Insight into reactive oxygen plasma characteristics and reaction mechanism on SRF accelerator plasma cleaning

Abstract

Reactive oxygen plasma treatment is an effective technique to eliminate hydrocarbon and improve the performance of Superconducting radio frequency (SRF) cavities. This work investigated the reaction mechanism between reactive oxygen plasma and hydrocarbon, surface reaction kinetics, and cleaning process optimization through experiments, analytical models, and numerical simulations. The experimental results declare that the reaction between oxygen plasma and hydrocarbon is dominated by ion-assisted chemical sputtering, and the hydrocarbon attenuates exponentially, increasing the work function exponentially. To study the surface reaction kinetic process in-depth, we proposed a plasma cleaning rate model based on the Langmuir-Hinshelwood theory. This study found that the plasma cleaning rate primarily depends on the sheath potential, electron temperature, O atoms density, O+ ions, and Ar+ ions densities. Furthermore, we did a control-parameter simulation and found that increasing gas pressures or O2 ratios are conducive to enhancing the chemical reaction rate between O atoms and hydrocarbon. Also, the power increase can enhance the physical effect of ions. It shows that increasing the gas pressure and power and reducing the oxygen content can achieve a better cleaning effect while reducing the radio frequency power loss caused by the oxide. Those results provide valuable guidance for optimizing the cleaning process, deepening the understanding of the cleaning mechanism, and improving the performance of SRF cavities.