Optical Investigation of a Low-Pressure Plasma in a Superconducting Radio-Frequency Cavity

The state-of-the-art cryomodules for particle accelerators use superconducting radio-frequency (SRF) cavities. It is known that accumulation of hydrocarbon impurities reduces the cavity surface work function, decreasing cryomodule performance metrics such as field emission, thermal stability, and accelerating gradients. An in-situ method of plasma processing has recently been developed to clean the cavities and restore cryomodule performance [1]. This study is to understand the properties of the low-pressure plasmas generated in a C100 SRF cavity and ultimately to improve the performance of plasma surface cleaning using optical emission spectroscopy. Using a 500 mm focal length Czerny-Turner monochromator coupled with a photomultiplier tube, emission spectra including the Balmer series hydrogen emission line Hα of Ar or Ar + 2% H2 plasma at a range of 50–150 mTorr were recorded. The effects of gas composition, gas pressure, mode frequency, and input power on the optical emission from the plasma were investigated.