Self-consistent simulations of multipacting in Superconducting Radio Frequencies

Multipacting continues to be an important issue in Superconducting Radio Frequency (SRF) cavities, particularly near waveguide couplers. Most modem simulations of multipacting are not self-consistent, using the fields from a purely electromagnetic simulation to drive the motion of multipacting electrons. This approach works well for the onset on multipacting but as the electron density increases in the cavity it can have an effect on the cavity mode. Recently VORPAL has demonstrated its ability to mode the electrodynamics of SRF cavities using finite difference time domain (FDTD) algorithms coupled with cut-cell treatments of the cavity boundaries based on the Dey-Mittra method. The FDTD approach allows us to easily incorporate multipacting electrons as PIC particles in the simulations. To allow multipacting simulations to be done with EM-PIC we have been developing particle boundaries for the cut-cells. Recently we have added particle removal boundaries at the particle sinks which will correct the unphysical build up of image charge at the boundaries. We have also modified the secondary electron routines in VORPAL so they are now aware of the cut-cell boundaries and use the normal to the boundary surface when emitting secondaries. Using these new particle boundary conditions we have started preliminary simulations of multipacting electrons in a SRF cavity.