Development of plasma processing for superconducting half-wave resonators

Several groups have demonstrated that plasma processing can help to mitigate degradation of the performance of superconducting radio-frequency cavities, making plasma processing a promising alternative to removal of cryomodules from the accelerator for refurbishment. Studies of plasma processing for quarter-wave resonators (QWRs) and half-wave resonators (HWRs) are underway at FRIB, where a total of 324 such resonators are presently in operation. Challenges for this effort include the high FPC mismatch at room temperature, the limited view of the cavity interior, and the absence of higher-order mode (HOM) couplers. A plasma processing trial on a QWR cryomodule, driving the plasma with HOMs via the fundamental power coupler (FPC) was done in January 2024. In parallel with QWR development, plasma processing tests are being done on HWRs, so far driving the plasma with the fundamental mode via a custom antenna or the FPC. Before-and-after cold tests suggest that plasma processing has good potential for reduction of field emission in HWRs.