Various cavity surface treatments have been found to significantly improve cavity quality factor, Q0, with one such treatment being nitrogen-doping (N-doping). N-doped 1.3 GHz cavities were the first found to exhibit anti-Q slope, the increase of Q0 with accelerating field, Eacc. However, even with the use of the same N-doping recipes, this anti-Q slope behavior has not been realized in sub-GHz frequency cavities. In this study, we measured the Q0 and surface resistance, RS, of our N-doped, single-cell, elliptical cavity for both the 644 MHz fundamental mode (FM) and 1.45 GHz higher-order mode (HOM). As a result of multi-mode measurements, the BCS and residual resistances, the temperature-dependent and temperature-in-dependent RF surface resistances, could be determined without the influence of cavity-to-cavity surface treatment variations. We will discuss the frequency-dependent behaviors of the BCS and residual resistances.

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.

Various cavity surface treatments have been found to significantly improve cavity quality factor, Q0, with one such treatment being nitrogen-doping (N-doping). N-doped 1.3 GHz cavities were the first found to exhibit anti-Q slope, the increase of Q0 with accelerating field, Eacc. However, even with the use of the same N-doping recipes, this anti-Q slope behavior has not been realized in sub-GHz frequency cavities. In this study, we measured the Q0 and surface resistance, RS, of our N-doped, single-cell, elliptical cavity for both the 644 MHz fundamental mode (FM) and 1.45 GHz higher-order mode (HOM). As a result of multi-mode measurements, the BCS and residual resistances, the temperature-dependent and temperature-in-dependent RF surface resistances, could be determined without the influence of cavity-to-cavity surface treatment variations. We will discuss the frequency-dependent behaviors of the BCS and residual resistances.