Modeling Breakdown and Electron Orbits in High‐Gradient Accelerating Cavities

Next‐generation rf accelerating cavities will employ very high‐gradient electric fields, greater than 100 MV/m, as well as strong magnetic fields. However, breakdown of accelerating structures due to high field gradients is a major limitation on these accelerating cavities. One possible mechanism for breakdown initiation is the rapid buildup of electrons due to field emission coupled with secondary electron emission. Multipacting may enhance this effect. In order to understand the physical processes of breakdown initiation and the effectiveness of potential mitigation techniques, researchers in the Muon Accelerator Program are experimenting with a simplified cavity, referred to as the Box Cavity, in which they will measure breakdown under high‐gradient rf with strong externally applied magnetic fields with different orientations. We present here simulation results for the box cavity including the effects of rf (805 MHz), magnetic fields, field‐dependent emission, secondary electron emission, and space charge, using the 3‐Dimensional plasma simulation code VORPAL. We measure the effect of different magnetic field strengths and orientations on electron orbits and buildup. We also simulate the effects of field emission in different parts of the box cavity, such as from corners of the box near the rf coupler, and measure the effects on electron buildup over many rf periods. These detailed and self‐consistent models will aid experimentalists to understand breakdown onset in high‐gradient metallic accelerating cavities.