WP 4: Crab CavityEd Ciapala, Erk Jensen

BE-RF, CERN

HL-LHC Meeting, CERN, 20 July 2010

Crab cavities: ContextMany 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!20 July

2010 2HL-LHC Meeting WP4: Crab Cavity

Local and Global Scheme Local Scheme: Global Scheme:

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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:• Requires novel Compact Cavities (194

mm separation), which are not validated;

• Requires 4 cavities per IP;• What if a cavity trips?

CERN StrategyAim ultimately at a significant luminosity

increase using the local scheme with compact crab cavities around the IPs.◦ This requires a substantial R&D program, but first

results are encouraging (see below).To mitigate the risk, and in parallel to the above,

design and implement a elliptical cavity in IR4 to obtain measurable luminosity increase in a global scheme.◦ Conceptual design exists (US-LARP), including coupler,

cryostat, wrong-order-mode dampers, ...◦ Space is available in IR4 (was foreseen for ACN200

system).◦ What could a test of the KEK crab cavity in SPS tell us?20 July

2010 HL-LHC Meeting WP4: Crab Cavity 4

Collaboration In order to encourage collaboration and to assure its

consistency and continuity, CERN has to take the lead in the world wide Crab Cavity development program. This is also a necessary strong sign to assure partners!

In the past, CERN was merely observer; results obtained by collaborators so far: ◦ US-LARP: design of elliptical cavity system, some 3 compact

designs, ◦ EuCARD WP 10.3: specifications, compact cavity design (Lancaster),◦ KEK-B positive results, compact cavity design (“Kota” cavity).

CERN is in the process of defining the strategy and a work plan (in parallel to ongoing efforts), which it will then adhere to.

Expected in September 2010: “Statement of Need”, Initial Cost & Schedule, work plan!

This will allow identification of work packages for partners. HL-LHC is one important element of the overall strategy

(focussing on “design” questions); other elements – coordinated with it – are: ◦ US-LARP and maybe APUL, EuCARD, the French “Grand Emprunt” in

Rhône-Alpes region, DoE SBIR & MTTR programs, ...)

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Elliptical cavity design

20 July 2010 HL-LHC Meeting WP4: Crab Cavity 6

Taken from Rama Calaga’s Talk at Chamonix 2010, http://indico.cern.ch/conferenceDisplay.py?confId=67839

Compact cavity designs

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EUCARD 4-rod cavity (Lancaster)

ODU Parallel Bar Cavity

SLAC Half-wave Spoke Resonator

KEK Kota Cavity

Taken from Graeme Burt’s Highlight Talk at EuCARD Annual Meeting, http://indico.cern.ch/conferenceDisplay.py?confId=73614

The possible issuesWill it work?Machine protection!

◦Major concern: single turn failure! Requires study!Collimation efficiency and hierarchy

◦ Initial calculations indicate that this might be OK – still, it requires more study!

Crab cavity induced noise◦Phase noise leads to beam offset – requires study!

Impedance, both longitudinal and transverse!Operational: how to make “invisible”, how to

commission? What happens during the ramp?20 July 2010 HL-LHC Meeting WP4: Crab Cavity 8

WP4: Possible design subjects (1/2)

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Tasks Collaborator

1Design optimization for novel schemes (minimum # of peripherals)

CI, LARP, CERN,KEK

2 Cavity and cryomodule specifications (LHC report) CERN, KEK, CNRS

3 Local IR layout and spatial integration CERN, KEK

4 Multipacting simulations of cavity & couplers CI, LARP

5 FEM simulations: mechanical & thermal aspects CI, LARP

6Fabrication and measurements on copper models: field profile, impedance, error sensitivity)

CI, LARP

7Demonstration of field gradients in compact cavities (4 structures)

LARP, CERN, CI

8 Q-slope, onset of field emission and quench LARP, CERN

9 Performance difference between 2 K & 4 K LARP, CERN

10 Coupler development and testing CERN, CEA

11 Tuner concepts and mock up on copper models CERN, CEA

12 Study of mech. resonances, microphonics CERN, CEA

13 Performance with couplers and horizontal cryostat CERN, CEA

14Performance with beam (e.g. SPS – impedance, collimation, …)

CERN, LARP

15 Interlocks and fast Feedback (other measures like Qext) CERN,

16 Common power and cryogenic systems across the IPs (?) CERN, CEA?

17 LLRF and controls – (phase control!) CERN, CI

Technology Issues

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WP4: Possible design subjects (2/2)

Tasks Collaborator

1RF behavior for cavity quench or turbulent behavior *

CERN, LARP, KEK

2Tracking simulations for loss maps for fast failure modes

CERN, LARP, KEK

3Beam-beam simulations to investigate instabilities, noise issues and DA

CI, LARP, CERN

4Impedance and growth rate estimates for specific HOMs to specify exact damping and feedback measures*

CERN, LARP

5Effect of non-zero dispersion and stable working points

CERN, LARP

6 Crab consistent optics CERN, LARP, KEK

Beam Issues

* Cavity design dependent

WP4: Possible Participants

CERNCockcroft Institute (U Lancaster)US-LARPKEKCEACNRS...

20 July 2010 HL-LHC Meeting WP4: Crab Cavity 11