1. Multipacting Simulations of TTF-III Coupler Components*#
L. Ge, C. Adolphsen, K. Ko, L. Lee, Z. Li, C. Ng, G. Schussman, F. Wang, SLAC, B. Rusnak, LLNL
Abstract: The TTF-III coupler adopted for the ILC baseline cavity design has shown a tendency to have long initial high power processing times. A possible cause for the long processing times is believed to be multipacting in various regions of the coupler. To understand performance limitations during high power processing, SLAC has built a flexible high-power coupler test stand. The plan is to test individual sections of the coupler, which includes the cold and warm coaxes, the cold and warm bellows, and the cold window, using the test stand to identify problematic regions. To provide insights for the high power test, detailed numerical simulations of multipacting for these sections will be performed using the 3D multipacting code Track3P.
Multipacting Simulations of TTF-III Components
TTF-III Coupler
Cold Coax and Bellows
Ceramic Window Region
RF In
RF Out
No multipacting activities in the bellows region
Electric
Field
Particle distribution at 2nd period
Particle distribution at 100th period
Multipacting map in cold coax w/ and w/o reflection
DESY TTF-III coupler experienced long processing time
Test stand built at SLAC to evaluate processing limitations
Multipacting simulations help identify processing barriers
A multipacting particle trajectory between ceramic window and taper region of inner conductor on the cold side
Impact energy and MP order versus input power for two-point multipacting particles between ceramic window and cold inner conductor
Coupler component test stand layout
Matching Taper
Delta as a function of RF input power and z axis location
Delta as a function of RF input power and MP order
During charging of SW cavity, reflection varies from 0% to 100 % . Partial SW field in coax shifts MP to lower order and power level.
Multipacting Simulation – Track3P
3D parallel high-order finite-element particle tracking code for dark current and multipacting simulations (developed under SciDAC)
Track3P
traces particles in resonant modes, steady state or transient fields
accommodates several emission models: thermal, field and secondary
MP simulation procedure
Launch electrons on specified surfaces with different RF phase, energy and emission angle
Record impact position, energy and RF phase; generate secondary electrons based on SEY according to impact energy
Determine “resonant” trajectories by consecutive impact phase and position
Calculate MP order (#RF cycles/impact) and MP type (#impacts /MP cycle)
Track3P benchmarked extensively
Rise time effects on dark current for an X-band 30-cell structure
Prediction of MP barriers in the KEK ICHIRO cavity
MP particle trajectory
Reflection: 0.5
Input power level: 160KW
Order: 5th order
Impact energy: eV
No Multipacting activities between coax pipes
High impedance coax supports MP at higher power level
Comparison of simulations and measurements
Track3P simulation
After high power processing uA mV
Electron pickup
Coax is the first component for high power processing
After initial processing, the vacuum and electron pickup signal show multipacting bands in agreement with simulations
Simulated power (kW)
170~190
230~270
350~390
510~590
830~1000
Power in Coupler (kW)
43~170
280~340
340~490
530~660
850~1020
klystron power (kW)
50~200
330~400
400~580
620~780
1000~1200
Secondary emission yield (R. Kirby)
* This work was supported by DOE contract No. DE-AC02-76SF00515.
# This work was performed under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.