2008/7/28 1

ILCDR08 Report- Electron Cloud Session -

2008.07.08 – 11 @Cornell Univ.

Y. Suetsugu, KEK

2008/7/28 2

Electron Cloud Group Charge(M. Palmer)

The charge to the Electron Cloud working group is to review the status of electron cloud simulations, both for electron cloud growth and for electron cloud induced beam dynamics, the benchmarking of the major codes against each other, and benchmarking of the codes against experiment. The group should also review the status of electron cloud measurement and mitigation techniques. Finally, the group should look at the world-wide experimental program and inputs that are required for the ILC and CLIC damping ring designs, paying particular attention to identifying tests that are needed as part of the CesrTA program.

__________________________________________________

This charge is quite ambitious; we made headway, but continuing discussions and real work will be required to fulfill the charge

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Four Working Groups

• Measurements– Electron cloud build up, effect on beam

• Simulations– Benchmarking, build up, effect on beam

• Mitigation techniques– Coating, groove, clearing electrode

• Experimental plans– Each Lab., CESR TA

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Talks in Measurement WG

• Cloud Build up– K. Kanazawa (KEK), R. Zwaska (Fermi Lab.) and S.

Greenwald (Cornell): RFA [Retarding Field Analyzer] monitors

– S. De Santis (LBNL): Microwave transmission• Effects on Beam

– K. Ohmi (KEK): Incoherent emittance growth– J. Flanagan (KEK): Coherent instability– R. Holtszpple (Alfred Univ.): Tune shift using

witness bunch

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Measurement (Kanazawa)

• RFA type electron detectors with Faraday cup or MCP or multi-strip anode are installed to KEKB LER.

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Measurement (Greenwald)

• Comparison between Cornell-type thin RFA and APS-type– Almost consistent each other

• Newly developed RFA for CESR-TA– Dipole Chamber RFA

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Measurement (Santis)

• Electron density can be estimated by measuring the phase shift of transmitting microwave

• Instruments were set upfor CESR-TA, following PEP-II

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• Head-tail instability: Synchro-Betatron sidebands– The behavior is consistent

with simulation

J. Flanagan, ECLOUD07

Effects on Beam (Flanagan)

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Effects on Beam (Holtzappie)

• Tune shift measurement using witness bunch• Use long trains e+/e- bunches to generate a electr

on cloud density. Place witness bunches at varying times after the generating train and measure the coherent tune shift of the witness bunch.

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Summary• Build up

– Thin RFA is prepared for wiggler section (B-magnet)– Microwave transmitting method with better hardware– Measurement in Q-magnet or in solenoid field: Future

issues• Effect on beam

– Measurement of coherent instabilities, incoherent emittance growth

– Tune shift– Beam size: Development of X-ray beam size monitor

• Provide valuable data for verification of simulation codes.

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Talks in Simulation WG

– G. Dugan: “Simulations at Cornell for CesrTA”– J. Calvey: “Simulations for RFA studies at CesrTA”– J. Crittenden: “Simulations for witness bunch studies at

CesrTA”– T. Demma: “Build-up of electron cloud in DANE in the

presence of a solenoid field”– C. Celata: “Electron cloud cyclotron resonances for short

bunches in magnetic fields”– K. Ohmi: “Study of electron cloud instabilities in CesrTA

and KEKB”

All but the last talk are about “build-up ecloud physics”

(M. Furman)

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What is to be done… (1)– Understand e-cloud build-up, decay, and spatial/energy

distribution– Benchmarking of build-up codes

• Bring the codes (ECLOUD, CLOUDLAND, POSINST) into agreement• “Rediffused electrons” likely to be the source of the discrepancy

– Simulate a few beam fill patterns• Obtain electron flux Je and dN/dE at RFAs• Obtain transverse distribution of e-cloud at dipoles• Fit basic SEY parameters to the above so as to agree with data• Predict Je, dN/dE, tune shift along train and transverse electron

distribution for other fill patterns– This subprogram will

• Characterize the e-cloud distribution around the machine• Increase the confidence in build-up codes

(M. Furman)

