an angle: where p is the electron momentum, e is the electron charge, and c is the speed of light. The rotation angle θ is proportional to the integral of the longitudinal magnetic field B z , which in turn is determined by the number of turns N in the solenoid coil and the coil current I , in accordance with Stokes' theorem: Our setup consists of a calibrated solenoid with a known number of coil turns, a pair of horizontal and vertical dipole trims placed upstream of the solenoid, and a YAG profile monitor downstream. We perform four measurements by scanning the electron beam horizontally and vertically using both positive and negative solenoid currents, while tracking the beam centroid on the YAG screen (see Fig. 14(a)). The angles between the beam trajectories produced by horizontal and vertical scans for opposite solenoid currents allow us to determine the beam momentum, full and kinetic energies, E and E k , of electrons as well as the voltage of the gun (where m is the mass of electron): Although it is possible to use a single dipole trim for this measurement, combining results from both horizontal and vertical scans helps cancel out astigmatism effects inherent to real-world profile monitor systems, thereby improving measurement accuracy. This method typically achieves a relative energy measurement accuracy of ~10 -3 . The charge per bunch was measured using an Integrated Current Transformer (ICT), a standard diagnostic tool in our accelerator system [50]. These measurements were cross-calibrated against readings from a Faraday cup to ensure accuracy. The charge per bunch, Q , along with the laser power, P l , was used to determine the quantum efficiency (QE) of the laser-illuminated photocathode area using the relation: where f l is the laser repetition frequency, and <img data-fr-image-pasted="true" width="90" height="11" src="data:image/png;base64,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" alt="Title: ℏ omega equals 2.33 space e V - Description: {"mathml":" ℏ ω = 2 . 33 e V ","origin":"MathType for Microsoft Add-in"}" class="fr-fic fr-dii"> is the laser photon energy. While we were able to accurately measure the QE of relatively large illuminated areas (on the order of several mm²), high-resolution QE maps—shown in Fig. 5(a) and 5(b) of the main text—were acquired in relative units, calibrated to the maximum QE observed within each scan. Additionally, we captured images of the photocathode emission using a YAG profile monitor (see Fig. 14(b)) by operating with a low-charge, high-repetition-rate electron beam. Although these images were not perfect, they provided supplementary information about the spatial distribution of QE on the GaAs photocathodes. Specifically, uniform illumination of the entire photoemitting area revealed QE degradation in regions that had been used to generate the electron beam over several days of operation. We also observed approximately twenty small (~100 μm in diameter) randomly distributed spots exhibiting zero QE. These localized QE defects could not be resolved using the standard QE scan, which has a spatial resolution of approximately 0.25 mm. Problems with the laser pulses Unfortunately, in our effort to optimize all other experimental conditions for the GaAs photocathodes, we failed to identify a critical issue with the laser's temporal profile before the start of the experiment. The system designed to combine six Gaussian sub-pulses (beamlets) with alternating polarization into a single, nearly flat-top pulse of 360 ps FWHM malfunctioned. Instead of producing a uniform pulse, it generated a signal composed of beamlets with nearly random amplitudes. We discovered the faulty temporal structure of the laser pulses—shown in Fig. 14(c)—only after completing the GaAs photocathode tests. Subsequent measurements revealed that the relative intensity of individual beamlets varied by more than an order of magnitude. 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Decker, and S. Belomestnykh, First experience with He conditioning of a superconducting rf photoinjector, Phys. Rev. Accel. Beams 25, 092001 (2022) E. Wang, V.N. Litvinenko, I. Pinayev, M. Gaowei, J. Skaritka, S. Belomestnykh, I. Ben-Zvi, J.C. Brutus, Y. Jing, J. Biswas, J. Ma, G. Narayan, I. Petrushina, O.Rahman, T. Xin, T. Rao, F. Severino, K. Shih, K. Smith, G. Wang, Y. Wu, Long lifetime of bialkali photocathodes operating in high gradient Superconducting Radio Frequency gun, Scientific Reports 11, 4477 (2021 I. Pinayev, Y. Jing, D. Kayran, V. N. Litvinenko, J. Ma, K. Mihara, I. Petrushina, K. Shih, G. Wang, Y. H. Wu, Using solenoid as multipurpose tool for measuring beam parameters, Review of Scientific Instruments 92, 013301 (2021), https://doi.org/10.1063/5.0015618 A. Arnold, H. Büttig, D. Janssen, T. Kamps, G. Klemz, W. Lehmann, U. Lehnert, D. Lipka, F. Marhauser, P. Michel et al., A high-brightness SRF photoelectron injector for FEL light sources, Nucl. Instrum. Methods Phys. Res., Sect. A 593, 57 (2008). A. Arnold and J. Teichert, Overview on superconducting photoinjectors, Phys. Rev. ST Accel. Beams 14, 024801 (2011). Status of the SLAC/MSU SRF Gun Development Project, J. Lewellen, C. Adolphsen, A. Arnold, Y. Choi, C. Compton et al, In Proceedings of North American Particle Accelerator Conference, NAPAC2022, Albuquerque, New Mexico, August 7-12 ,2022, WEPA03, DOI: 10.18429/JACoW-NAPAC2022-WEPA03 D. M. Gassner, V. Litvinenko, R. Michnoff, T. Miller, M. Minty, I. Pinayev, Coherent Electron Cooling Proof of Principle Instrumentation Design, in Proc. of Beam Instrumentation Workshop, April 15-19, 2012, Newport News, Virginia, USA, weap01, http://accelconf.web.cern.ch/AccelConf/BIW2012/papers/weap01.pdf Additional Declarations There is NO Competing Interest. 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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-6536191","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":459299055,"identity":"290b8d17-d3ae-4df9-bbfe-b0362832458c","order_by":0,"name":"Vladimir 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Laboratory","correspondingAuthor":false,"prefix":"","firstName":"Gang","middleName":"","lastName":"Wang","suffix":""},{"id":459299075,"identity":"7af97b67-5203-45c0-871a-0937436901ab","order_by":20,"name":"Dan Weiss","email":"","orcid":"","institution":"Brookhaven Ntional Laboratory","correspondingAuthor":false,"prefix":"","firstName":"Dan","middleName":"","lastName":"Weiss","suffix":""}],"badges":[],"createdAt":"2025-04-26 16:40:15","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6536191/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6536191/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41567-025-03138-7","type":"published","date":"2026-02-02T05:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":83220701,"identity":"8f7f4f2b-4070-4eea-beb9-066b94360758","added_by":"auto","created_at":"2025-05-21 10:21:14","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":86115,"visible":true,"origin":"","legend":"\u003cp\u003eLayout of the continuous wave (CW) CeC SRF gun system. The gun consists of a 113 MHz quarter-wave (QW) SRF niobium cavity (operating at 4K and cooled by liquid helium), a photocathode storage and transfer system, and a room-temperature cathode stalk, which serves as a holder for the cathode and serves as a half-wave RF choke. RF power is supplied to the cavity through a fundamental power coupler (FPC), a hollow coaxial tube that also fine-tunes the cavity frequency. Electrons are generated by illuminating the photocathode with green laser pulses (532 nm, 360 psec) delivered via an off-axis mirror in the laser cross. The electrons are accelerated (right to left) from the cathode in the cavity's accelerating gap (~16 cm) and are focused by the gun solenoid located 0.65 meters downstream of the cathode.\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6536191/v1/7fc9a011ee187c546425b4c3.jpg"},{"id":83222345,"identity":"8408e07a-16dd-4f06-ae5e-b854d1671523","added_by":"auto","created_at":"2025-05-21 10:37:14","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":83181,"visible":true,"origin":"","legend":"\u003cp\u003eThe SRF gun cathode system: (a) The cathode puck transferred into is operational position at the end of the cathode stalk, which is maintained at room temperature by circulating water. The stalk is separated from the body of the SRF gun niobium cavity operated at temperature of 4 Kelvin. The stalk served as half-wave RF choke transferring both the electric current and the heat induced in the cathode puck by the RF electro-magnetic field. The cathode puck is maintained by the end-effector attached to the long horizontal transfer arm. (b) A cut-off of the end-effector before connection to the cathode puck. It showing the inner RF fingers which serve as electric and thermal connection between the parts. (c) Photograph of the cathode pack attached to the end-effector during the transfer. The external RF fingers serve as electric and thermal connection between the end-effector and the cathode stalk.