Modeling Plasma Discharge Cleaning of SRF Cavities

Superconducting radio frequency (SRF) cavities, such as the continuous electron beam accelerator facility (CEBAF) 5-cell cavity, rely on high field generation, but surface contamination of the superconducting vessel walls due to initial chemical treatment and regular operation can severely impact the field strength through field emission and surface arcing. Therefore, efficient and effective cleaning of the cavity surfaces is necessary to maintain optimal acceleration gradients. In-situ cleaning techniques involve generating plasma discharges that remove the impurities through mostly chemical and some physical processes, and simulation of this plasma generation is necessary to predict and understand the cleaning process. Hybrid kinetic/fluid simulations including surface and bulk reactions allow for the modeling of the plasma generation, removal of impurities from surfaces, and subsequent impurity transport out of the device. The plasma parameters within the device also provide insight into the potential damaging effects of the plasma on the RF window and other sensitive hardware. In phase I, we showed that VSim can successfully and accurately simulate the electromagnetic mode gen- eration within SRF cavities. We also demonstrated that the plasma generation via ionization cascade can be modeled including impact ionization, elastic electron scattering, and recombination processes vital to the plasma behavior. In phase II, we plan to improve the reaction framework further by including surface interactions and reaction diagnostics. A fluid solver will be introduced along with timescale compression to allow for the simulation of long time scales (ie. milliseconds) so that impurity transport can be observed. These new software features will be integrated into the VSim GUI and documentation. Finally, simulations of interest and relevance to the JLAB experimental plasma cleaning project will be performed including investigation of power requirements, plasma excitation via HOM couplers, and the effectiveness of the cleaning process in removing impurities from the device. Commercial applications and other benefits: The modeling capability that will be implemented is relevant to many problems, including low-temperature plasma treatment of materials ranging from plastics to biological tissue. Plasma surface interactions are relevant to semiconductor etching for microprocessor production.