Feasibility study of plasma processing for the FRIB energy upgrade 5-cell superconducting cavity

A 5-cell elliptical superconducting radio-frequency (SRF) cavity (644 MHz, beta = 0.65) is being developed at Michigan State University for the proposed FRIB driver linac energy upgrade (FRIB400). The cavity is not equipped with higher-order mode (HOM) couplers and the fundamental power coupler (FPC) coupling strength is relatively weak (loaded bandwidth = 32 Hz) due to CW operation, so a key challenge is to perform effective cavity plasma processing while avoiding plasma ignition in the FPC region; FPC ignition could otherwise lead to copper sputtering onto the niobium cavity surfaces or damage to the FPC ceramic window. In this study, we examined the feasibility of plasma processing by driving HOMs through the FPC. With a combination of TE111 passband modes, we were able to sustain plasma in each of the 5 cells without FPC ignition. We will present measurements of the plasma density as a function of drive mode, input RF power, drive RF frequency shift, and gas pressure, and will discuss an optimized cleaning recipe for future FRIB400 cryomodules.