Three-Dimensional Self-Consistent Simulations of Multipacting in Superconducting Radio Frequency Cavities

78764S Superconducting radio frequency (RF) accelerator cavities lose power to stray electrons, especially when the electron motion is in resonance with the fields and the electrons strike the cavity surface repeatedly (multipacting). One of the main tools for studying multipacting is numerical simulation, but none of the codes used presently has sufficiently realistic models of all the physical processes to model the problem fully. One main limitation of present modeling approaches is the lack of three-dimensionality. Related to this is the need for parallel computing to handle the increased computational needs of running in three dimensions. Currently, new designs for superconducting RF cavities are typically tested by building physical prototypes and examining their performance. This makes using a large variety of different geometries for different accelerating sections problematic due to the high cost of prototyping all the different designs. This project will add needed models and features to the VORPAL plasma simulation code so it can function as a virtual prototyping tool for understanding multipacting in superconducting RF cavities. In Phase I, a variable weight mesh will be added to VORPAL to accurately model the electromagnetic fields near surfaces of complex geometries. Next, niobium will be added to the available metals in the POSINST secondary emission routines, which VORPAL can access through the CMEE (Computational Modules for Electron Effects) library. With these added capabilities, proof-of-principle simulations of multipacting in the high beta SNS (Spallation Neutron Source) cavity will be run, and results will be compared to experimental data. Commercial Applications and Other Benefits as described by the awardee: There is a strong potential for the use of free-electron lasers in areas of defense and surface processing. Because superconding RF cavities are major component of free-electron lasers, the new design tool should provide low cost prototyping for these industries.