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: 542-544 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.