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Simulations for the LCLS Photo-Injector

This article discusses simulations and optimizations for the LCLS photo-injector, including nominal tuning, sensitivity studies, and benchmarking with Parmela. Various parameters such as laser beam characteristics, solenoid positions, and field strengths are explored. The results show that the LCLS requirements for emittance and pulse characteristics can be met with the present design.

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Simulations for the LCLS Photo-Injector

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  1. Simulations for the LCLS Photo-InjectorC.Limborg, SSRL / SLACApril 24, 2002 • LCLS Photo-injector Simulations • Nominal tuning • Sensitivity study • Benchmarking Parmela C.Limborg SSRL

  2. RF Gun : •  • E(MV) • Balance •  th • Goal: • projected < 1.2 mm.mrad • slice < 1.0 mm.mrad • for 80 slices out of 100 • at 150MeV, for 1nC, 10ps pulse, and “jitter” errors • Optimization in 19 D • parameter space • Laser Beam: • Longitudinal • (length, rise time, flatness) • Transverse • (r, uniformity) • Energy  charge • Linac Solenoid : • Position • Length • Field • Linacs : • Position • Field • Spacing • Gun Solenoid : • Position • Length • Field Matching Section Linac Center Line Sector 20 Linacs C.Limborg SSRL

  3. Optimization of LCLS photo-injector • Preliminary tuning with Homdyn then refined optimization with PARMELA-LANL • 1nC, 10ps flat top (0.7 ps rise time), transverse uniform, th = 0.3 mm.mrad • For all above cases, projected less than 1 mm.mrad • Good margin for errors • Very similar projectedvalues for 140MV/m gun C.Limborg SSRL

  4. Nicely converging beam • No emittance growth in Matching Section due to space charge when aspect ratio of beam is 10:1 ; checked with 3D Parmela-LANL computation Units as indicated with colors C.Limborg SSRL

  5. slice for 99 slices 97% part. i < 1 mm.mrad 95% part. i < 0.9 mm.mrad 71% part. i < 0.8 mm.mrad Much better than slice goal slice rad for 100 slices (ps)  (ps) C.Limborg SSRL

  6. Sensitivity study : Effect of individual parameters Balance 3% is ok Charge  of 10% ok Gun field  < 0.5MV/m Bsolenoid  < 0.4% ok Phase 3 ok rspot size 0.1 mm ok C.Limborg SSRL

  7. Sensitivity study: Combination of errors Individual parameters increase emittance from 0.92 mm.mrad to 1.0 mm.mrad proj = 2.51 mm.mrad for worst combination slice mm.mrad for 100 slices (ps) Run Number C.Limborg SSRL

  8. Sensitivity study - Combination of errors (“jitter” type) Variations included in simulations are larger than specifications  = 1.25 mm.mrad for worst combination slice mm.mrad for 100 slices ps C.Limborg SSRL

  9. Sensitivity: Various Thermal emittances slice (mm.mrad) slice (mm.mrad) C.Limborg SSRL

  10. Sensitivity: Rise time of pulse Distribution density (a.u) (mm.mrad) • Distributions are built from stack of Gaussians; rise time is rms of Gaussians • (th = 0.6 mm.mrad) (ps) C.Limborg SSRL

  11. Sensitivity: Transverse uniformity • Radial modulation of emission intensity on cathode spot • Rectangular grid (“checker”) to be studied with 3D • Reference deck includes longitudinal modulation, • r = 1mm; with uniform spot proj = 0.98 mm.mrad • proj increases by 10% when radial modulation is 40% peak-to-peak • Measurements on GTF cathode indicate less than 20% variation peak-to-peak in emission (See J.Schmerge “Experimental results”) • Laser Non-uniformity will be less than 20% (See P.Bolton “Laser”) Emission Spot on Cathode C.Limborg SSRL

  12. Benchmarking Parmela • GTF data are well reproduced by Parmela • Parameters • Cathode Field = 110MV/m • gun  40 • r = 1mm • Bsolenoid 2kG • Linac Gradient = 8.55 MV/m • Cathode-to-Linac Dist. = 90 cm • Gaussian Pulses PARMELA  Experiment  • Quad Scan with Parmela gives 20% smaller emittance with 5% truncation of tails (similar truncation used in experiment) C.Limborg SSRL

  13. Benchmarking Parmela • Parmela in good agreements with experiments • Parmela explains results for 2ps and 4ps pulses • Booster mismatch worse for the 4 ps than for the 2ps • With 5.5 MV/m instead of 8.5 M/m => emittances reduce to 1.3 mm.mrad and 1.0 mm.mrad respectively for the 2ps and 4 ps cases at 300pC • Work under way • Systematic quad scan (computing contribution from space charge- See also J.Schmerge “Experimental results”) • Longitudinal booster phase scan (Longitudinal measurements) • Slice experiments analysis(both SLAC and BNL) • PIC codes • Good agreement Magic2D , Maxwell-T, PARMELA-LANL, PARMELA-UCLA for test problem of first 40 ps leaving cathode • Full simulation of gun in collaboration with NLC team C.Limborg SSRL

  14. Conclusions • LCLS Requirements are met for a 1nC, 10 ps pulse • Goal of  projected < 1.2 mm.mrad, slice < 1.0 mm.mrad is met with present design • With thermal = 0.5 mm.mrad • With combined errors meeting tolerance specifications • For non-uniformity of transverse spot size up to 40% • For longitudinal pulse with rise time < 0.7 ps • Confidence in Parmela • From good agreement with experimental results • From comparison with PIC codes • Plans for year • Finalize sensitivity study • Finalize comparison with PIC codes • Perform more comparisons with experiments C.Limborg SSRL

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