Multipacting Simulation for the Muon Collider Cooling Cavities*

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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

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• 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 gt 1 for copper

• Resonant trajectories present above 14 MV/m
• Impact energy form 150 eV to 20 keV
• Multipacting barriers determined using coppers
  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

1. Track3P is a fast and efficient particle tracking
   tool to simulate multipacting and dark current.
2. Simulation of mulitpacting helps understand RF
   heating and breakdown in muon cavities.
3. Simulation has shown no multipacting activities
   inside the 805 MHz magnetic insulated cavity.
   However, potential multipacting trajectories were
   found in the beampipe region.
4. 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.