Modeling Plasma Discharge Cleaning of SRF Cavities

Superconducting radio frequency (SRF) cavities, such as the continuous electron beam accelerator facility (CEBAF) 5-cell cavity, are designed with the goal of achieving large accelerating gradients- At present, the acceleration gradients are limited by field emission and surface arcing, which can be traced back to surface contamination- It has been proposed that this contamination can be removed through discharge cleaning – the creation of in situ plasma discharges that through both chemical and physical processes can remove the contaminants- In one approach, the plasma is created by gas inflow from one end with electromagnetic energy in through the coupler- Such an approach can be applied without disassembly- However, the internal plasma is difficult to diagnose, given the limited visual access to the cavity interior; hence there is a desire to compute the generated plasma to allow the researchers to know what is going on- As well, there is a desire to explore different setups in terms of coupling and gas inflow- Creating different setups is expensive, and so prior to doing so, it is desired to have modeling data to inexpensively predict what will happen- Plasma discharges in SRF cavity geometry will be simulated to compute the distribution function for each plasma species- This will lead to a better understanding of how RF power characteristics and gas flow affect the plasma and so allow optimization of the cleaning procedure-