ILC Damping Ring Collective Instability update:

Electron Cloud

Mauro Pivi

SLAC

ILC Vancouver Meeting

July 19-22, 2006

• History back to Nov 2005, DR recommendation

• What is changed by simulations, since then:

- increased confidence that 2 x 6 km is safely below instability threshold

- increased confidence on remedies, 1 DR may be feasible

• DR possible scenarios and risks: 2 DR vs 1 DR.

Layout

Damping ring recommendation

Simulated single bunch instability (SBI) thresholds and electron cloud build-up densities for peak secondary electron yield SEY=1.2 and 1.4. All TESLA wiggler aperture. PEHTS and POSINST codes benchmarked with ECLOUD, CLOUD_LAND, HEADTAIL.

A large bunch spacing is desirable to limit the build-up of the electron cloud

Task Force 6. K. Ohmi, M. Pivi, F. Zimmermann, Nov 2005.

Recent results for low Q (N=1x1010)K.Ohmi (KEK)

• Ecloud: Threshold of electron cloud, 1.4x1011 m-3.• Ion: Feedback system can suppress for 650 MHz (3ns

spacing), number of bunch in a train 45, and gap between trains 45ns..

Recent results: wiggler aperture increaseRecent results: wiggler aperture increaseRecent results: wiggler aperture increaseRecent results: wiggler aperture increase

2 x 6km DR: Beneficial effect of increasing the wiggler chamber aperture.

Margin of safety below the single-bunch instability (SBI) threshold.

Motivations for electron cloud studies and R&D

• For ILC Damping Ring:– If the Secondary electron yield (SEY) can be reduced in

magnets, a smaller positron (6 km) ring can be feasible– promising cures in magnet regions as thin micro-fins and

clearing electrodes need further R&D and full demonstration in accelerators

• For Super B factory:– Higher currents shorter bunch spacing

• For KEKb:– KEKb Annual Report 2005: "The electron cloud effect still

remains the major obstacle to a shorter bunch spacing, even with the solenoid windings “ [1].

• For LHC

[1] http://www-kekb.kek.jp:16080/pukiwiki/index.php?Documents

Extensive program worldwide incl. KEK, UK, Frascati, IHEP

DR component optimization: wigglers, fast kickers; (Cornell)studies of the use of CESR as a DR test facility (in 2008)

Damping Ring Design and Optimization (ANL)Lattice design and optimization; studies of ion instability in

the APS ring; design of a hybrid wiggler

E-cloud, SEY, FII simulations, experiments in PEP-II, KEKB, CESRc and Dafne rings (SLAC, KEK, CERN, Cornell, Frascati)

ATF damping ring experiments (SLAC, LBNL, Cornell)

Lattice designs for damping rings and injection/extraction lines; characterization of collective effects; stripline kickers for single-bunch extraction at KEK-ATF (LBNL)

Damping ring R&D

Remedies simulation summary (see also next talk by L.

Wang)

-20 -10 0 10 20

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L. Wang CLOUD_LAND code

P. Raimondi, M. Pivi POSINST code

0 Voltage 100 Voltage

Bunch spacing = 6ns ! Bunch spacing = 1.5ns !!

Clearing electrodes R&D

• Suppress the electron cloud in BEND and WIGGLER (QUAD) section: Perfect !

• Prototypes installation in LHC test dipole.

CERN ad Texas Univ. stripe electrode design.

• Warning: ion-clearing electrodes (alumina) in Daphne generate

impedance and overheating, need to be removed.

• Control the generation of HOM, transverse impedance, resistive wall impedance and RF heating.

• Preliminary longitudinal impedance measurements at LBNL loss

factor k=5e9 V/C.

• Optimized design should be tested in beam line with similar beam

parameters R&D at PEP-II, Cornell, KEK.

LHC electrode design

• Very low SEY can be achieved.

• Triangular fin (grooves) SEY << 1 in magnets (see L. Wang talk)

• Rectangular groove SEY < 1 in magnets and field free.

• Fin-chamber (challenging) extrusion completed: four 2 meter long chambers will be installed in PEP-II at SLAC in fall 2006.

• Resistive wall impedance enhanced by 47% (K. Bane, G. Stupakov).

Fin chambers

PEP-II fin-chamber

Scenario Collective instability

Pro’s Con’s Risks R&D

1 DR e- cloud above SBI threshold

50% cost reduction, Reliability

Need a cure for electron cloud in magnets

Novel cures for electron cloud are technically not feasible

Increase electron

cloud R&D »

2 DR e- cloud below SBI threshold

e- cloud SBI free, Upgradeability, Flexibility

Costs None Later, after R&D, possibly decrease to 1 DR

Possible ILC positron DR scenarios

Risks:

• In the case of 1 DR: novel cures for electron cloud could not be technically feasible due to i) conditioning is not sufficient, ii) grooves or clearing electrodes generate HOM, large resistive wall impedance, large transverse impedance, and RF heating.

