Cryogenic Distribution System and Its Entropy-Based Optimization on the Example of the Polish FEL Facility

Polish Free Electron Laser (PolFEL), which is currently under construction in the National Centre for Nuclear Research in Świerk near Warsaw, will comprise an electron gun and from four to six cryomodules, each accommodating two 9-cell TESLA RF superconducting resonant cavities. To cool the superconducting resonant cavities, the cryomodules will be supplied with superfluid helium having a temperature of 2 K. Other requirements regarding the cooling power of PolFEL result from the need to cool the power couplers for the accelerating cryomodules (5 K) and thermal shields, which limit the heat inleaks due to radiation (40 K – 80 K). The machine will use several thermodynamic states of helium, such as two-phase superfluid helium, supercritical helium and low-pressure helium vapours. Supercritical helium will be supplied from a cryoplant by a Cryogenic Distribution System (CDS): transfer line and valve boxes, where it will be thermodynamically transformed into a superfluid state. This article presents the architecture of the CDS, discusses several design solutions that could have been optimised with the use of Second Law analysis and presents the design methodology of the chosen CDS elements.