\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6536191/v1/9e3c3d5417035e610efd32e4.jpg"},{"id":83221988,"identity":"08fa1273-9faa-46b4-a30a-5a548ce654e1","added_by":"auto","created_at":"2025-05-21 10:29:14","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":73844,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Parts of the GaAs cathode assembly comprised of a polished titanium cup, modified body of Mo cathode puck with central cut to house the GaAs substrate. (b) Assembled GaAs cathode placed in the holder for activation in the cathode preparation system.\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6536191/v1/ca602cab420826d2b2fd17f0.jpg"},{"id":83221990,"identity":"3cd73bd0-7fb0-465e-872b-8b45b53f8969","added_by":"auto","created_at":"2025-05-21 10:29:14","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":72803,"visible":true,"origin":"","legend":"\u003cp\u003eGaAs cathode transfer into the SRF gun: (a) Vacuum level (in torr) in the horizontal manipulator arm during cathode transfer on May 15, 2024; (b) Image of the GaAs cathode inserted into the SRF gun with shiny cathode cap (20 mm in diameter and 5 mm cathode opening). The circle around the cathode is the tip of the cathode stalk with the gun cavity nose visible further out; (c) The same image with scattered laser light illuminating the metal cup and the 0.5 mm diameter laser spot in the center of the open GaAs photocathode area. This set-up was used to measure QE map of the cathode.\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6536191/v1/fd854e48fc823d262f8ec71e.jpg"},{"id":83220706,"identity":"f03b8210-22d1-40ce-8cff-0f331f60a07a","added_by":"auto","created_at":"2025-05-21 10:21:14","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":93346,"visible":true,"origin":"","legend":"\u003cp\u003eMeasured QE maps of the GaAs photocathode #1 (a) and #2 (b). The photoemission area is limited to 5 mm circular opening of the cathode cup. The color map is in relative units with peak QE value of 0.1%. \u0026nbsp;(c) Measured dependence of maximum charge per bunch extracted from the GaAs cathode #1 as function of the gun’s operating voltage (dots) and linear fit (line) of the data. The fit has 1.2 pC/kV slope. Error bars extend from minimum to maximum measured charge at points of measurements. Intermediate points (dots) are measurements during transition between set voltage points.\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6536191/v1/9da61de2ad9ea8d718ee1a51.jpg"},{"id":83220707,"identity":"7bc1f7cf-2afd-4773-ad81-6db627fff261","added_by":"auto","created_at":"2025-05-21 10:21:14","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":80615,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Measured SRF gun voltage during tests with two GaAs photocathodes: the first cathode was used from May 15 to May 21, 2024, and the second cathode was operated from May 22 to June 4, 2024; (b) Typical measurements of the electron beam kinetic energy as a function of laser phase. The error bars indicate the uncertainty in beam energy measurement at each of the 16 RF phase settings for the laser pulse arrival. Details of the energy measurements are provided in the Methods, with further explanation available in [46]; (c) Measured charge per bunch using GaAs photocathode #2 on May 25, while operating the SRF gun between 0.9 and 1.07 MV.\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6536191/v1/1aa93f9fbaeec35772dbdce0.jpg"},{"id":83221991,"identity":"b73575ca-744c-4d7c-9bdf-265a351b5213","added_by":"auto","created_at":"2025-05-21 10:29:14","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":58666,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of GaAs photocathode quantum efficiency (QE): (a) as a function of time, and (b) as a function of extracted charge. The photocathode was illuminated using 532 nm green laser. Black circles represent QE measurements under CW illumination, while red circles correspond to illumination with a 1% duty cycle. Dashed lines show the best exponential fits to the data: in panel (a), the extracted QE lifetimes are 18 hours (CW) and 24 hours (1% duty cycle); in panel (b), the corresponding charge lifetimes are 80 mC (CW) and 1.3 mC (1% duty cycle).\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6536191/v1/10f8407831b0240464a719d4.jpg"},{"id":83221992,"identity":"381f320a-a5e8-4a19-88b1-fd44b3836c1f","added_by":"auto","created_at":"2025-05-21 10:29:14","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":95582,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of GaAs photocathode quantum efficiency (QE) with Cs-O activation without (top graph) and with Te interlayer (bottom graph).\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6536191/v1/e396d20945fd3f6b38e4b853.jpg"},{"id":83221994,"identity":"3ea22bdf-9dc9-419e-ab0f-50374512a574","added_by":"auto","created_at":"2025-05-21 10:29:14","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":90039,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of quantum efficiency for GaAs photocathode with Te interlayer and excess Cs.\u003c/p\u003e","description":"","filename":"9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6536191/v1/bd6b574107d8b1ec1824d974.jpg"},{"id":83220717,"identity":"0bebbfdf-5828-453d-9336-df6795a0bffc","added_by":"auto","created_at":"2025-05-21 10:21:14","extension":"jpg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":88863,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of quantum efficiency of two GaAs photocathodes used for our SRF gun tests.\u003c/p\u003e","description":"","filename":"10.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6536191/v1/447e20841aaa8d6e685e8908.jpg"},{"id":83222346,"identity":"2d68d2cc-73c0-4a7f-8022-e8c4769ae065","added_by":"auto","created_at":"2025-05-21 10:37:14","extension":"jpg","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":87386,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of quantum efficiency of first (a) and second (b) GaAs photocathodes used for operating in the SRF gun. The QE lifetime can be estimated to ~ 54 hours for the first and ~ 231 hours for second photocathodes.\u003c/p\u003e","description":"","filename":"11.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6536191/v1/58ca9195b958ba5b4e0ce6ac.jpg"},{"id":83220727,"identity":"a0b32d88-6611-457d-af37-3892d26d2dc6","added_by":"auto","created_at":"2025-05-21 10:21:14","extension":"jpg","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":69322,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Simulated electric field profiles in the SRF gun operating at 1.25 MV for various cathode recess depths. At the standard 10.5 mm recess used in the experiment, the electric field at the cathode surface is approximately 12 MV/m. (b) ANSYS simulation of temperature rise in a GaAs photocathode with a 100 μm crystal thickness, under SRF gun operation at 1.25 MV.\u003c/p\u003e","description":"","filename":"12.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6536191/v1/999e2b44c9c480d136d0d8ac.jpg"},{"id":83220721,"identity":"5e34bd2d-8fcf-47b6-915a-b06351001b65","added_by":"auto","created_at":"2025-05-21 10:21:14","extension":"jpg","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":92890,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Process of mitigating multipacting and associated vacuum pressure excursions through periodic scanning of the gun solenoid current. The bottom graph shows the SRF gun voltage, the middle graph displays the solenoid current (in amperes), and the top graph presents the evolution of vacuum pressure (in Torr) measured by the FPC cold vacuum gauge. The horizontal axis represents time in days. (b) Typical vacuum spikes in the FPC region, with vacuum pressure (in Torr) shown in the bottom graph, and corresponding spikes in the flow rate of evaporated helium (in g/sec) shown in the top graph. The horizontal axis indicates time in hours.\u003c/p\u003e","description":"","filename":"13.