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What is to be done… (2)– Understand effects of the e-cloud on the beam– Available codes: HEADTAIL, WARP, PEHTS, CMAD,…– CesrTA e-cloud R&D is driven by the necessity to preserve a

very low beam emittance. This will bring intense scrutiny of e-cloud codes that compute effects on the beam

– To a large degree, this subprogram can proceed in parallel with the build-up subprogram

• Just assume a value for the e-cloud density near the beam and proceed• Look at single bunch (coherent and incoherent) effects• Multi-bunch coherent effects

– As the build-up subprogram provides more information on the e-cloud around the ring, refine the understanding of the e-cloud effects on the beam

(M. Furman)

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Talks in Mitigation WG

– F. Caspers (CERN) : Microwave transmission recent measurements in the SPS and LHC Measurements?

– M. Pivi (SLAC): Mitigations experiments at SLAC – R. Zwaska (Fermilab): Plans for a resistive electrode– Y. Suetsugu (KEK): Experiment on clearing electrode

at KEKB positron ring– M. Palmer: Mitigation studies presently included in

the CesrTA program

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ECLOUD2 – Grooved Chambers Performance: M. Pivi

M. Pivi et al, SLAC

Electron cloud signal in two smooth (flat) TiN-chambers and two grooved TiN-chambers installed in PEP-II.

Electron cloud signal in stainless steel chamber.

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Conditioning surfaces in PEP-II: M. Pivi, SLAC

(M. Pivi)– Stability of TiN coating

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Clearing Electrodes in KEKB: Y. Suetsugu, KEK(M. Pivi)

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Mitigation tests in Cesr TA: M. Palmer, Cornell

(M. Pivi)

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Preliminary table to be completed as input for Technical Design Phase. Goal is to turn all Red colors to Green in the next two years.

Other mitigations under development! (ex: Carbon coating CERN)

Recommendation for mitigation as input for DR design: Discussion All

DR element % ring Antechamber Coating Additional Mitigation

Remarks

DRIFT in STRAIGHT

33 No NEG Solenoid Groove if necessary

DRIFT in ARC

56 Downstream of BEND only

NEG Solenoid Groove if necessary

BEND 7 Yes TiN Grooves and Electrodes

WIGG 3 Yes TiN Electrodes and Grooves

QUAD 1 Downstream BEND / WIGG

TiN Grooves and Electrodes

(M. Pivi)

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Summary• Successful R&D program on electron cloud

mitigations.• TiN coating has been demonstrated to have an SEY

below the instability threshold. Work continues to address a few remaining issues.

• Yet, requirements at future colliders (2 picometer emittance in the ILC DR, e.g.) are challenging.

• Hence, close collaboration between labs to develop complementary mitigation techniques is needed to further suppress the electron cloud effect.

(M. Pivi)

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Talks in Experimental Plan WG

– G. Dugan (Cornell): Cesr-TA experimental plans– Y. Suetsugu (KEK): Experimental Plan at KEKB

Positron Ring:Grooved Surface, and Clearing Electrode Ver.2

– K. Kanazawa (KEK): Plan of measuring cloud density in the solenoid field and in the quadrupole field

– W. Fischer (BNL): EC plans in connection with eRHIC

– General discussion on key experiments for experimental planning-focused on code validation and mitigation techniques-all

(G. Dugan)

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CESR EC experimental areas• L3 Straight Experimental area

– Instrument large bore quadrupoles and adjacent drifts

– Install of PEP-II experimental hardware (including chicane) in early 2009

– Provide location for installation of test chambers

• Arc experimental areas – Instrument dipoles and adjacent

drifts– Provide locations for installation of

test chambers, in drifts where wigglers were removed.

• L0 Wiggler Experimental area– All wigglers in zero dispersion

regions for low emittance– Instrumented wiggler straight and

adjacent sections

(G. Dugan)

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Experimental Setup

• Use the same location

Test chamber with antechambers

BeamGate Valve Gate Valve

Wiggler magnetsB = 0.75 T

R47

Beam

[Groove]

[Monitor]

Mag

neti

c field

Experimental Plan at KEKB Positron RingGrooved Surface, and Clearing Electrode Ver.2

Y. Suetsugu, H. Fukuma, KEK

M. Pivi and W. Lanfa, SLAC

(G. Dugan)

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Plan of measuring cloud density in the solenoid field and in the quadrupole field

K. Kanazawa (KEK)

SOLENOID

• Given a solenoid field and the position of detection, the energy of measured electrons is automatically selected (=the volume is automatically defined).

