The anomalous skin effect and copper cavity operation at cryogenic conditions

A quarter-wave coaxial cavity was extensively investigated, to learn more about an RF operation of copper components at cryogenic temperatures. A focus is put on frequencies with relevance in ion linac acceleration. The 340 MHz test cavity was built from bulk copper, with a top-lid from copper-plated stainless steel. The quality - factor measurements were performed in a liquid helium cryostat during the warming up phase of the cavity.The cavity was at first measured after just polishing the bulk copper surface. In the next step this surface was copper-plated. After annealing for one hour at 400 deg Celsius, cavity measurements were repeated. A geometric model based on a spherical Fermi - surface and using the equivalent skin-layer model allows to calculate the real part of the surface impedance, which is relevant for the RF power losses in the cavity walls. An exact solution for this conduction electron model in skin layers was derived. It is compared with measurements and with predictions from the traditional diffusion model as formulated by Reuter, Sondheimer and Chambers. An ion linear accelerator with copper cavities operated at cryogenic temperatures around 40 deg Celsius is discussed: An overall advantage in case of low repetition rate, short pulse linear accelerators like ion synchrotron injectors is expected. Simulations on heat conduction in pulsed operation give promising results.