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6536191/v1/642580fa0ed046fa08a2bb4f.jpg"},{"id":83221999,"identity":"5787fd04-b97c-434b-a09c-1ccb039592b2","added_by":"auto","created_at":"2025-05-21 10:29:14","extension":"jpg","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":95543,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Typical output from beam energy measurements conducted during the experiment. Red and pink squares represent beam centroid positions corresponding to 16 settings of the horizontal and vertical trim magnets, respectively, ranging from -0.4 A to +0.4 A, with the solenoid current set at -2.65 A. Green and blue squares show equivalent measurements at a solenoid current of +2.65 A. Lines of matching colors represent linear fits to the data. The resulting rotation angle of approximately 62.2° corresponds to a kinetic beam energy of 0.68 MeV. The difference between vertical and horizontal scans indicates astigmatism in the system.\u003c/p\u003e\n\u003cp\u003e(b) The accelerator’s focusing system was configured to image the electron beam distribution emitted from the photocathode onto a YAG profile monitor located 3.5 meters downstream of the gun. While the left and center portions of the image appear sharp, the right edge is distorted and clipped due to imperfections in the beam transport system. The 5 mm diameter emitting area is projected with approximately 4× magnification onto a YAG screen with a 25.4 mm diameter, indicated by the dotted circle.\u003c/p\u003e\n\u003cp\u003e(c) One of the recorded laser temporal profiles, captured after the pulse-shaping issue was identified.\u003c/p\u003e\n\u003cp\u003eThis laser malfunction prevented us from optimizing the beam dynamics and achieving the low emittance values typically observed in our SRF gun [20].\u003c/p\u003e","description":"","filename":"14.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6536191/v1/c8775d64e718eeb35949b02a.jpg"},{"id":101739856,"identity":"01384ddb-c45c-4904-8864-7f7dc23bbccb","added_by":"auto","created_at":"2026-02-03 08:07:17","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1952388,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6536191/v1/71d92361-87f9-49d8-8d1f-565e9044c7b2.pdf"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Breakthrough in Generation of Polarized Electron Beams: Unveiling the World's First RF Electron Gun with GaAs Photocathode","fulltext":[{"header":"I. Polarized electron guns","content":"\u003cp\u003ePolarized electrons have been and continue to be important research tools in atomic physics and condensed matter research [1]. The opportunities presented by polarized electron beams in high-energy and nuclear physics led to the development of polarized electron sources in the 1970s – see review in [2]. After a brief period of using atomic sources for polarized electrons, a significant breakthrough occurred in 1974 with the discovery of GaAs photocathodes, which generate polarized electrons when illuminated by circularly polarized infrared photons [3-5].\u003c/p\u003e\n\u003cp\u003eAfter that, numerous GaAs sources were built and successfully used in high-energy and nuclear physics experiments [6-13]. In addition, novel techniques were developed for GaAs-based sources to achieve electron polarization approaching 85% and high QE [14-15]. Compared to other photocathodes, GaAs photocathodes exhibit extreme sensitivity to residual gas content and pressure, as well as to ion back-bombardment of the cathode surface [12,16]. The most important consequence of these requirements is that operational polarized GaAs sources [6-13,17-19] are low-voltage (50-320 KV), low-gradient (~ 5 MV/m) DC electrostatic electron guns. These guns provide an excellent vacuum level at 10⁻¹² torr level to ensure a long lifetime for the cathode's quantum efficiency (QE). Additionally, they all operate at relatively low accelerating voltages to minimize dark current emission, which could otherwise compromise the system’s vacuum, increase ion back-bombardment, and reduce the quantum efficiency (QE) lifetime.\u003c/p\u003e\n\u003cp\u003eIt is well known that generating high-quality electron beams requires a high accelerating gradient as well as a high accelerating voltage, i.e., sufficient beam energy at the gun exit [17-20]. In contrast with electrostatic guns, RF guns can operate with very high gradients and accelerating voltage measured in megavolts (MV). When combined with the negative affinity of GaAs photocathodes, this could enable the generation of electron beams with exceptionally low transverse emittances [12]. This is why the physics community has been seeking a method to operate GaAs photocathodes inside an RF gun as sources of polarized electrons for future colliders [22-28]. Unfortunately, all previous attempts to operate an RF gun with GaAs photocathodes failed [29-34]. In the best case, the cathode's quantum efficiency (QE) survived only a few RF cycles [30].\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThere are three main challenges that cause most RF guns to operate only with very robust photocathodes, such as metal, Cs\u003csub\u003e2\u003c/sub\u003eTe, or CsK\u003csub\u003e2\u003c/sub\u003eSb.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe first challenge is the relatively poor vacuum in most normal-conducting (room temperature) high-gradient RF guns, where high power loss in the cavity’s walls leads to outgassing [32-33]. This problem made normal conducting RF guns incompatible with GaAs photocathodes. In contrast, properly designed superconducting RF (SRF) guns, operating at cryogenic temperatures (from 2K to 4.5K), can provide vacuum conditions suitable for long lifetime of GaAs photocathodes.\u003c/p\u003e\n\u003cp\u003eThe second challenge is to eliminate both multipacting—a resonant process where an electron avalanche develops on the cavity surface [35 and references therein]—and dark current, the uncontrolled cold emission of electrons [36-38]. Both effects significantly accelerate the degradation of the photocathode's quantum efficiency (QE).\u003c/p\u003e\n\u003cp\u003eThe third challenge is the significant decline in photocathode quantum efficiency at the cryogenic temperatures typical for SRF guns [39]. To address this, a specialized system known as the cathode stalk is required to thermally insulate the photocathode from the gun cavity walls. This system, equipped with an RF choke, must ensure adequate electrical and thermal conductivity to maintain the photocathode at the required temperature. Failure to do so may lead to the evaporation of volatile materials, such as cesium, from the photocathode surface, which could degrade the performance of the SRF cavity.\u003c/p\u003e"},{"header":"II.\tSRF electron gun at Brookhaven National Laboratory","content":"\u003cp\u003eAll these challenges were overcome in May 2024 when we successfully demonstrated the sustained operation of two GaAs photocathodes in our SRF gun, shown in Figure 1.\u003c/p\u003e\n\u003cp\u003eThe gun was constructed as part of the SRF accelerator for a Coherent Electron Cooling (CeC) experiment at the Relativistic Heavy Ion Collider (RHIC) [40-43] and was brought into operation ten years ago. The successful operation with GaAs photocathodes was achieved after years of operation, resolving numerous challenges, implementing system improvements, and developing dedicated operating modes. After finding solution how to overcome multipacting problems [35], developing methods of conditioning out dark current [44], the gun started demonstrating reliable months-long operation with CsK\u003csub\u003e2\u003c/sub\u003eSb photocathodes [45] at 1.25 MV accelerating voltage and generating world-brightest CW electron beams [20].\u003c/p\u003e\n\u003cp\u003eThe months-long lifetime of the CsK₂Sb photocathodes, which are sensitive to all three effects mentioned above, was a very encouraging indicator that the CeC SRF gun could sustain operation with the more demanding GaAs counterparts. However, several remaining challenges still posed a risk to a successful test.\u003c/p\u003e\n\u003cp\u003eFirst, the cathode transfer ultra-high vacuum (UHV) system was designed for operating with CsK₂Sb cathodes that is typically insufficient for more sensitive GaAs. Furthermore, unlike typical DC polarized guns, where GaAs photocathodes are prepared in a system connected to the gun, our SRF gun is located in the RHIC tunnel\u0026mdash;a radiation-protected area with restricted access, available only once every two weeks for a limited time. Our photocathodes are prepared at the laboratory located few kilometers away from the gun - see the corresponding description in the Methods. They are then transferred into a portable vacuum chamber, known as the photocathode\u0026rsquo;s \u0026ldquo;garage,\u0026rdquo; which is sealed with a vacuum valve. The sealed chamber is transported to the RHIC tunnel, where it is connected to the SRF gun\u0026rsquo;s cathode transfer system load-lock chamber. After baking the load-lock chamber, the valve can be opened, allowing the photocathode to be transferred into the cathode stalk using three vacuum manipulator arms\u0026mdash;one vertical and two horizontal\u0026mdash;with a total transfer length of approximately two and a half meters.