A world record accelerating gradient in a niobium superconducting accelerator cavity

A two-cell, 3 GHz, niobium superconducting accelerator cavity has sustained a continuous wave (CW) accelerating gradient of 34.6 MV/m, with a corresponding peak surface electric field of 100 MV/m, record performances in each category for a superconducting accelerator cavity. Field emission (FE) loading of the cavity initially limited the cavity to E_acc=21 MV/m (E_peak=60 MV/m). The record field was achieved by reducing the FE loading through high peak power (HPP) RF processing of the cavity. Analysis of previous results of the HPP experimental program indicated that maximum fields under both pulsed and CW conditions were limited by thermal breakdown, which is related to the surface magnetic field in the cavity. The two-cell cavity shape was chosen to bypass the thermal breakdown limitations by reducing the ratio of peak surface magnetic field to peak surface electric field, from a value of H_peak/E_peak=23 Oe/(MV/m) in the previous cavity, to 14 Oe/(MV/m) in the two-cell cavity. A simple thermal model accurately simulates the pulsed breakdown