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Summary: Measurements of Electron Cloud Effects

K. Harkay

ECLOUD07, Daegu, Korea, Apr 2007

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Comments represent my main impressions from the workshop – experimentalist’s perspective

Apologies in advance for not mentioning everyone’s excellent results

I’d like to acknowledge the hard work of all the speakers at the workshop!

Much thanks for discussions with and insights from C. Celata, S. Heifets, S. Kato, J. Flanagan, M. Furman,

R. Macek, K. Ohmi, M. Pivi, L. Schachter, and many others

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Discussion topics

Novel experiments Progress in cures/mitigation Price to pay with cures (impedance)max < 1 (!!!)

What do we need to pay more attention to? Final thoughts

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Topics covered by the following talks:

R. Macek, Summary of e-p feedback mini-workshop W. Fischer, Summary of ECL2 mini-workshop S. Cousineau, E-p instab at SNS J. Flanagan, EC and spec. lumi. at KEKB W. Fischer, EC msmt at RHIC R. Zwaska, EC at FNAL MI A. Molvik, Msmnts EC with solenoid, quad magnetic transport R. Macek, EC gen, trapping in quad at PSR S. Casalbuoni, Pressure rise and beam heat load on cold chamber M. Kireeff Covo, EC density msmnt at HCX K.-I. Kanazawa, Msmnt EC density at KEKB T. Ieiri, Msmnt EC wake effects at KEKB S.-Y. Zhang, Proton emitt. growth at RHIC

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Topics covered by the following talks (cont):

F. LePimpec (M. Pivi), SEY msmnts at SLAC S. Kato, SEY msmnts at KEKB W. Fischer, Molecular desorption on SS from high energy ion

beams M. Pivi, ILC R&D in PEP2, SEY, grooved chambers M. Nishiwaki, SEY msmnts coated and uncoated chambers

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Novel experiments to characterize EC

EC trapped in quads (R. Macek) EC dynamics in solenoid/quad transport (A. Molvik) EC density (M. Kireeff Covo, K.-I. Kanazawa) EC wakefield (T. Ieiri, M. Palmer) Parasitic single bunch studies at KEKB (J. Flanagan)

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EC trapped in quads

Studied electrons in quadrupoles using RFA and sweeping electrode (Macek et al.)

Ultralong EC lifetime, 2-3 orders magnitude longer (60-100 s) in quads than in drifts (~100ns)

Ejection into drift spaces due to EB Drive out of quad (sweeper), see increase quad RFA;

drift RFA decreases after a few turns

Electrostatic mirrors to control ejection

Multipactor 25x less in quads, signal 1.5 times higher; implies >35x more seed electrons in quads vs. drifts

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EC dynamics in solenoid/quad transport

HCX measures ions and electrons from all sources to characterize the EC and beam potential, data for benchmarking code (Molvik, Kireef Covo et al.)

Used RFAs, clearing electrodes, solenoid, GESD, to characterize EC

Varied clearing electrodes in solenoid and between solenoids (ring) to study control of trapping and to study effect of trapped electrons in heavy ion beam

New diagnostic: image light emitted by ion-induced desorbed gas

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EC density

KEKB experiments (Kanazawa)– Use RFA to select EC energy to select desired

volume in chamber– Drift: High energy cutoff selects area with radius r

centered on beam– Dipole: computed volume selected by RFA– Error sources: finite bunch length; resonantly

trapped EC doesn’t reach wall (Schachter, Loiacono/Harkay (PAC05))

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EC density

HCX exp (Kireeff-Covo)– Configured RFA for ion selection or electron

selection (4 grids that can be biased)– Used combination of RFA and clearing electrodes to

deduce evolution of EC density in long ion beam pulse

– Results likely insightful for long proton bunches, perhaps also to understand time scales of evolution of EC effects in PSR

– Results may be applicable to short e+ bunches as well in regime when long bunch train acts like single long bunch

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EC wakefield

Bunch train, witness bunch study (Ieiri)– Detailed tune shift and tune spread study to

characterize electron cloud density and lifetime– Good qualitative agreement with expectation– Detailed comparison with analytical EC wakefield to

be done

Bunch train/witness bunch studies at CESR (Palmer) – Tune shift measurements comparing e+ and e-– EC overshoot for e- beam– Understanding is needed, simulations, etc

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Parasitic single bunch studies at KEKB

Machine study time almost nonexistent at HEP colliders

How to study electron clouds and instabilities? Clever parasitic studies at KEKB (Flanagan et al.) Drive bunch injected into operational pattern, study

single bunch effects as EC, bunch current decays

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Progress in cures/mitigation

Control of SE emission from warm or cold surface, in drifts or magnetic fields

Criteria: drift- vs. dipole-dominated; primary (PE) vs. secondary (SE) electron dominated– Drifts: Coating (TiN, NEG) and solenoids– Bends: antechamber (PEP2) or grooves (LHC) to

mitigate photoemission, conditioning (pre 2007, limited to max ~1.3 (SS) at SPS)

– Wiggler: all of the above SC undulator heating at ANKA: short bunch agrees with

RW heating; long bunch NOT (possible evidence of EC) (Casalbuoni). See ECLOUD02/04 for CERN data on conditioning of cold surface.

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Cures/mitigation techniques of choice: present

NEG + bunch pattern: RHIC (Zhang, Fischer)– Surface roughness also reduces EC from beam loss– NEG also good for pumping desorbed gas?

Solenoid: KEKB

– Do not use/need low max coatings – PE dominated? Chamber conditioning gives low max?

TiN+solenoid: PEP2, SNS (also clearing electrode at foil)– TiN not enough, also needs solenoids (Cu in straights)

– Interesting to test max conditioning with radiation/beam to see if as effective in PEP2 as KEKB results show – turn off solenoids (Heifets)

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Cures/mitigation techniques of choice: future

Antigrazing ridges– For photoelectrons (LHC)– Showers issue for collimator/beam loss application

(Fischer, RHIC) Grooves (rectangular/triangular (“fins”): for ILC DR

(Pivi)– Considered “holy grail” to allow one e+ ring instead

of two (shorter bunch spacing) (pre 2007)– PEP2 tests disappointing (rect. fins)

Clearing electrodes: high interest at CERN, elsewhere (Fischer, ECL2)

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Clearing electodes

Promising results from simulations Many uncertainties in mechanical construction Many questions of max and electrical conductivity of

glass/enamel substrate Looking forward to studies in machines

Fischer listed references back to 1970s on experience measuring and curing ion-induced gas desorption. Useful to have in-depth review of past experience using clearing electrodes.

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Price to pay for mitigation techniques

Impedance/HOMs– Electrodes (0.5 /m km = large (Pivi))– Grooves (adds 30% in case of ILC DR (Bane))– Glass/Enamel coatings

Percent of circumference that needs coverage for effect? (see past experience at, e.g. ISR, PSR, etc)

Mechanical complexity/cost– How to repair if damaged (vent chamber!)

But…

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Biggest surprises of workshop (to me)

max < 1 for conditioned surfaces (LePimpec, