Rf losses in a high gradient cryogenic copper cavity

Abstract

The development of high brightness electron sources can enable an increase in performance and reduction in size of extreme X-ray sources such as free electron lasers (FELs). A promising path to high brightness is through larger electric fields in radio-frequency (rf) photoinjectors. Recent experiments with 11.4 GHz copper accelerating cavities at cryogenic temperatures have demonstrated 500 MV/m surface electric fields with low rf breakdown rates. However, when the surface electric fields are larger than 300 MV/m, the measured cavity quality factor, Q0, decreases during the input rf pulse by up to 30%, recovering before the next rf pulse. In this paper, we present an experimental study of the rf losses, manifested as degradation of Q0, in a copper cavity operated at cryogenic temperatures and high gradients. The experimental conditions range from temperatures of 10-77 K and rf pulse lengths of 100-800 ns, using surface electric fields up to 400 MV/m. We developed a model for the change in Q0 using measured field emission currents and rf signals. We find that the Q0 degradation is consistent with the rf power being absorbed by strong field emission currents accelerated inside the cavity.