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Working Group 3 Summary

Working Group 3 Summary. Padamsee, Ohmi and Calaga, S. Peggs. Design crab cavity for these beam parameters. Layouts Discussed. Free space = 30 - 60 m. Lateral Space ~ 45 cm ??. Small crossing angle (1 mr) does not work, need more space.

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Working Group 3 Summary

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  1. Working Group 3 Summary Padamsee, Ohmi and Calaga, S. Peggs

  2. Design crab cavity for these beam parameters

  3. Layouts Discussed

  4. Free space = 30 - 60 m Lateral Space ~ 45 cm ?? Small crossing angle (1 mr) does not work, need more space

  5. Gupta:Quad Pairs for (not so) Large Crossing Angle, 4mrad, why not 8 mrad? Consider the two counter-rotating beams with the first going through a quad. How close the second beam can be? It is 200 mm for the geometry on the right Displaced quads with the first beam in the quad and counter rotating beam just outside the coil in a field free region. Minimum X-ing angle is determined by how close the other beamline can come 50 m free space !, 45 cm lateral Might work for 800 MHz? Later

  6. Most Advanced KEK - Three Damping Strategies • Coaxial damper is very effective is damping TM010 mode (f = 413 Mhz, MHz, Q < 70), TE111 mode (f = 650, 677 Mhz, Q < 20 • Squashed cavity very effective in raising frequency of unwanted TM110 from 500 MHz to 700 MHz I.e. above the cut-off frequency of large beam pipe. • Large beam pipe removes all modes f > 1 GHz • Filling factor is low • < 0.1 m • We need to come up with different concept with larger filling factor • Multi-cells?? • Each HOM becomes n HOMs • Trapped modes

  7. Cavity Radial Size (43 cm)!!

  8. KEK-B Cryomodule Size 1.5 m

  9. Argonne ConceptSingle Cells, More Cells in Cryomodule. Improve Filling Factor Input coupler Rejection filter Qext - 1100 Power extraction from coax Use 2-cells

  10. LBNL Waveguide Damping Concepts Q ≈ 2000

  11. LBNL ConceptsMulti-cells, Waveguide Dampers Waveguides to damp LOM, HOM and unwanted dipole mode • Q ≈ 1500, not < 100 • One Monopole mode (0-mode) is trapped due to cavity symmetry • Difficult to be damped either by coaxial insert or waveguides in 3-cell • Consider asymmetry and larger beam pipes

  12. 2-Cell super-structure with damping • Two 2-cell cavity with waveguide in between beam pipe to damp unwanted dipole mode • Damping TE11 mode in beam pipe • Effective in damping unwanted dipole mode • The waveguide does not couple strongly with the LOMs • Waveguide near beam iris to damp unwanted dipole mode (TM) directly • Strong damping on unwanted dipole mode • Modest damping to LOM, 0 mode Best Q’s are still ≈ 1000

  13. Reduced iris to maximize R/Q of mode Open beam pipe to increase damping to Q ≈ 100

  14. Tolerances and Other Issues

  15. Sychro-Betatron Coupling Consequences • Crab cavity causes an increase synchroton tune (for Qx/y < 0.5). Preset tunes 0.31 is OK. • Instabilities predicted above 1/2 integer betatron tunes. Dispersion makes it worse, may still be OK (Boaz). • With pair of crab cavities effect is reduced.

  16. Tolerances at 1 mr and 400 MHz

  17. Compare to ILC

  18. Field and Phase Stability RequirementsFor Near-Future Projects Different accelerators have different requirements for field stability! • approximate RMS requirements: • 1% for amplitude and 1 deg for phase (storage rings, SNS, JPARK) • 0.1% for amplitude and 0.1 deg for phase (linear collider, LCLS) • down to 0.01% for amplitude and 0.01 deg for phase (XFEL, ERL light sources) From Matthias Liepe, Cornell, PAC 05

  19. Example: Digital I/Q Control Vector Modulator Klystron Master Oscillator  90° X  X 0° Icontrol Cavity Qcontrol Imeas, Qmeas DSP/ FPGA DAC ADC DAC IF RF LO X Down Converter

  20. LLRF for J-PARC (KEK) • LLRF system for pulsed n.c. proton linac of J-PARC • FPGA based I / Q control • Field stability exceeds specs (±1% in amplitude and ±1 degree in phase ) • Base for STF LLRF system Mixer&I/Q DSP/FPGA ± 0.08% ± 0.04 deg S. Michizono et al. Shown for absolute phase

  21. Cornell LLRF System for ERL operation at QL = 1.2108 Very good field stability demonstrated with 5 mA beam: A/A  1·10 - 4 12.4 12.3 accelerating field [MV/m] 12.2 12.1 0 0.2 0.4 0.6 0.8 1 time [sec] 11   0.02 deg 10.5 10 phase [deg] 9.5 9 0 0.2 0.4 0.6 0.8 1 time [sec] How much better can we do for relative cav-cav relative phase ??

  22. Upcoming Test

  23. Final Recommendations • Size calibration: Crab mode at 400 MHz means fundamental TM010 mode is at 259 MHz…ouch ! • Take a very hard look at 800 MHz, is emittance growth due to non-linearity of RF acceptable? • Use R12 = 45 m => V = 37 MV for 8 mrad • Use advanced gradient = 10 MV/m • Active length = 3.7 m x 4 (regions) • Filling factor = 0.3 => 12 m • Phase tolerance is x2 more relaxed.

  24. Can This Work (Maybe 2 Cavities if necessary) Forward beam RF Cavity TM010 Dampers Dampers Return Beam

  25. Advantages • TM010 is the Lowest Order Mode in the Cavity • No Degenerate Mode to Worry about • Damping HOMs easier • Easier manufacturing • Will the longitudinal E field cause a problem ??

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