Plasma Processing of FRIB Low-Beta Cryomodules using Higher-Order-Modes

TL;DR

This work demonstrates the development and deployment of in-situ plasma processing to mitigate field emission in FRIB low-beta SRF cavities, leveraging higher-order mode (HOM) excitation to ignite and sustain plasma through the fundamental power coupler (FPC). A four-round processing protocol on a spare QWR cryomodule and an in-tunnel trial on a FRIB QWR cryomodule show cavity-by-cavity improvements in FE onset after processing, with some cavities requiring additional RF conditioning. The study also explores dual-drive HOM strategies to achieve more uniform plasma distributions, reducing ignition thresholds and enabling controlled plasma redistribution across cavity lobes. If these methods scale, in-situ plasma processing could shorten maintenance downtime, improve accelerating gradients, and extend cavity performance in FRIB’s linac.

Abstract

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, with before-and-after bunker tests and subsequent installation into the linac tunnel. The first in-tunnel plasma processing trial was completed in September 2025. For both cryomodules, before-and-after cold tests showed a significant increase in the average accelerating gradient for field emission onset after plasma processing for some cavities. In parallel with the cryomodule trials, the use of dual-drive plasma is being explored with the goal of improving the effectiveness of plasma processing.

Figures (6)

• Figure 1: Diagram of the gas supply and pumping system. The gas flows from Cavity 1 to Cavity 8. CGC: compressed gas cylinder; PRV: pressure regulation valve; SR: safety relief valve; MFC: mass flow controller; GF: gas filter; P1, P2, P3: pressure sensors; WDC: warm diagnostics chamber; GV: gate valve; MV: metering valve; LV: leak valve. TMP1, TMP2: turbo-molecular pumps; FP: fore-line pump; HF: HEPA filter; SIV: sentry isolation valve. RGA: residual gas analyzer. Magenta: valves set up to close if power is lost.
• Figure 2: (a) Pumping connection to the downstream warm chamber. (b) gas supply connection to the upstream warm chamber. (c) gas supply/pumping cart, RF cart, and cryomodule.
• Figure 3: Plasma processing time for each cavity, including the originally-planned processing rounds and bonus rounds after the pressure excursion.
• Figure 4: Field emission performance of the cryomodule (a) before the warm-up for plasma processing (July 2025); (b) after plasma processing (September 2025); (c) after additional CW/pulse conditioning on Cavity 7 and Cavity 4 (September 2025). Upper axis: $E_p$ = peak surface electric field. Dashed green line: design field.
• Figure 5: Reduction in the ignition threshold power for the $\sim 1$ GHz quadrupole mode with ignition-assist from a dipole mode at $\sim 690$ MHz.