Argonne National Laboratory is collaborating with MSU, HZDR, and SLAC on the design, fabrication, and testing of a prototype superconducting radiofrequency (SRF) gun for the LCLS-II-HE upgrade at SLAC. The gun cavity is a quarter-wave resonator with a frequency of 185.7 MHz. Despite careful calibration of the cavity field probe, there are still uncertainties in the RF measurements taken to determine quality factor and field level in the cavity. One way to independently check the RF measurements is to calculate the field level from the x-ray energy spectrum generated by field emission during testing. X-ray measurements were done with a sodium iodide detector. This paper presents results of x-ray energy spectrum measurements and compares it to the RF measurements of cavity field level at 18 MV/m and 21 MV/m. Numerical simulations are also presented to understand the acceleration and dynamics of field-emitted electrons.

Several groups have demonstrated that plasma processing can help to mitigate degradation of the performance of superconducting radio-frequency cavities. Plasma processing provides an 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. Plasma processing tests were done on several QWRs using the fundamental power coupler (FPC) to drive the plasma. Driving the plasma with a higher-order mode (HOM) shows promise, as it allows for less mismatch at the FPC. Before-and-after cold tests showed a significant reduction in field emission X-rays with judicious application of plasma processing. The first attempt at plasma processing of FRIB QWRs in a cryomodule is planned for December 2023/January 2024. A repeat bunker test of the cryomodule is planned to assess the results.