RF PULSE CONDITIONING TO REDUCE FIELD EMISSION IN FRIB SRF CRYOMODULES

The superconducting radio-frequency (SRF) linear accelerator (linac) for the Facility for Rare Isotope Beams (FRIB) has been operating for user experiments since May 2022, using 104 quarter-wave resonators (80.5 MHz) and 220 half-wave resonators (322 MHz) operating with peak surface electric fields (E_peak) ranging from 27 MV/m to 33 MV/m. Field emission (FE) can limit the maximum accelerating gradients in SRF cavities and may worsen over time during long-term operation. RF pulse conditioning has been a useful technique to reduce FE in FRIB cryomodules for commissioning and user operations. In this paper, we will present RF pulse conditioning results and analysis for about 50 cavities, performed in cryomodule bunker tests and the linac tunnel. During conditioning, we observe”electrical breakdown”, a rapid (<1 µs) collapse of the field, which usually leads to a significant reduction in FE X-rays. On the other hand, when a thermal breakdown (∼ms decay time, also known as quench) occurs, opportunities for further conditioning at higher field are limited and FE X-rays may not be significantly improved.