使用高阶模进行FRIB低β低温模块的等离子体处理
Plasma Processing of FRIB Low-Beta Cryomodules using Higher-Order-Modes
AI总结：
本文研究了FRIB低β谐振腔的等离子体处理技术，提出利用高阶模驱动等离子体以克服基频功率耦合器失配问题，实验证明该方法显著提升了部分谐振腔的场致发射起始加速梯度。
摘要：
在SNS和CEBAF中，隧道内等离子体处理后SRF加速器性能得到提升。针对FRIB四分之一波和半波谐振腔（QWRs, HWRs）的等离子体处理开发于2020年启动。在单个QWRs（beta = 0.085）和HWRs（beta = 0.53）上进行等离子体处理已被证明能显著降低场致发射。FRIB谐振腔面临的挑战是基频功率耦合器（FPC）的耦合强度相对较弱（这是为实现高效的连续波加速而选择的），这在室温等离子体处理过程中会产生大量失配。对于FRIB QWRs，使用高阶模（HOMs）驱动等离子体有助于减少FPC失配并提高等离子体密度。2024年1月在一个备用FRIB QWR低温模块上进行了首次等离子体处理试验，包括处理前后的掩体测试以及随后安装至直线加速器隧道中。首次隧道内等离子体处理试验于2025年9月完成。对于这两个低温模块，处理前后的冷测试表明，经过等离子体处理后，部分谐振腔的场致发射起始平均加速梯度显著增加。与低温模块试验同步，正在探索双驱动等离子体的使用，旨在提高等离子体处理的有效性。
英文摘要：
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.
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