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Compensation of Transient Beam-Loading in CLIC Main Linac

Compensation of Transient Beam-Loading in CLIC Main Linac. Oleksiy Kononenko, Alexej Grudiev IWLC, October 20, 2010. Contents. Motivation Calculation of unloaded/loaded voltages Optimization of the pulse shape Spread minimization for BNS damping and transient in the subharmonic buncher

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Compensation of Transient Beam-Loading in CLIC Main Linac

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  1. Compensation of Transient Beam-Loading in CLIC Main Linac Oleksiy Kononenko, Alexej Grudiev IWLC, October 20, 2010

  2. Contents • Motivation • Calculation of unloaded/loaded voltages • Optimization of the pulse shape • Spread minimization for BNS damping and transient in the subharmonicbuncher • Effects of the charge jitters in drive and main beams • Conclusions

  3. Motivation: CLIC Performance Issue In order to have luminosity loss less than 1%, the RMS bunch-to-bunch relative energy spread must be below 0.03% *CLIC-Note-764, private conversations with Daniel Schulte (CERN)

  4. Beam Loading: Steady State *Beam loading for arbitrary traveling wave accelerating structure. A. Lunin, V. Yakovlev

  5. Energy Spread Minimization Scheme • Unloaded Voltage in AS • fix phase switch times in buncher • generate corresponding drive beam profile • take into account PETS (+PETS on/off) bunch response • calculate unloaded voltage Klystron (reference) pulse • Loaded Voltage in AS • calculate AS bunch response • calculate total beam loading voltage • - add to unloaded voltage Energy Spread Minimization varying buncher delays

  6. Electric Field Distribution for Port and Plane Wave Excitations Considering T24 CLIC main accelerator structure

  7. Accelerating Voltage for the Port excitation and Beam Impedance Eportz (z,f) → [ exp ( ± i *z *ω/c ) ] → [ ∫ dz ] → VU (f) Epwz(z,f) → [ exp ( ± i *z *ω/c ) ] → [ ∫ dz ] → V(f) → [IHFSS = 2*π*r * E0 / Z0] → Z(f)

  8. Envelopes of the Time Response for the Port Excitation and Wake Potential

  9. CLIC Drive Beam Generation Complex *CLIC-Note-764

  10. Drive Beam Combination Steps fbeam= 4 * 3 * 2* fbuncher 12 GHz 3 GHz 1 GHz 0.5 GHz

  11. PETS: Single Bunch Response *kindly provided by Alessandro Cappelletti, Igor Syratchev (CERN)

  12. PETS: Generated Rectangular Pulse No delays, just nominal (~240ns) switch times in buncher trise ≈ 1.5 ns

  13. Rectangular Pulse in Main Linac Optimizing injection time one can optimize the energy spread down to the level of 6% only

  14. Schematic Pulse Shape for CLIC

  15. Optimization Algorithm • Brief Description: • 1. Fix injection time • 2. Generate delays • 3. Find the minimal energy spread and optimal delays • 4. Repeat 2. starting from the optimal delays

  16. Energy Spread Calculation Vbeam(t) Vunloaded r(t) pCLIC(t)

  17. Energy Spread Optimization Utility

  18. Optimized Pulse Shape Corresponding switch delays in buncher

  19. Optimized Energy Spread along the Main Beam RMS bunch-to-bunch relative energy spread is around 0.03%

  20. Model Improvements • For BNS damping it is necessary to inject bunches a bit (10 - 30 deg) off-crest • Take into account transient in the subharmonicbuncher during DB phase switch

  21. Energy Spread Dependence on the Injection Phase CLIC constraint

  22. Optimal Switch Delays for the Different Injection Phases

  23. Transient in the SubharmonicBuncher During DB Phase Switch

  24. Energy Spread Dependence on the Buncher Switch Time Currently in CTF3

  25. Optimal Switch Delays for the Different Buncher Switch Times

  26. Study of the Charge JitterInfluence on the Energy Spread • Gaussian drive/main beams charge distribution with relative rms spread of 0.1% • “White noise” jitter of the charge along the drive/main beams

  27. Drive Beam Charge Spread Effect CLIC constraint Constraint of 0.1% charge spread in drive beam(D. Schulte, CERN)is ok for the energy spread minimization

  28. Main Beam Charge Spread Effect CLIC constraint Constraint of 0.1% charge spread in main beam (D. Schulte, CERN)is ok for the energy spread minimization

  29. Conclusions • Developed pulse shape optimization method allows to reach acceptable level of 0.03% in the main beam energy spread • Performing optimization for the different possible buncher switching times and injection phases the same CLIC acceptable level of energy spread is reached 3. Randomly distributed along the bunch train 0.1% rms spread charge jitters in drive and/or main beams don’t increase the final energy spread in the main beam

  30. Thank You for the Attention!

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