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Crab Cavities

Crab Cavities . R. Calaga, E. Ciapala, E. Jensen/CERN

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Crab Cavities

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  1. Crab Cavities R. Calaga, E. Ciapala, E. Jensen/CERN and many more: R. Appleby, T. Baer, J. Barranco, I. Ben-Zvi, G. Burt, S. Da Silva, J. Delayen, L. Ficcadenti, R. De Maria, B. Hall, Z. Li, A. Grudiev, R. Rimmer, J. Tückmantel, J. Wenninger, V. Yakovlev, F. Zimmermann, …

  2. Crab Cavities – context • Many bunches require non-zero crossing angle to avoid parasitic collisions and to reduce beam-beam effects; • With non-zero crossing angle, luminosity gain by squeezing beams further is small (red curve below). • Crab cavities can compensate for this geometric effect and thus allow for a luminosity increase of about 50 % at β* of 25 cm. • In addition, crab cavities provide an ideal knob for luminosity levelling; • This allows optimizing for integratedrather than peak luminosity! HiLumi LHC Public Session

  3. Crab Cavities History: 1988 to 2009 Elliptical Technology R. Palmer, 1988, LC K. Hosoyama, 2010 HiLumi LHC Public Session

  4. Crab Cavities for LHC L. Xiao et al. Proposed 2005 US-LARP First concentrated on elliptical cavities ~250 mm outer radius HiLumi LHC Public Session Y. Yakovlev et al.

  5. Local vs. Global Scheme • Global Scheme: • Local Scheme: • Advantages: • Only one cavity per beam; • Larger beam separation near IP4; • Elliptical cavity of known technology. • Disadvantages: • Constraining betatron phase advance; • Requires larger collimator settings; • Works only for H or V crossing; • Only 800 MHz or higher fits. • Fit only in IR4 • Advantages: • Individual luminosity control at each IP; • Adapted to H or V crossing; • Orbit perturbed only locally; • Could work lower f – better performance. • Disadvantages/concerns: • Requires novel Compact Cavities (194 mm separation), well advancing, but not yet validated; • Requires 4 cavities per IP; • What if 1 cavity trips? HiLumi LHC Public Session

  6. Compact Crab Cavities are needed! The nominal LHC beam separationin the LHC is 194 mm;Conventional (elliptical) cavities scale with λ – they are too large even at 800 MHz! … but at higher f,the RF curvatureisnon-linear! This is a real challenge! HiLumi LHC Public Session

  7. Progress with Compact Crab Cavities They appeared in LHC-CC08 (in the box “Exotic Designs”); seriously considered from 2009. Decision to focus on Compacts: LHC-CC09 They made remarkable progress since then. Truly global effort: FNAL, SLAC, BNL, KEK, LBNL, ODU/JLAB, ULANC & CERN … something exciting and fascinating happened: many new ideas were born that seemed to make the impossible possible: HiLumi LHC Public Session

  8. Truly global design effort R. Calaga, SRF2011 This on-going design effort is now the core of WP4! HiLumi LHC Public Session

  9. SLAC (&ODU/JLAB): Double-ridged cavity Double ridge cavity – now teamed up with ODU/JLAB. Excellent! Field flatness < 0.6% @ ± 10 mm first OOM far away, HOM damping relatively simple (below cut-off) HOM below (stringent) impedance budget. Zenghai Li HiLumi LHC Public Session

  10. ODU/JLAB (&SLAC) : Parallel bar to double ridged waveguide – evolution J. Delayen, S. da Silva HiLumi LHC Public Session

  11. Progress with ODU/JLAB/SLAC design We don’t know exactly how flat the field has to be, but there is a handle to control field flatness. Flattening field profile OK: MP: cavity quite clean; issue maybe in the couplers – under study! Engineering design has started: sensitivity to pressure variation done. Prototype “square outer conductor”; size 295 mm OK @ 3 MV, marginal for 5 MV First CU, then Nb prototypes: HiLumi LHC Public Session

  12. Prototype status HiLumi LHC Public Session

  13. BNL: ¼ wave cavity Beam Compact and simple, mechanically stable. Synergy with eRHIC (181 MHz) Large separation to next HOM (theor. factor 3, realistically 1.4, high-pass filter enough!) Non-zero longitudinal field – issue? Easy tuning. Field flatness OK (<1% over ± 20 mm) MP: easy to condition through. Topology similar to double ridge! Technology is at hand (S. Bousson) I. Ben-Zvi, R. Calaga HiLumi LHC Public Session

  14. 4-rod cavity: Evolution from JLAB proposal to ULANC Design H. Wang, R. Rimmer CEBAF separator supported by G. Burt, B. Hall HiLumi LHC Public Session

  15. ULANC (CI/DL): LHC-4R Successful field flatteningprofile led to new shape: MP studied – OK for cleancavity, MP free after discharge cleaning (with SEY 1.25) Arrival of Aluminium prototype at Daresbury Lab/Cockcroft Institute G. Burt, B. Hall, R. Rimmer (JLAB) HiLumi LHC Public Session

  16. Aluminium Prototype • Bead-pull measurements are being performed on a to scale aluminium prototype. • Coupler ports present to allow verification of damping. HiLumi LHC Public Session

  17. 4R-LHC: Fabrication techniques Nb sheets, multiple pressed sections; EBW complicated. Offset rods, slanted rods to make EBW easier. End plates from 1 Nb solid; Wire-etch twoend-plates from 1 Nb block – modifiedmodify shape to make compatible with EDM.  HiLumi LHC Public Session

  18. Comparing 400 MHz compacts 500 HiLumi LHC Public Session

  19. Concern – Field linearity: R. Appleby, R. De Maria, A. Grudiev, J. Barranco Studied for example with multipole expansion. Effect of B(2) on tune shift dominating; with the above estimates ξ < 7E-4. HiLumi LHC Public Session

  20. Concern – Machine Protection T. Baer, J. Wenninger • Requirement: Stay below 1 MJ in 5 turns! • For upgraded optics, one gets 4 σ offset at CC voltage maximum. (10 MV kick, single cavity) • Dynamics dominated by Qext. (τ= 1 ms for 1E6) • up to 0.5 σ per turn! 2.2 σ after 5 turns. • Voltage failure – bunch centre not affected • Phase failure – bunch centre affected • Scenarios to stay below 1 MJ loss in 5 turns: • Highlyoverpopulatedtailsobserved: • In horizontal plane about4%of beam beyond4σmeas. • Correspondsto≈20MJ with HL-LHC parameters. • Collimationsystemdesignedfor fast accidentallossofupto1MJ. • Hollow electron lens to deplete tails gives additional failure margin. HiLumi LHC Public Session

  21. Concern – RF Phase Noise HiLumi LHC Public Session

  22. Overall planning HiLumi LHC Public Session

  23. After LHC-CC11 (earlier this week) Useful discussions on clarifying specifications: (2 K or 4.5 K?, HOM damping, tuners, geometry constraints, …) We will have the cavity specifications complete by April 2012 We need tests, both in SPS and in LHC Point4 - these tests will need be specified precisely. Try to use cavities & cryostats as close to the final in IP1 & IP5.. Joint design of cryomodule will start, involving cavity designers, CERN & outside cryo experts. We made good progress during the last days The planning is very challenging for the SPS & LHC tests! HiLumi LHC Public Session

  24. Conclusion Crab cavities are a key component of HL-LHC. The Design-Study of Compact Crab Cavities has made significant and encouraging progress over the last year. We have to get our act together for timely tests – the schedule is challenging! The team working on LHC Crab Cavities is truly international and working together well! HiLumi LHC Public Session

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