RF heating in cavity-bellows of CW SRF cryomodule

Abstract

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 cooled down through a relatively slow process of conduction cooling by adjacent superconducting cavities. 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 superconducting cavities, resulting thermal runaway could disrupt performance of the cavities and therefore, overall operation. For this reason, the paper presents a comprehensive assessment of RF heating resulting from most potent dynamic heating mechanisms in the interconnecting cavity bellows of 1.3 GHz, continuous wave, cryomodule of the Linac Coherent Light Source-II linear accelerator. Studies are performed for two types of the bellows surfaces i.e. stainless-steel and copper. The results show that RF heating at stainless-steel surface is about four times higher than at copper surface. In this work we also develop a simplified analytical model to analyse the bellows surface temperature growth resulting from RF heating. The model shows a good agreement with numerical analysis and indicates a temperature growth of up to 70 K in stainless-steel bellows in the steady state. Calculations also show that a thin copper coating of 15 μm on stainless-steel surface not only reduces RF heating but also confines the temperature growth well within 10 K. The studies presented here attribute practical importance guiding to characterize copper coating requirements for non-superconducting components in a cryomodule and allowing to model a more relevant cryogenic heat load budget accounting for bellows heating.