Report on first plasma processing trial for a FRIB quarter-wave resonator cryomodule

TL;DR

This work addresses field-emission-driven performance degradation in FRIB QWR cryomodules by developing and validating an in-situ plasma processing approach. Using two higher-order modes to ignite and densify plasma in a spare FRIB QWR cryomodule, the study demonstrates significant reductions in field-emission X-rays and faster conditioning of a high multipacting barrier after processing. The findings indicate that in-tunnel plasma processing is a viable path to reduce refurbishment downtime and labor for FRIB, with the TEM 5λ/4 mode effective for FE mitigation and the dipole mode aiding multipacting conditioning. The work lays the groundwork for future tunnel-based trials and broader HOM exploration to enhance overall SRF performance during long-term operation.

Abstract

Plasma processing has been shown to help mitigate degradation of the performance of superconducting radio-frequency cavities, providing 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 the Facility for Rare Isotope Beams (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, with promising results. Driving the plasma with a higher-order mode allows for less mismatch at the FPC and higher plasma density. The first plasma processing trial for FRIB QWRs in a cryomodule was conducted in January 2024. Cold tests of the cryomodule showed a significant reduction in field emission X-rays after plasma processing.

Figures (18)

• Figure 1: Measured field emission X-rays at $E_a = 10$ MV/m in cold tests of FRIB $\beta_m = 0.086$ QWRs before and after "on-the-bench" plasma processing with an FPC set for maximum coupling. The dashed line indicates the X-ray sensor background level.
• Figure 2: Measured FPC coupling factor ($\beta_1 = Q_0/Q_{\mathrm{ext,1}}$) as a function of frequency for some of the modes in a FRIB $\beta_m = 0.086$ QWR, with the FPC set for maximum coupling (red squares) or nominal coupling (black diamonds).
• Figure 3: Sectional views of the FRIB $\beta_m = 0.086$ QWR with intensity maps (blue = low, red = high) of the electric field magnitude for (a) the 404 MHz TEM mode and (b) the 605 MHz dipole mode. The side views (left) show the beam ports and drift tube. The front ("beam's eye") views (right) show the RF ports for the input/FPC and pickup probe couplers below the beam line.
• Figure 4: Schematic of the gas supply and pumping system for cryomodule plasma processing. CGC: compressed gas cylinder; FP: fore-pump; GF: gas filter; GV: gate valve; HF: HEPA filter; MFC: mass flow controller; LV: leak valve; MV: metering valve; P: pressure sensor; PRV: pressure regulation valve; RGA: residual gas analyzer; SR: safety relief valve; TMP: turbo-molecular pump; VP: viewport. Cyan: signals recorded by the data acquisition system. Gray: components turned off for plasma processing.
• Figure 5: Schematic of the RF system for cryomodule plasma processing. A: attenuator(s); C: circulator with load; D: directional coupler; DD: dual-directional coupler; G: RF signal generator; HLA: high-level amplifier; LNA: low-noise signal amplifier; pA: picoammeter; S: power sensor; SSA: spectrum analyzer; SW: RF switch; T: bias T; VNA: network analyzer. Signal paths are shown in black (RF) and green (dc). Cyan: signals recorded by the data acquisition system. Magenta boxes: components used for the software interlock.