Multipacting Simulation for the Muon Collider Cooling Cavities*

1. Impact energy of resonant particles vs. field level w/o external B field High impact energy (heating?) w/ 2T external axial B field SEY > 1 for copper Impact energy too low for MP SEY > 1 for copper • 2 types of resonant trajectories: • Between 2 walls – particles with high impact energies and thus no MP • Around iris – MP activities observed below 1 MV/m 2T Resonant trajectory w/ 2T B field at 10 degree w/ 2T transverse B field SEY > 1 for copper SEY > 1 for copper • 2 types of resonant trajectories: • Between upper and lower irises • Between upper and lower cavity walls • Slight MP activities observed above 6 MV/m 2T 2T Multipacting Simulation for the Muon Collider Cooling Cavities* L Ge, Z Li,C Ng, K Ko, SLAC R.B. Palmer, BNL D Li, LBNL The muon cooling cavity for the Muon Collider operates under strong external magnetic fields. It has been observed that the external magnetic field can enhance multipacting activities and dark current heating. As part of a broad effort to optimize the external magnetic field map and cavity shape for minimal dark current and multipacting, we used SLAC Track3P to analyze mutipacting issues. Track3P is a 3D parallel finite-element tracking code which has been successfully used to predict multipacting phenomena in many accelerator components such as the ILC ICHIRO cavity. Here we present the multipacting simulation results for the 200 MHz cavity and the 805 MHz magnetic insulated cavities. 805 MHz Magnetic Insulated Cavity Introduction • Magnetic insulation: design cavity surface to follow external magnetic field lines 1 • High gradient RF cavities required in muon cooling channel • Will be operated under strong magnetic field • Multipacting activities and dark current heating are important issues in cavity design • Multipacting simulations help identify processing barriers • Track3P is used to simulate multipacting and dark current phenomena in 200 MHz and 805 MHz cavities Simulation parameters • 3T maximum magnetic field • Field gradient scanned: 1MV/m - 50MV/m • Scan interval: 1 MV/m • Total number of processors: 512 • Total run time: 1 hour • Multipacting and Dark Current simulation tool: Track3P • 3D parallel high order finite element particle tracking code • Curved elements fitted to the curvature boundary • Trace particles in resonant modes, steady state or transient fields • Accommodate several emission models: thermal, field and secondary • Extensively benchmarked against measurements and theories Snapshots of particle distributions at 28 MV/m Initial particles emitted on all surfaces Non-resonant particles Multipacting region 200 MHz Pillbox Cavity Impact energy of resonant particles vs. field level A multipacting particle trajectory Effects of External Magnetic Field on Multipacting SEY > 1 for copper • Resonant trajectories present above 14 MV/m • Impact energy form 150 eV to 20 keV • Multipacting barriers determined using copper’s SEY curve • One point, one to three order MP at different locations and field levels • Multipacting occurs in beampine region • Impact energy: 535 eV • Particle hits cavity wall once in two periods Summary and Future Work • Track3P is a fast and efficient particle tracking tool to simulate multipacting and dark current. • Simulation of mulitpacting helps understand RF heating and breakdown in muon cavities. • Simulation has shown no multipacting activities inside the 805 MHz magnetic insulated cavity. However, potential multipacting trajectories were found in the beampipe region. • Further multipacting and dark current simulations will be carried out to improve cavity design and to optimize external magnetic field profile. • 2 types of resonant trajectories: • One-point impacts at upper wall • Two-point impacts at beampipe • MP activities observed above 1.6 MV/m *Work supported by US Department of Energy under contract DE-AC02-76SF00515.