Multiyear Longitudinal Analysis of Vacuum Breakdown and RF Conditioning in the C-Band Linac of SwissFEL

SwissFEL is an X-ray free-electron laser (XFEL) that has been in regular user operation since December 2017. The GeV-scale electron beam is driven by a 420-m-long linear accelerator (linac) consisting of 27 radio frequency (RF) modules. Each RF module has four C-band accelerating structures that operate at an average accelerating gradient of 30 MV/m. These accelerating structures were realized through a series production using ultrahigh-precision machining and brazing that saw the structures fabricated on-tune with a production yield rate of over 99%. After installing each of the accelerating structures into its respective RF module, the high-power conditioning began, taking between 300 and 600 million RF pulses to achieve the nominal accelerating voltage of 240 MV per RF module. During the user operation, the breakdown rate inside the RF modules began to fall as a result of RF conditioning incidental to the nominal high-power operation. This reduction in breakdown rate followed a power-law trend that saw it reducing by over three orders of magnitude during the first three years of user operation, from $10^{-6}$ to less than $10^{-9}$ breakdown per pulse per meter. A postprocessing analysis of the breakdowns’ locations found that they cluster around the input RF coupler and central regular cells of the accelerating structures. Given that this analysis consists of 100 accelerating structures, each with the same RF design and method of fabrication, this analysis represents one of the largest datasets of RF conditioning and vacuum breakdowns ever produced for a single accelerating structure design.