\u003c/p\u003e\n\u003cp\u003eThese operations demand an extremely high vacuum. Achieving the required vacuum level involved multiple upgrades to both the cathode deposition system and the portable vacuum chamber. Additionally, the entire cathode transfer system\u0026rsquo;s vacuum setup had to be overhauled, including the installation of two 1,500 liter-per-second non-evaporable getter (NEG) pumps. We reached an acceptable\u0026mdash;though not ideal\u0026mdash;level of vacuum for GaAs cathodes. To verify this, we conducted a partial GaAs cathode transfer and observed a fivefold decrease in its quantum efficiency (QE). Although this degradation was significant, the remaining QE was sufficient for our proof-of-principle test.\u003c/p\u003e\n\u003cp\u003eSecond, our SRF gun was designed to operate with CsK\u003csub\u003e2\u003c/sub\u003eSb photocathodes, which are deposited on the polished front surface of the molybdenum (Mo) puck with two side groves for the puck transfer in the system and the inner stub that is used to conduct the electric current and the heat induced in the cathode to the end-effector and to the water-cooled stalk \u0026ndash; see Figure 2.\u003c/p\u003e\n\u003cp\u003eExcellent heat and electrical current transfer to the stalk are critically important for proper operation of the SRF gun and the photocathodes. Overheating of the cathode puck will result in evaporation volatile photocathode material, which would result in increase of dark current or even arcing and complete failure of the gun. In addition, deposition of the photocathode material inside the SRF gun will result in multipacting, which would kill the QE of the photocathode. Furthermore, poor current transfer could result in the arcing, vacuum bursts and destruction of the photocathode.\u003c/p\u003e\n\u003cp\u003eIn contrast with CsK\u003csub\u003e2\u003c/sub\u003eSb photocathodes, the GaAs photocathode uses a cesiated GaAs crystal substrate, which should be connected to the cathode puck both electrically and thermally. We modified two Mo pucks as shown in Figure 3 (a) to house thin (100 to 300 microns) GaAs crystals. The crystals were connected to the body of the puck using Indium and were held in place by molybdenum cups that were crewed onto the pucks. The cups have openings with 5 mm diameter used to activate the GaAs (see methods for details).\u003c/p\u003e\n\u003cp\u003eThird, we took every possible measure to ensure good thermal contact between the GaAs substrate and the Mo puck. We also conducted simulations to assess the heat generated in the substrate and its resulting temperature rise. The simulation predicted 5 W of RF losses in the GaAs substrate when operating the gun at its nominal voltage of 1.25 MV. Our standard SRF gun turn-on routine, used with CsK₂Sb photocathodes, involves a rapid initial voltage jump to approximately 200 kV (to prevent vacuum excursions caused by multipacting at low voltages) followed by a fast voltage ramp to 1 MV. However, previous failures with GaAs cathodes in RF guns introduced enough uncertainty that we chose not to attempt immediate operation at high voltages. Instead, we developed operation modes at lower voltages, starting as low as 52 kV, while maintaining excellent vacuum conditions\u0026mdash;details of these developments are provided in the corresponding Methods section. We began our GaAs photocathode test at 52 kV, where estimated heat losses in the crystal were below 1 mW, and then gradually increased the operating voltage.\u003c/p\u003e\n\u003cp\u003eFourth, one of SRF gun important advantages is that it provides excellent vacuum. Its body operated at temperature of 4K serving as a very powerful cryogenic vacuum pump that freezes all gases including hydrogen. The frozen gasses attach to the surface and can be released only when the temperature rises to the evaporation level. Situation is similar for cold-to-room-temperature transition in the front and the back of the SRF gun cavity. The transitions are cooled by helium gas evaporating by the SRF cavity. The helium gas propagates trough channels attaches to the transitions and gradually warms up to the room temperature. Temperature gradient results is distribution of frozen residual gas species, originating in warm section of accelerator, along the surface of the transition according to their temperatures of freezing. Variations of the helium pressure would result in variation of its flow, changes in the temperature profile and corresponding vacuum pressure bursts. Such bursts were observed in our SRF gun system and were acceptable for operation with CsK\u003csub\u003e2\u003c/sub\u003eSb photocathodes but would be intolerable for the GaAs. This was the reason that our cryogenic group developed mode of operating the cryogenic system that prevented variations of the helium gas flow and eliminated the vacuum bursts.\u0026nbsp;\u003c/p\u003e"},{"header":"III. Operation of SRF gun with GaAs photocathodes","content":"\u003cp\u003eCompletion of improvements mentioned above and opening in the CeC operation schedule allowed us to demonstrate sustained operations of our SRF gun with GaAs photocathodes.\u003c/p\u003e\n\u003cp\u003eTwo GaAs photocathodes (which we will call number #1 and #2) with initial QE of 4.8% in green (532 nm) light were produced in March 2024. The vacuum suit with these cathodes was moved to RHIC tunnel and connected to the SRF gun cathode transfer system on April 2, 2024. After finishing the bake-out of the load-lock system on April 8, 2024, we performed our planned \u0026ldquo;survival test\u0026rdquo; by transferring the cathode #1 out of the vacuum suit to the vertical transfer arm, lowered to the tip of the long horizontal transfer arm and back to the suit. The cathode survived but its QE dropped to estimated value ~ 1% - the accuracy was limited by imperfect QE measurement system incorporated into transfer system. The cathode was held in the load-load chamber awaiting completion of the preparation for the actual tests.\u003c/p\u003e\n\u003cp\u003eAll necessary SRF gun operating modes were initially tested using a blank Mo puck. On May 15, 2024, Cathode #1 was transferred into the gun. Time allocated for this experiment was limited to twenty days, after which we must return to regular CeC program.\u003c/p\u003e\n\u003cp\u003eMeasurements taken before the transfer showed that the #1 cathode\u0026apos;s QE had dropped to approximately 0.5%, a twofold decrease over 37 days. Following this measurement, the cathode was first transferred vertically using a manual system. After positioning it onto the end-effector of the long horizontal arm, we used a computer-controlled motor system (referred to as the \u0026ldquo;tractor\u0026rdquo;) to slowly move the cathode into its final position. This slow movement was crucial for minimizing vacuum spikes caused by friction (see Fig. 4(a)). The horizontal transfer process took about one hour.\u003c/p\u003e\n\u003cp\u003eInitial measurements, conducted at a very low bunch charge to avoid saturation, revealed a degradation of the GaAs photocathode quantum efficiency (QE) to 0.04% during the transfer. This clearly demonstrated that our cathode transfer system, originally designed for other photocathodes, does not meet the stringent requirements for GaAs. This finding suggests that future polarized SRF guns should be designed with a more reliable\u0026mdash;and likely simpler\u0026mdash;cathode transfer system, possibly combined with a local GaAs cathode activation system.\u003c/p\u003e\n\u003cp\u003eThe primary goal of our experiment was to demonstrate the operation of an SRF gun with a GaAs photocathode. We gradually increased the operating voltage to 1 MV while carefully monitoring vacuum conditions, dark current, and liquid helium consumption. Throughout this process, we observed no QE degradation. Instead, the QE increased to 0.08% after one day of CW operation and to 0.093% after two days. We attributed this improvement to the electron beam effectively cleaning contaminants from the cathode surface that had been introduced during the transfer.