RF heating in cavity-bellows of CW SRF cryomodule

Abstract

In a cryomodule, bellows are commonly used to connect two adjacent superconducting cavities. It provides cavities a mechanical flexibility facilitating for thermal contraction or expansion during temperature transitions. While heat from superconducting cavities are removed from helium flowing through their jackets, non-superconducting bellows are cool-down through an indirect process of conduction cooling. An excessive heat deposition to the bellows could results in unsustainable rise in temperature that could lead to a thermal runaway. Because of the bellows proximity to the cavities, resulting thermal runaway could disrupt performance of the cavities and therefore, overall operation. Consequently, a careful evaluation of RF heating in bellows needs to be performed especially for a Continuous Wave (CW) and high intensity beam operation where beam induced, and operating fields heating could be substantial. For this reason, the paper presents a comprehensive analysis of RF heating mechanisms in bellows of 1.3 GHz cryomodule of the Linac Coherent Light Source-II (LCLS-II) linear accelerator (linac). The paper details RF heating estimations for two type of bellows i.e. stainlesssteel and copper bellows and describes simplified analytical approach developed to evaluate temperature distribution across the bellows. Since copper does not exhibit desired material property for the bellows at cryogenic temperature, inner copper coated stainless-steel bellows is an ideal practical alternative. The paper also converses implications of coating on conduction resistance of the multilayer system that enables to determine the coating thickness.