Plasma characterization and modulation techniques for 1.3 GHz, 9-cell superconducting rf cavity cleaning

Plasma cleaning is extensively employed in superconducting radio-frequency (SRF) accelerators to mitigate field emissions induced by hydrocarbon contaminants. This study explores the plasma physical characteristics and modulation techniques in a commonly utilized 1.3 GHz 9-cell tesla-shaped SRF cavity through the integration of optical emission spectroscopy and a plasma fluid model. Investigations of argon and neon plasma demonstrate that argon generates a higher electron density due to its lower ionization energy, while neon plasma exhibits higher electron temperature and potential that promotes the decomposition of oxygen and the rate of surface chemical reactions. Furthermore, the research reveals that the electron density, temperature, and electric potential of the neon plasma within the cavity exhibit a centrally symmetric distribution, with maximum values of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mn>1</a:mn><a:mo>×</a:mo><a:msup><a:mrow><a:mn>10</a:mn></a:mrow><a:mrow><a:mn>15</a:mn></a:mrow></a:msup><a:mtext> </a:mtext><a:mtext> </a:mtext><a:msup><a:mrow><a:mi mathvariant="normal">m</a:mi></a:mrow><a:mrow><a:mo>−</a:mo><a:mn>3</a:mn></a:mrow></a:msup></a:mrow></a:math>, 0.53 eV, and 24.5 V, respectively. Subsequently, the effects of power, gas pressure, and frequency on the physical properties of the plasma are systematically analyzed. Among these, the 640 nm spectral line was selected as a qualitative indicator of glow intensity and plasma state changes within the cavity due to its strongest peak intensity. Findings indicate that increasing power and gas pressure enhance electron density and 640 nm spectral line intensity while reducing electron temperature. Additionally, comparative analysis of plasma properties in cells 1-9 reveals that most cells exhibit higher electron temperature and electron density at frequency II. (Frequency I serves as the transfer mode for plasma transfer, while frequency II acts as the stable mode for plasma stabilization.) Consequently, plasma cleaning of the 1.3 GHz 9-cell cavity using neon gas at this frequency is anticipated to yield superior cleaning outcomes. These findings enhance our understanding and control of plasma behavior within the SRF cavity and provide valuable insights for optimizing plasma cleaning processes. Published by the American Physical Society 2024