\u003c/p\u003e\n\u003cp\u003eWhile operating at 500 kV, we performed a QE map scan of the cathode and found that the QE varied by a factor of two, with a local maximum reaching approximately 0.1% (see Fig. 5(a)). We had sufficient laser power to saturate the cathode and observed a nearly linear relationship between the maximum extracted charge and the gun\u0026apos;s accelerating voltage (see Fig. 5(c)). For laser spot with 5 mm diameter the slope of this curve was 1.2 nC/MV, and when operating at voltages above 1 MV, we successfully generated bunch charges exceeding 1 nC per bunch. Our initial successes were followed by a series of technical system failures, which caused brief but measurable vacuum spikes. These spikes led to a gradual decrease in QE to 0.033%, 0.025%, and 0.021% over three days.\u003c/p\u003e\n\u003cp\u003eThe turning on of the SRF gun to initial voltage of 52 kV did not generate any vacuum spikes and after illuminating the photocathode with our green laser we observed first electrons bunches with charge ~ 100 picocoulombs (pC).\u003c/p\u003e\n\u003cp\u003eTo demonstrate that our success with GaAs photocathode #1 was not merely due to luck, we replaced it with cathode #2 on May 22, 2024. During the transfer, the integrated pressure in the manipulator measured at 8.5\u0026times;10⁻⁷ torr-seconds, and cathode #2 had an initial QE of 0.1%. We used this cathode to test operation at higher voltages (see Fig. 6(a)), along with tracking QE values, QE map evolution, and measuring electron beam parameters.\u003c/p\u003e\n\u003cp\u003eDetails of the photocathode\u0026rsquo;s QE evolution can be found in the corresponding section of the Methods. Our analysis showed that the QE lifetime is approximately 54 hours for the first photocathode and around 231 hours for the second.\u003c/p\u003e\n\u003cp\u003eFollowing 1.4 MV quench event, we resumed operation at our standard voltage of 1.25 MV. We used the remaining five days of the test period to investigate some of the unexpected behaviors we had observed during the two weeks of operation: (a) significant non-uniformity and various structural patterns in the transverse beam profiles and (b) large measured transverse beam emittances, when compared with typical values observed in previous SRF gun operations [20].\u003c/p\u003e\n\u003cp\u003eWe found that while QE non-uniformity contributed to uneven transverse beam density, the primary cause of the transverse emittance blow-up was the failure of the laser system to deliver flat-top 360 ps pulses. This issue was resolved after the completion of our tests (see Methods for details).\u003c/p\u003e"},{"header":"IV. Possible future steps","content":"\u003cp\u003eWhile we successfully demonstrated more than a threefold increase in accelerating voltage and achieved a reasonable — for a proof-of-principle — QE lifetime for two GaAs photocathodes, not all aspects of our experiment were without issues:\u003c/p\u003e\n\u003cp\u003e·\u0026nbsp; \u0026nbsp; \u0026nbsp;The vacuum level in the cathode transfer system was insufficient to preserve the QE of GaAs photocathodes. As a result, each cathode transfer led to a significant reduction in QE and introduced strong QE non-uniformity.\u003c/p\u003e\n\u003cp\u003e·\u0026nbsp; \u0026nbsp; \u0026nbsp;The laser system’s performance was inadequate for a proper evaluation of the generated electron beam quality.\u003c/p\u003e\n\u003cp\u003eA six-month operation of the CeC SRF gun is planned for 2025, with a primary focus on demonstrating Coherent electron Cooling. The laser system has since been repaired and will now provide 360 ps flat-top pulses. While there is a possibility that the CeC SRF gun could be used for another GaAs photocathode test, improvements to the cathode transfer system's vacuum conditions are unlikely. Consequently, we can expect the initial QE to be around 0.1%. However, the repaired laser system will enable accurate evaluation of the beam quality generated from GaAs cathodes.\u003c/p\u003e\n\u003cp\u003eThe CeC SRF gun relies on liquid helium from the RHIC cryogenic system. RHIC will cease operations at the end of 2025 to enable construction of the future Electron-Ion Collider in the RHIC tunnel. This will eliminate the possibility of using the CeC SRF gun for future GaAs tests, leaving the Rosendorff SRF gun [47, 48] — which has struggled to operate even with robust CsK₂Sb photocathodes — as the only remaining operational SRF gun in 2025.\u003c/p\u003e\n\u003cp\u003eFortunately, a new quarter-wave (QW) SRF gun is currently under construction at Michigan State University for the LCLS II CW X-ray FEL [49]. This new SRF gun could potentially serve as the next testing platform for exploring the GaAs photocathode technology.\u003c/p\u003e\n\u003cp\u003eThe authors would like to acknowledge the continuous support from the administration of the Collider-Accelerator and Instrumentation Departments at BNL, as well as the RHIC and CeC operations groups. Special thanks go to William Weldon and Rudy Begay (Instrumentation Department, BNL), Ron Napoli and Kirk Sinclair (Collider-Accelerator Department, BNL), and Jyoti Biswas (Electron-Ion Collider, BNL) for their significant contributions to this project. This research was supported by the DE-SC0021426 award from the Office of Nuclear Physics and by Brookhaven Science Associates, LLC under Contract No. DE-SC0012704 with the U.S. Department of Energy.\u0026nbsp;\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003eGaAs photocathodes preparation\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003eGaAs Puck Design and Activation Procedure\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe GaAs puck was designed to maintain full compatibility with the existing infrastructure used for growing and transporting alkali antimonide photocathodes to the SRF gun. To enable the use of GaAs-based photocathodes while shielding the sharp edges of the sample from the gun\u0026rsquo;s internal electric fields, the puck consists of two molybdenum components.\u003c/p\u003e\n\u003cp\u003eThe main body of the puck includes one of the two grooves needed for sample manipulation and features a pocket on its top surface to hold a small 7\u0026times;7 mm\u0026sup2; GaAs sample. The second groove is provided by a cap that screws onto the top of the body. This cap, mechanically polished to a mirror finish, has a 5 mm diameter hole in its center, allowing the GaAs crystal surface to be exposed to Cs, O, and Te during activation, as well as to laser illumination for electron extraction (see Figure 3 in the main text).\u003c/p\u003e\n\u003cp\u003eBefore final assembly, both the body and cap are high-pressure rinsed to remove particulates and then sealed in a dedicated transporter within a cleanroom. The GaAs crystals are chemically etched to remove surface oxides, followed by rinsing with deionized water. The assembly of the GaAs sample into the puck also takes place in the cleanroom. A thin indium foil (30 \u0026micro;m thick) is inserted between the GaAs crystal and the puck body. During the heat cleaning process, this indium foil melts, improving thermal conductivity between the GaAs crystal and the puck.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003ePreliminary Experiments on GaAs Activation and Lifetime\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePreliminary experiments were conducted to estimate the photocathode lifetime within our growth system and to identify the dominant mechanisms contributing to quantum efficiency (QE) degradation under the vacuum conditions of our activation chamber. These conditions range from a steady-state vacuum of ~1\u0026times;10⁻\u0026sup1;\u0026sup1; Torr to ~1\u0026times;10⁻\u0026sup1;⁰ Torr during activation.\u003c/p\u003e\n\u003cp\u003eA GaAs sample with p-type doping of 1\u0026times;10\u0026sup1;⁹ cm⁻\u0026sup3; was chemically etched with 35% HCl, rinsed with deionized water, and then transferred to the vacuum system. The sample was heat-cleaned at 580 \u0026deg;C for two hours to remove surface oxides and then twice activated to negative electron affinity (NEA) using Cs and oxygen. QE evolution was measured using a green laser (532 nm) at both 100% and 1% duty cycles.\u003c/p\u003e\n\u003cp\u003eThese experiments aimed to determine whether the observed QE lifetime was primarily affected by chemical poisoning or by ion back bombardment. Given that the photocathode bias was maintained at -50 V and the average extracted current was in the microampere range, ion back bombardment could not be ruled out as a significant factor. The results, shown in Figure 7, indicate that similar QE lifetimes were observed over time, suggesting that further investigation is required to isolate the dominant degradation mechanism.