Crab Cavities

R. Calaga, E. Ciapala, E. Jensen/CERNand 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, …

HiLumi LHC Public Session 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 integrated

rather than peak luminosity!

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HiLumi LHC Public Session 3

Crab Cavities History: 1988 to 2009

R. Palmer, 1988, LC

Elliptical Technology

K. Hosoyama, 2010

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HiLumi LHC Public Session 4

Crab Cavities for LHC• Proposed 2005 US-LARP• First concentrated on elliptical cavities

Y. Yakovlev et al.~250 mm outer radius

L. Xiao et al.

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HiLumi LHC Public Session 5

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.• 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?

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HiLumi LHC Public Session 6

Compact Crab Cavities are needed!• 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!

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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:

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HiLumi LHC Public Session 8

Truly global design effort

R. Calaga, SRF2011This on-going design effort is now the core of WP4!

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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.

HiLumi LHC Public Session

Zenghai Li

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HiLumi LHC Public Session 10

ODU/JLAB (&SLAC) : Parallel bar to double ridged waveguide – evolution

J. Delayen, S. da Silva

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11

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:

HiLumi LHC Public Session

We don’t know exactly how flat the field has to be, but there is a handle to control field flatness.

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HiLumi LHC Public Session 12

Prototype status

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13

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 flatness OK (<1% over ± 20 mm)• MP: easy to condition through.• Topology similar to double ridge!• Technology is at hand (S. Bousson)

HiLumi LHC Public Session

I. Ben-Zvi, R. Calaga

Beam

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HiLumi LHC Public Session 14

4-rod cavity: Evolution from JLAB proposal to ULANC Design

supported by

H. Wang, R. RimmerCEBAF separator

G. Burt, B. Hall

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15

ULANC (CI/DL): LHC-4R• Successful field flattening

profile led to new shape:• MP studied – OK for clean

cavity, MP free after discharge cleaning (with SEY 1.25)

• Arrival of Aluminium prototype at Daresbury Lab/Cockcroft Institute

HiLumi LHC Public Session

G. Burt, B. Hall, R. Rimmer (JLAB)

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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.

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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.

HiLumi LHC Public Session

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Comparing 400 MHz compacts

HiLumi LHC Public Session

500

Values for 400 MHz, 3 MV integrated kick

Double ridge (ODU/SLAC)

LHC-4R (ULANC)

¼ Wave (BNL)

Cavity radius [mm] 147.5 143/118 142/122

Cavity length [mm] 597 500 380

Beam Pipe radius [mm] 42 42 42

Peak E-field [MV/m] 33 32 47

Peak B-Field [mT] 56 60.5 71

RT/Q [Ω] 287 915 318

Nearest OOM [MHz] 584 371-378 575

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HiLumi LHC Public Session 19

Concern – 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

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Concern – 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 additional failure margin.

HiLumi LHC Public Session

T. Baer, J. Wenninger

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HiLumi LHC Public Session 21

Concern – RF Phase Noise

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HiLumi LHC Public Session 22

Overall planning

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LHC 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

possible?

LHC IR4

LS3: Installation of HL-LHC HW, LHeC

2015 2022LS2:

Collimation,

2020 2021 20232016 2017 2018 2019

Fall back solution

dixit L. Tavian

SPS

2011 2012 2013 2014LS1: Splice Consolid.,

Collimation IR3

Fall-back solution

advance!

advance!

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!

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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!

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