Update on Crab Cavity: Impedance (at fundamental), RF noise, operational scenario
Compact Crab Cavity Status
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Transcript of Compact Crab Cavity Status
Compact Crab Cavity Status
E. Jensen for WP4
R. Appleby, T. Baer, J. Barranco, I. Ben-Zvi, G. Burt, R. Calaga, E. Ciapala, S. Da Silva, J. Delayen, L. Ficcadenti, R. De Maria, B.
Hall, Z. Li, A. Grudiev, R. Rimmer, J. Tückmantel, J. Wenningerand many more … (excuses)
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 integrated
rather than peak luminosity!
Local vs. Global Scheme• Local Scheme:• Global 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.
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?
now discarded
Compact Crab Cavities are in need!• The nominal LHC beam separation
in 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 curvatureis non-linear!• This is a real
challenge!
Progress with Compact Crab Cavities• They appeared in LHC-CC08 (in the box “Exotic
Designs”); seriously considered from 2009.• They made remarkable progress since then.• Truly global effort:
FNAL, SLAC, BNL, KEK, LBNL, ODU/JLAB, ULANC & CERN
Truly global design effort
R. Calaga, SRF2011
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.
LHC-CC11, CERN, 15 Nov 2011
Zenghai Li
ODU/JLAB (&SLAC) : Parallel bar to double ridged waveguide – evolution
J. Delayen, S. da Silva
Progress with ODU/JLAB/SLAC design• 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:
LHC-CC11, CERN, 15 Nov 2011
Prototype status
BNL: ¼ wave cavity• 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 flattening OK (<1% over ± 20 mm)• MP: easy to condition through.• Topology similar to double ridge!• Technology is at hand (S. Bousson)
LHC-CC11, CERN, 15 Nov 2011
I. Ben-Zvi, R. Calaga
4-rod cavity: Evolution from JLAB proposal to ULANC Design
supported by
ULANC (CI/DL): LHC-4R• Flattening field
profile led to new shape:
• Aluminium prototype arrived:
• MP studied – OK for cleancavity, MP free after discharge cleaning (with SEY 1.25)
• Bead-pull OK, • Couplers and HOM damper
studies started.
LHC-CC11, CERN, 15 Nov 2011
G. Burt, B. Hall, R. Rimmer (JLAB)
Aluminium Prototype• Beadpull measurements are being
performed on a to scale aluminium prototype. • Coupler ports present to allow
verification of damping.
4R-LHC: Fabrication techniques
1. Nb sheets, multiple pressed sections; EBW complicated.
2. Offset rods, slanted rods to make EBW easier.3. End plates from 1 Nb solid; Wire-etch two
end-plates from 1 Nb block – modifiedmodify shape to make compatible with EDM.
LHC-CC11, CERN, 15 Nov 2011
Comparing 400 MHz compacts
LHC-CC11, CERN, 15 Nov 2011
400 MHz,3 MV kick
500
Common concerns
• Field linearity
• Power coupler
• HOM impedance/HOM coupler/HOM damping
•Multipactor
• Fabrication techniques
•Machine protection
• RF phase noise
Field linearity:• Studied for example with multipole expansion.
• Effect of B(2) on tune shift dominating; with the above estimates ξ < 7E-4.
ODUCAV SRHW KEKCAV UKCAV QWAVER FRSCAV
Vz(x=0) [kV] 0.0 -2.1 - 2.5i -4 +1378i 0.0 0 +85.7i -0.1 -0.2i
Vx [MV] 5 5 5 5 5 5
B(2) [mTm/m] 0 0 -0.04i -32.7 - 0.1i 0.02 + 0i 25 + 0i 0 +108i
B(3) [mTm/m2 ] 1250 + 0i 229 + 0i 250 - 0i 2452 - 0.5i 464 + 0i -233 +1i
B(4) [mTm/m3] 0 0 266 - 5i 0 540 +0i -189 -14209i
R. Appleby, R. De Maria, A. Grudiev, J. Barranco
Machine Protection• 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:• Highly overpopulated tails observed:• In horizontal plane about 4% of beam beyond 4σmeas.• Corresponds to ≈20MJ with HL-LHC parameters.• Collimation system designed for fast accidental loss of up to 1MJ.
• Hollow electron lens to deplete tails gives add’l failure margin.
LHC-CC11, CERN, 15 Nov 2011
T. Baer, J. Wenninger
RF Phase Noise
Overall planningLHC operation (draft)
EuCARDDS HiLumi LHC PDR TDREuCARD2 (planned)
Compact Crab CavityValidation
MilestoneCompact Cavity Technology validationTechnical Design
Milestone Decision on: Local scheme with Compact CCBeam TestsConstructionCommissioningElliptical Crab CavityTechnical DesignConstruction
Milestone Decision on: Global scheme with Elliptical CCP4 cryo upgrade 2 K or 4.5 K?Prepare IR4Commissioning
Infrastructure LHCPlanningP4 cryo upgradePrepare IR4 for testsPrepare IR1 & IR5 could this be advanced?
Infrastructure SPSPlanningPreparation (Coldex)SPS CC cryoBeam test EllipticalBeam test Compact
dixit L. Tavian
SPS
2011 2012 2013 2014LS1: Splice Consolid.,
Collimation IR3
dixit L. Tavian
possible?
LHC IR4
LS3: Installation of HL-LHC HW, LHeC
2015 2022LS2:
Collimation,
2020 2021 20232016 2017 2018 2019
Fall back solutionFall-back solution
Testing-commissioning