\u003c/p\u003e\n\u003cp\u003eWhen the same QE data are plotted as a function of the extracted charge in Fig. 7 (b), it becomes evident that QE decay occurs much more rapidly\u0026mdash;by more than a factor of 50\u0026mdash;when the sample is operated with a 1% duty cycle compared to continuous (100%) illumination. This strongly suggests that under our experimental conditions, chemical poisoning is the dominant mechanism responsible for QE degradation, rather than ion back bombardment.\u003c/p\u003e\n\u003cp\u003ePreliminary experiments were also performed using a modified activation procedure involving a tellurium (Te) interlayer between two Cs\u0026ndash;O layers. The GaAs sample was initially activated by alternating exposures to Cs and oxygen. After reaching the maximum QE, a slight excess of Cs was deliberately deposited by continuing the Cs exposure, which resulted in a small decrease in QE, as shown in Figure 8.\u003c/p\u003e\n\u003cp\u003eIt was expected that the excess Cs would gradually react with residual oxygen in the vacuum chamber over the following hours, leading to a further increase in QE. This effect was indeed observed over the next two days, as QE measurements taken every 24 hours confirmed a gradual improvement (see Figure 9).\u003c/p\u003e\n\u003cp\u003eForty-eight hours after the initial Cs and oxygen activation, it was observed that further Cs deposition led to a decrease in the QE of the GaAs photocathode. At this point, tellurium was deposited onto the photocathode surface until no measurable photocurrent was detected under 532 nm laser illumination (see Figure 9). Subsequent Cs and oxygen exposure resulted in a renewed QE peak, reaching levels comparable to those achieved in the previous activation cycle. As before, the surface was slightly overexposed to Cs vapor, and over the following two days, the QE gradually recovered to its maximum value.\u003c/p\u003e\n\u003cp\u003eAll measurements described thus far were conducted with the photocathode inside the activation vacuum chamber. To evaluate the photocathode\u0026rsquo;s robustness against vacuum degradation\u0026mdash;particularly from mechanical motion\u0026mdash;we attempted to transfer the activated photocathode into the preparation chamber of the activation system. During the transfer, vacuum levels degraded rapidly to the 10⁻⁹ Torr range due to outgassing from the manipulator\u0026rsquo;s bellows. After the transfer was completed, the QE was measured again and found to have dropped to approximately 1.5% (see Figure 9).\u003c/p\u003e\n\u003cp\u003eThe QE of the GaAs photocathode also exhibited degradation with a decay time of approximately one hour when the magnetic manipulator was used to transfer the photocathode from the preparation chamber to the docking chamber, in preparation for insertion into the portable vacuum chamber (we call it a garage). This portable chamber is used to transport the activated photocathode to the SRF gun loading system. However, since the transfer typically takes only a few minutes, the resulting decrease in photocurrent was minimal and considered negligible.\u003c/p\u003e\n\u003cp\u003eThese measurements enabled us to determine the optimal maximum speed at which the linear manipulators can be operated to minimize QE loss due to the transient gas load released during the transfer process.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePhotocathodes lifetimes in the SRF gun\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFor operation in the SRF gun, two GaAs photocathodes were prepared using the activation method involving a Te interlayer, as described previously. This technique produced photocathodes with very similar final QE values, each reaching approximately 4% following the intentional deposition of a slight excess of Cs at the end of the activation process (see Figure 10). For the operation in the SRF gun two GaAs photocathodes were prepared activating the surface with a Te interlayer as described in the previous paragraph.\u003c/p\u003e\n\u003cp\u003eBoth photocathodes were transferred from the activation chamber to the preparation chamber and subsequently to the garage docking station in a carefully controlled manner, ensuring that vacuum levels remained below the 10⁻\u0026sup1;⁰ Torr range throughout. This procedure successfully preserved the QE of both cathodes, with no significant degradation observed after the transfer to the preparation system.\u003c/p\u003e\n\u003cp\u003eOnce in the garage chamber, QE measurements were performed using a collecting anode that could be positively biased. However, due to the geometric configuration\u0026mdash;where the anode is positioned to the side of the cathodes\u0026mdash;and the large incident angle of the laser beam, accurate QE estimation was not possible at this stage. Nevertheless, given the short transfer time, the consistently low vacuum levels (with the garage chamber maintained in the 10⁻\u0026sup1;\u0026sup1; Torr range), and the controlled handling, we assumed that the QE of both photocathodes remained largely unaffected during the transfer.\u003c/p\u003e\n\u003cp\u003eThe ultra-high vacuum (UHV) garage chamber was then transported to the RHIC tunnel and connected to the load-lock of the SRF gun\u0026apos;s cathode loading system. Immediately following the connection, photoemission from the photocathodes was confirmed, with photocurrent values comparable to those measured before transporting the garage.\u003c/p\u003e\n\u003cp\u003eThe load-lock system of the SRF gun was vacuum bake-out at approximately 150 \u0026deg;C for one week following connection. After the bake-out, one of the two photocathodes was transferred from the garage into the SRF gun\u0026rsquo;s loading chamber, where its QE was measured and found to have decreased to around 1%, with an estimated 1/e decay time of approximately 183 hours (see Figure 11(a)). The photocathode remained in the loading chamber, under vacuum at ~1\u0026times;10⁻\u0026sup1;\u0026sup1; Torr, for about 37 days as preparations for SRF gun beam operation progressed.\u003c/p\u003e\n\u003cp\u003eJust before being loaded into the SRF gun, the QE of the first cathode was measured again and had further decreased to approximately 0.5%, corresponding to an estimated 1/e decay time of about 1,281 hours (see Figure 11(a)). Once in the SRF gun, the accelerating voltage was gradually ramped up. The first reliable QE measurement\u0026mdash;free of image charge-induced saturation\u0026mdash;showed a QE of about 0.08%. Over the following days, the photocathode was operated at varying accelerating voltages, with several additional QE measurements taken. A 1/e decay time of approximately 54 hours was estimated during this operational phase.\u003c/p\u003e\n\u003cp\u003eAs the QE of the first cathode continued to decline, it was removed and replaced with the second photocathode, which was then inserted into the gun.\u003c/p\u003e\n\u003cp\u003eQE measurements for the second photocathode were conducted primarily while it was installed in the SRF gun, so its full degradation history is not as well documented as that of the first cathode.\u003c/p\u003e\n\u003cp\u003eHowever, the initial QE in the gun was comparable to that of the first photocathode. As cavity operation became more stable and vacuum excursions during injector operation were less severe, a longer 1/e QE decay time was observed.\u003c/p\u003e\n\u003cp\u003eThe QE gradually decreased to 0.08% on May 23 during overnight operation at 410 kV, and further declined to 0.065%, 0.04%, and 0.031% on May 24, 27, and 28, respectively. Two unsuccessful attempts on May 25 and 27 to operate above 1 MV were followed by sustained SRF gun operation at 1.35 MV on May 29. However, an attempt to reach 1.4 MV resulted in a cavity quench, likely caused by GaAs cathode overheating and Cs contamination of the gun\u0026rsquo;s cavity.\u003c/p\u003e\n\u003cp\u003eNotably, we observed that QE increased from 0.031% to 0.04% when the RF voltage was raised from 0.6 MV to 1.1 MV. We attribute such increase to a rise in the GaAs substrate temperature, which likely reduced the photocathode\u0026rsquo;s work function. This assumption aligns with the observed cavity quench at 1.4 MV, which corresponded to a 62% increase in RF losses within the GaAs crystal, when compared with operating at 1.1 MV.\u003c/p\u003e\n\u003cp\u003eThis decay time was estimated to be approximately 231 hours (see Figure 11(b)).