1. 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
w/ 2T transverse B field
SEY > 1 for copper
High impact energy (heating?)
Impact energy too low for MP
w/o external B field
w/ 2T external axial B field
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
Impact energy of resonant particles vs. field level
w/ 2T B field at 10 degree
Between upper and lower irises
Between upper and lower cavity walls
Slight MP activities observed above 6 MV/m
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.