Application of sub-cooled superfluid helium for cavity cooling at linac-based free electron lasers, energy recovery and proton linacs

Abstract

In order to build a compact linear accelerator, high acceleration gradients of superconducting radio frequency (SRF) cavities have to be achieved. In many large accelerators, e.g. XFEL, CEBAF or SNS, operational limitations are caused either by a limit on available overall cooling power of refrigerators or on cooling capabilities of sc cavities. So, for the further improving of sc cavity cooling, it is possible to increase either a quality factor (Q 0) or to improve a heat transfer at the cavity surfaces. Application of a sub-cooled superfluid helium gives several advantages, e.g. higher heat flux densities, longer time for onset of a film boiling regime and shorter recovery time, reduced Kapitza resistances, etc. In the present paper, application of sub-cooled superfluid (sf) helium for linac-based Free Electron Lasers, Energy Recovery and Proton Linacs is considered. In order to limit the present discussion, its application to CEBAF/SNS-style cryomodules is discussed in detail. For operation at higher RF power levels, further cooling improvements of a fundamental power coupler are needed and design modifications also presented.

Design of injectors and stay-alone cryostats with superconducting cavities for high RF powers applications

“…It is worth particularly to underline that the improvement of cavity operation is not possible without advantages in the operation of fundamental and HOM couplers. For high power operation, these components need to be actively cooled [2][3][4][5][6][7][8][9][10] as well as thermal intercept between cavities and these components has to be often applied. This significantly complicates the overall cryogenic process scheme; see reference [3] for more details.…”

“…For high power operation, these components need to be actively cooled [2][3][4][5][6][7][8][9][10] as well as thermal intercept between cavities and these components has to be often applied. This significantly complicates the overall cryogenic process scheme; see reference [3] for more details.…”

“…Typically, one or two cryomodules with few cavities are foreseen and also could be used as injectors. Figure 4 shows an example of 9-cell TESLAstyle cavity with two fundamental couplers (similar to TTF/Sacley 1 ones) and one HOM coupler [3].…”

“…a sub-cooled superfluid (sf) helium instead of boiling helium at saturated conditions. Due to increased RF power at cavities, mainly fundamental and High Order Mode (HOM) couplers must be also modified [1][2][3].…”

“…In the previous papers, modifications of TESLA-style as well as CEBAF/SNS cryomodules in order to apply a sub-cooled cooling scheme and, therefore, to operate cavities at larger cryogenic heat loads was presented [2,3]. Detailed discussion of main advantages of sub-cooling scheme, e.g.…”