\u003c/p\u003e\n\u003cp\u003eThe fourfold improvement in QE lifetime observed with the second photocathode was the result of enhanced SRF gun control systems and the avoidance of failures that occurred during the initial learning curve with the first cathode.\u003c/p\u003e\n\u003cp\u003eOn the final day of operation, a power dip triggered a gun trip, resulting in a vacuum spike that reached the 10⁻⁶ Torr range. This incident caused a sudden drop in QE by nearly an order of magnitude. With the QE reduced to the 10⁻⁴ range, the available laser power was insufficient to extract more than a few tens of picocoulombs. Consequently, the GaAs photocathode was removed and replaced with a standard alkali antimonide photocathode for the remainder of the run.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePreparations of the SRF gun for operating with GaAs photocathodes\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePossible overheating of GaAs photocathodes, leading to contamination of the SRF gun cavity by cesium (Cs), posed a significant risk and required a cautious approach. To mitigate this, we developed a detailed procedure that began with operating the gun at the lowest possible voltage, followed by a gradual voltage increase while closely monitoring vacuum and radiation conditions in the gun area.\u003c/p\u003e\n\u003cp\u003eOur SRF gun utilizes cathodes recessed by 10.5 mm from the cavity nose to provide the necessary focusing for the generated electron beams. This configuration reduces the peak electric field at the cathode surface by a factor of two compared to a non-recessed cathode (see Figure 12(a)). To evaluate potential heating, we first simulated temperature increases due to dielectric losses in the GaAs crystal at the SRF gun\u0026rsquo;s operating frequency of 113 MHz, as well as resistive losses in the molybdenum (Mo) pack (see Figure 12(b)).\u003c/p\u003e\n\u003cp\u003eWe employed p-doped GaAs crystals with thicknesses of 100 \u0026mu;m and 300 \u0026mu;m for cathodes #1 and #2, respectively. Since specific data on GaAs dielectric losses at the 113 MHz frequency of the SRF gun is not available, we extrapolated existing data to estimate\u003cimg data-fr-image-pasted=\"true\" width=\"75\" height=\"13\" src=\"data:image/png;base64,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\" alt=\"Title: tan delta almost equal to 10 to the power of negative 4 end exponent - Description: {\u0026quot;mathml\u0026quot;:\u0026quot;\u003cmath style=\\\u0026quot;font-family:stix;font-size:16px;\\\u0026quot; xmlns=\\\u0026quot;http://www.w3.org/1998/Math/MathML\\\u0026quot;\u003e\u003cmstyle mathsize=\\\u0026quot;16px\\\u0026quot;\u003e\u003cmi\u003etan\u003c/mi\u003e\u003cmi\u003e\u0026#x3B4;\u003c/mi\u003e\u003cmo\u003e\u0026#x2248;\u003c/mo\u003e\u003cmsup\u003e\u003cmn\u003e10\u003c/mn\u003e\u003cmrow\u003e\u003cmo\u003e-\u003c/mo\u003e\u003cmn\u003e4\u003c/mn\u003e\u003c/mrow\u003e\u003c/msup\u003e\u003c/mstyle\u003e\u003c/math\u003e\u0026quot;,\u0026quot;origin\u0026quot;:\u0026quot;MathType for Microsoft Add-in\u0026quot;}\" class=\"fr-fic fr-dii\"\u003e. The resulting estimate predicted that GaAs dielectric losses of approximately 0.5 W for the 100 \u0026mu;m crystal and ~1 W for the 300 \u0026mu;m crystal dominate the overall losses.\u003c/p\u003e\n\u003cp\u003eSimulations predicted temperature increases of approximately 14 K for 100 \u0026mu;m and 28 K for 300 \u0026mu;m GaAs crystals, raising serious concerns about potential overheating. This necessitated initiating gun operation at previously unexplored voltages below 100 kV. Based on our prior experience [35], there was a significant likelihood of multipacting activity in the SRF gun\u0026rsquo;s Fundamental Power Coupler (FPC) region at voltages below 200 kV\u0026mdash;particularly in the presence of magnetic fields from the gun\u0026rsquo;s focusing solenoids. Any such multipacting would lead to a rise in vacuum pressure, which would inevitably degrade the quantum efficiency (QE) of the GaAs cathodes.\u003c/p\u003e\n\u003cp\u003eTo address this, we dedicated the week of May 8\u0026ndash;14, 2025, to cleaning up potential multipacting zones and suppressing associated vacuum spikes in the FPC. This was accomplished by performing periodic bi-polar scans of the solenoid current, ramping it to the maximum values required for each specific SRF gun voltage (see Fig. 13(a)).\u003c/p\u003e\n\u003cp\u003eIn parallel, the RF group developed a new gun turn-on procedure featuring a flexible voltage target. As a result of these efforts, we successfully established multipacting-free operation of the SRF gun across a wide voltage range\u0026mdash;from 52 kV to 1 MV\u0026mdash;with FPC vacuum levels in the low 10⁻\u0026sup1;⁰ Torr range.\u003c/p\u003e\n\u003cp\u003eAs described in the main text, the SRF gun cavity transitions\u0026mdash;from room temperature to cryogenic temperatures\u0026mdash;are cooled by evaporated helium gas. This gas travels through a spiraling tube surrounding the transition vacuum chamber, moving from the cold end to the room-temperature end. The resulting temperature gradient causes residual gas components from the warmer regions to condense onto the vacuum chamber walls where their respective freezing points are reached. Any reduction in the helium gas flow leads to an increase in wall temperature and the release of previously frozen gases\u0026mdash;a phenomenon illustrated in Fig. 13(b). While such gas bursts were tolerable during operation with CsK₂Sb photocathodes, they were unacceptable for use with GaAs. The RHIC cryogenics team refined the SRF gun cryo-system controls to eliminate fluctuations in helium flow, thereby preventing pressure spikes and ensuring stable vacuum conditions.\u003c/p\u003e\n\u003cp\u003eWith all preparations complete, we proceeded with testing GaAs photocathodes in the SRF gun. The initial test began at an SRF gun voltage of 52 kV, which ensured negligible power losses\u0026mdash;on the order of milliwatts\u0026mdash;and a sub-Kelvin temperature rise in the GaAs crystals.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMeasuring parameters of electron beam\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe employed well-established methods to measure key electron beam parameters from the SRF gun, including beam energy, bunch charge, and beam emittance.\u003c/p\u003e\n\u003cp\u003eTo measure the electron beam energy, we used a precise method described in [46], which is based on the principle that a solenoid rotates the plane of particle motion by an angle:\u003c/p\u003e\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\" width=\"232\" height=\"73\"\u003e\u003c/p\u003e\n\u003cp\u003ewhere \u003cem\u003ep\u0026nbsp;\u003c/em\u003eis the electron momentum, \u003cem\u003ee\u003c/em\u003e is the electron charge, and \u003cem\u003ec\u003c/em\u003e is the speed of light. The rotation angle \u003cem\u003e\u0026theta;\u003c/em\u003e is proportional to the integral of the longitudinal magnetic field \u003cem\u003eB\u003csub\u003ez\u003c/sub\u003e\u003c/em\u003e, which in turn is determined by the number of turns \u003cem\u003eN\u003c/em\u003e in the solenoid coil and the coil current \u003cem\u003eI\u003c/em\u003e, in accordance with Stokes\u0026apos; theorem:\u003c/p\u003e\n\u003cp\u003e\u003cimg 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\" width=\"394\" height=\"96\"\u003e\u003c/p\u003e\n\u003cp\u003eOur setup consists of a calibrated solenoid with a known number of coil turns, a pair of horizontal and vertical dipole trims placed upstream of the solenoid, and a YAG profile monitor downstream. We perform four measurements by scanning the electron beam horizontally and vertically using both positive and negative solenoid currents, while tracking the beam centroid on the YAG screen (see Fig. 14(a)).