Speaker
Description
Improvement in SRF accelerator performance after in-tunnel plasma processing has been seen at SNS and CEBAF. Plasma processing development for FRIB quarter-wave and half-wave resonators (QWRs, HWRs) was initiated in 2020. Plasma processing on individual QWRs (beta = 0.085) and HWRs (beta = 0.53) has been found to significantly reduce field emission. A challenge for the FRIB cavities is the relatively weak fundamental power coupler (FPC) coupling strength (chosen for efficient continuous-wave acceleration), which produces a lot of mismatch during plasma processing at room temperature. For FRIB QWRs, driving the plasma with higher-order modes (HOMs) is beneficial to reduce the FPC mismatch and increase the plasma density. The first plasma processing trial on a spare FRIB QWR cryomodule was done in January 2024; before-and-after bunker tests showed a significant increase in the average accelerating gradient for field emission onset after plasma processing. The plasma-processed cryomodule was installed into the FRIB linac in August 2024; the in-situ performance was similar to that of the post-plasma-processing bunker test. Additional development work is ongoing, with the goal of a first in-tunnel plasma processing trial during the summer 2025 linac maintenance period.
Funding Agency
This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of High Energy Physics, under Award Number DE-SC0023633 and DE-SC0018362.
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