• In the case of 2 DRs: risks are low provided that e- cloud simulation predictions are giving good estimates.

SBI = single bunch instability

M. Pivi, ILC Meeting Vancouver 19-23 July 2006

• The 2 x 6km ring is safely below single-bunch instability threshold and can

accommodate rather large secondary electron yield values.

• In a single 6km ring, the electron cloud develops near threshold.

• High confidence given now by simulations on possible cures, including grooves and

clearing electrodes. Need a full technical feasibility demonstration.

• Scenario 1: Decrease to 1 DR and increase R&D effort. (Sugg: by design 2nd

ring built-in; be ready to install a 2nd ring. Not making it impossible).

• Scenario 2: Maintain 2 DR. Later, depending on R&D results, decrease to 1 DR.

Scenarios summary

At the KEKB Positron Ring Test chambers (Cu, TiN-coated and NEG-coated)

were installed in the KEKB positron ring. 3.5 GeV positron, stored beam current ~1.7 A.

Number of electrons near the beam orbit was measured using a special electron monitor.

SR of 1x1016 photons/s/m/mA was irradiated at side wall.

Incident angle ~8 mrad.

Electron monitor

Y.Suetsugu et al. KEK 2005.12.06

At the KEKB Positron Ring

Y.Suetsugu et al. KEK 2006.07.06

Positron Measurements• Positrons @ 5.3 GeV• Single train of 45 bunches

with 14 ns spacing– NOTE: at highest bunch

currents, filling of bunches

> #20 no longer uniform

• Plots– Top: Bunch Tune (kHz) vs

Bunch

– Bottom: BPM ADC level vs Bunch (note missing bunches at high bunch currents)

The use of the HERA Electron Ring

in Conjunction with ILC Damping Rings

Damping Ring Collaboration Meeting May9, 2006

F. Willeke, DESY

•Long term perspective

•Short term goals

•DR design examples

•Schedule

•Long term perspective

•Short term goals

•DR design examples

•Schedule

Possible Not-too-Far-Term DR Studies in HERA

• Storage of 250mA of positron with a bunch-spacing of 6-16 ns,

• study of electron-cloud issues, testing of remedies

• Demonstration of 1pm vertical emittance

• Demonstration of effective bba procedures

• Polarization test measurements

• De-install n.c. 500MHz cavities, RF feedbacks, 250MHz bandwidth MB damper improved HOM couplers at SCC

•Improved BPM electronics Additional BPMs Additional BBA circuitryLow Measurement equipment

ISSUE

To be discussed and closely co-ordinated with GDE and DR collaboration!

Additional Equipment Needed

F. Willeke, DESY Damping Ring Collaboration Meeting May 9, 2006

Thanks ! To the contributors to this presentation M. Palmer (Cornell), S. De Santis (LBNL) F. Willeke (DESY), K. Suetsugu (KEK), K. Bane, P. Raimondi, L. Wang (SLAC), F. Zimmermann (CERN)

and to DR collaboratorsD. Arnett, G. Collet, R. Kirby, N. Kurita, B. Mckee, M. Morrison, P. Raimondi, T. Raubenheimer, J. Seeman, L. Wang, K. Bane, G. Stupakov (SLAC), D. Rubin, D. Rice, L. Schachter, J. Codner, E. Tanke, J. Crittenden (Cornell), J. Gao (HIPEP), A. Markovic et al. (Rostock Univ.), M. Zisman, S. De Santis, C. Celata, M. Furman, J.L. Vay (LBNL), K. Ohmi, Y. Suetsugu (KEK), F. Willeke, R. Wanzenberg (DESY), E. Benedetto, F. Zimmermann, J.M. Jimenez, J-P. Delahaye (CERN), A. Wolski (Cockroft Uniiv.), B. Macek (LANL), C. Vaccarezza, S. Guiducci, R. Cimino (Frascati), …

July 19-22, 2006

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Frequency (MHz)

Longitudinal impedance bench measurements LBNL

Experimental setup - coaxial wire method

Initial results: peaks spacing is ~379 MHz i.e. a wavelength equal to twice the length of the test electrode (/2 resonance). Our test pipe cutoff is around 3 GHz.

Z// 2Zc ln(S21DUT / S21

REF )

Walling log formula for distributed impedances

378.75 MHz

k 5 10 9 V / C

Loss factor (back of the envelope estimate)

(Ohm

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S. De Santis, M. Pivi July 2006

Rectangular fins:t = fin thicknessp = fin pitch

Resistive wall impedance increases by 47% for PEP-II fin-chamber design.

K.Bane and G. Stupakov 19-23 July 2006