\u003c/p\u003e\n\u003cp\u003eThe angles between the beam trajectories produced by horizontal and vertical scans for opposite solenoid currents\u003c/p\u003e\n\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\" width=\"201\" height=\"82\"\u003e\u003c/p\u003e\n\u003cp\u003eallow us to determine the beam momentum, full and kinetic energies, \u003cem\u003eE\u003c/em\u003e and \u003cem\u003eE\u003csub\u003ek\u003c/sub\u003e\u003c/em\u003e, of electrons as well as the voltage of the gun (where \u003cem\u003em\u003c/em\u003e is the mass of electron):\u003c/p\u003e\n\u003cp\u003e\u003cimg 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\" width=\"745\" height=\"69\"\u003e\u003c/p\u003e\n\u003cp\u003eAlthough it is possible to use a single dipole trim for this measurement, combining results from both horizontal and vertical scans helps cancel out astigmatism effects inherent to real-world profile monitor systems, thereby improving measurement accuracy. This method typically achieves a relative energy measurement accuracy of ~10\u003csup\u003e-3\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eThe charge per bunch was measured using an Integrated Current Transformer (ICT), a standard diagnostic tool in our accelerator system [50]. These measurements were cross-calibrated against readings from a Faraday cup to ensure accuracy. The charge per bunch, \u003cem\u003eQ\u003c/em\u003e, along with the laser power, \u003cem\u003eP\u003csub\u003el\u003c/sub\u003e\u003c/em\u003e, was used to determine the quantum efficiency (QE) of the laser-illuminated photocathode area using the relation:\u003c/p\u003e\n\u003cp\u003e\u003cimg 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width=\"463\" height=\"96\"\u003e\u003c/p\u003e\n\u003cp\u003ewhere\u0026nbsp;\u003cem\u003ef\u003csub\u003el\u003c/sub\u003e\u0026nbsp;\u003c/em\u003eis the laser repetition frequency, and\u003cbr\u003e\u003cimg data-fr-image-pasted=\"true\" width=\"90\" height=\"11\" 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alt=\"Title: ℏ omega equals 2.33 space e V - Description: {\u0026quot;mathml\u0026quot;:\u0026quot;\u003cmath style=\\\u0026quot;font-family:stix;font-size:16px;\\\u0026quot; xmlns=\\\u0026quot;http://www.w3.org/1998/Math/MathML\\\u0026quot;\u003e\u003cmstyle mathsize=\\\u0026quot;16px\\\u0026quot;\u003e\u003cmi\u003e\u0026#x210F;\u003c/mi\u003e\u003cmi\u003e\u0026#x3C9;\u003c/mi\u003e\u003cmo\u003e=\u003c/mo\u003e\u003cmn\u003e2\u003c/mn\u003e\u003cmo\u003e.\u003c/mo\u003e\u003cmn\u003e33\u003c/mn\u003e\u003cmo\u003e\u0026#xA0;\u003c/mo\u003e\u003cmi\u003ee\u003c/mi\u003e\u003cmi\u003eV\u003c/mi\u003e\u003c/mstyle\u003e\u003c/math\u003e\u0026quot;,\u0026quot;origin\u0026quot;:\u0026quot;MathType for Microsoft Add-in\u0026quot;}\" class=\"fr-fic fr-dii\"\u003e\u0026nbsp;is the laser photon energy.\u003c/p\u003e\n\u003cp\u003eWhile we were able to accurately measure the QE of relatively large illuminated areas (on the order of several mm\u0026sup2;), high-resolution QE maps\u0026mdash;shown in Fig. 5(a) and 5(b) of the main text\u0026mdash;were acquired in relative units, calibrated to the maximum QE observed within each scan.\u003c/p\u003e\n\u003cp\u003eAdditionally, we captured images of the photocathode emission using a YAG profile monitor (see Fig. 14(b)) by operating with a low-charge, high-repetition-rate electron beam. Although these images were not perfect, they provided supplementary information about the spatial distribution of QE on the GaAs photocathodes. Specifically, uniform illumination of the entire photoemitting area revealed QE degradation in regions that had been used to generate the electron beam over several days of operation.\u003c/p\u003e\n\u003cp\u003eWe also observed approximately twenty small (~100 \u0026mu;m in diameter) randomly distributed spots exhibiting zero QE. These localized QE defects could not be resolved using the standard QE scan, which has a spatial resolution of approximately 0.25 mm.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eProblems with the laser pulses\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eUnfortunately, in our effort to optimize all other experimental conditions for the GaAs photocathodes, we failed to identify a critical issue with the laser\u0026apos;s temporal profile before the start of the experiment. The system designed to combine six Gaussian sub-pulses (beamlets) with alternating polarization into a single, nearly flat-top pulse of 360 ps FWHM malfunctioned. Instead of producing a uniform pulse, it generated a signal composed of beamlets with nearly random amplitudes.\u003c/p\u003e\n\u003cp\u003eWe discovered the faulty temporal structure of the laser pulses\u0026mdash;shown in Fig. 14(c)\u0026mdash;only after completing the GaAs photocathode tests. Subsequent measurements revealed that the relative intensity of individual beamlets varied by more than an order of magnitude. It is also likely that the pulse structure drifted during the experiment, with the pulse occasionally being dominated by just one or two beamlets.\u003c/p\u003e\n\u003cp\u003eThis laser malfunction prevented us from optimizing the beam dynamics and achieving the low emittance values typically observed in our SRF gun [20].\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eJ. 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Michel et al., A high-brightness SRF photoelectron injector for FEL light sources, Nucl. Instrum. Methods Phys. Res., Sect. A 593, 57 (2008).\u003c/li\u003e\n\u003cli\u003eA. Arnold and J. Teichert, Overview on superconducting photoinjectors, Phys. Rev. ST Accel. Beams 14, 024801 (2011).\u003c/li\u003e\n\u003cli\u003eStatus of the SLAC/MSU SRF Gun Development Project, J. Lewellen, C. Adolphsen, A. Arnold, Y. Choi, C. Compton et al, In Proceedings of North American Particle Accelerator Conference, NAPAC2022, Albuquerque, New Mexico, August 7-12 ,2022, WEPA03, DOI: 10.18429/JACoW-NAPAC2022-WEPA03\u003c/li\u003e\n\u003cli\u003eD. M. Gassner, V. Litvinenko, R. Michnoff, T. Miller, M. Minty, I. Pinayev, Coherent Electron Cooling Proof of Principle Instrumentation Design, in Proc. of Beam Instrumentation Workshop, April 15-19, 2012, Newport News, Virginia, USA, weap01, http://accelconf.web.cern.ch/AccelConf/BIW2012/papers/weap01.pdf\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6536191/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6536191/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003ePolarized electron beams play critical role in fundamental physics research by providing additional observables and opening new channels of discoveries. This discovery potential is well-known and is successfully used in high-energy and nuclear physics research. Recently, more conventional branches of science, such as ultra-fast electron microcopy, started exploitation unique features of polarized electrons. Currently GaAs crystals illuminated by circular polarized IR lasers remain the best choice for generating polarized electrons. All existing polarized sources are an electrostatic electron guns providing extremely high vacuum (XHV) conditions for survival of GaAs photo-emissivity. But limits in accelerating voltage and its gradient limits both the quality and quantity of available beams from these guns. These are the reasons why accelerator community was and is attempting to extend this technology to the radio-frequency electron guns, which are capable of accelerating beams with significantly higher accelerating gradients and total accelerating voltage. Unfortunately, all previous attempts of operating GaAs photocathodes in RF guns were unsuccessful. In this paper, we report on successful operation of GaAs photocathode in superconducting RF gun, describe in detail the accelerator system, used techniques, evolution of the GaAs quantum efficiency, and parameters of the generated electron beam.\u003c/p\u003e","manuscriptTitle":"Breakthrough in Generation of Polarized Electron Beams: Unveiling the World's First RF Electron Gun with GaAs Photocathode","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-05-21 10:21:09","doi":"10.21203/rs.3.rs-6536191/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-physics","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"nphys","sideBox":"Learn more about [Nature Physics](http://www.nature.com/nphys/)","snPcode":"","submissionUrl":"","title":"Nature Physics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Research","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"f7f82be2-6e14-4411-bfb9-528dc1d731e5","owner":[],"postedDate":"May 21st, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":48791506,"name":"Physical sciences/Physics/Techniques and instrumentation/Design, synthesis and processing"},{"id":48791507,"name":"Physical sciences/Physics/Particle physics/Experimental particle physics"}],"tags":[],"updatedAt":"2026-02-03T08:07:09+00:00","versionOfRecord":{"articleIdentity":"rs-6536191","link":"https://doi.org/10.1038/s41567-025-03138-7","journal":{"identity":"nature-physics","isVorOnly":false,"title":"Nature Physics"},"publishedOn":"2026-02-02 05:00:00","publishedOnDateReadable":"February 2nd, 2026"},"versionCreatedAt":"2025-05-21 10:21:09","video":"","vorDoi":"10.1038/s41567-025-03138-7","vorDoiUrl":"https://doi.org/10.1038/s41567-025-03138-7","workflowStages":[]},"version":"v1","identity":"rs-6536191","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6536191","identity":"rs-6536191","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}