Electrons accelerated by a bunch along X-axis reach the detector.

X-axis

QUADRUPOLE

(G. Dugan)

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Key questions for experimental planning

• Cloud build up-What experiments will best pin down the SEY model parameters, particularly the number of rediffused electrons? The photoelectron generation model parameters?

•Fit of cloud saturation as measured by RFA to SEY peak and SEY yield at zero energy.

•Fit RFA energy-differential current and/or tune shift as a function of beam current and time to disentangle photoelectron parameters from SEY yield parameters. Transverse shape of RFA current measurement can be sensitive to SEY model parameters.

•RFA measurements in quadrupole can be important since they are at peak beta values.

•Improved RFA time resolution is important.•TE wave transmission can measure growth and decay of average cloud density in a local area of the ring.

(G. Dugan)

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Key questions for experimental planning

• Effect on beam-How can we test that the effects of the “pinch” are being properly modeled?

•Head-tail instability threshold. •Bunch length dependence of tune shift measurements

-How can we best establish confidence in the instability predictions? The predictions for emittance growth?

•Scaling-Threshold of head-tail instability-dependence on beam size? Dependence is stronger on sync tune, avg beta, chromaticity. Should be roughly independent of emittance.

•Emittance growth-need transverse feedback, turn by turn beam size measurements. Difficult measurement. Codes that can be validated here: WARP, Ohmi’s PEHTS, HEADTAIL

(G. Dugan)

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Key questions for experimental planning

• Other bench measurements (e.g. SEY secondary spectrum) which could help establish code parameters?

•Measure (using Auger spectrometer) secondary spectrum of initial (and irradiated?) chamber samples.

•Measure <15 eV part of SEY curve-light sources? Try to think of other ways to do this.

•XPS to measure photoelectron spectrum (or with synchrotron light beam line from CHESS?) and photon reflectivity vs. energy, angle…data may exist for the latter.

(G. Dugan)

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Key questions for experimental planning

• Mitigation Techniques-What additional experiments are needed to establish high confidence in the proposed mitigation techniques to be used in the ILC damping ring?

•Repeat measurement of EC cloud suppression for TiN and NEG.•Investigate chamber exposure to gases such as SF6, N2, O2. •Try carbon coating proposed for SPS-durability under SR radiation•Long term lifetime of TiN-PEP II and KEK can provide data. •Clearing electrodes in wiggler and dipole, and grooves in dipole. Grooves for wiggler chamber should continue to be studied.

•Investigate feasibility of measuring transverse wake from single grooved chamber (and electrodes) using local field probes.

(G. Dugan)

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Summary – Cesr TA: A superb and ambitious e-cloud R&D program– Essential resources are in place

• Hardware• Diagnostics and simulation tools• Operational expertise• Knowledge, flexibility and maturity of the machine

– e+ / e–, almost arbitrary fill pattern,…• Knowledge of certain relevant e-cloud parameters• Dedicated beam time

– Close collaboration with outside experts is highly desirable to make rapid and sustained progress

– I have a suspicion that 2 years will not be enough to achieve all the desired goals

– Nevertheless, I am quite confident of a large degree of success, both for Cesr TA in particular, and for the e-cloud field in general.

(M. Furman)

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Comments from the CesrTA PM• Working Group Summaries

– Electron Cloud• CesrTA offers a unique opportunity to benchmark simulation codes

over a wide range of parameters and to study both EC growth and beam dynamics issues that are critical for damping ring performance

• Very promising recent results on mitigation techniques– Much remains to be done to ensure a viable (and economical) solution

for the damping rings– Mitigation techniques and ring design must be integrated into an overall

design• Significant questions remain on surface physics issues

– CesrTA offers a chance flexibly explore the “integrated effect”– Collaboration support for further surface science studies could be very

beneficial• CesrTA program will aggressively pursue a broad range of experiments

to characterize the EC parameters that are necessary to provide confidence in the damping ring design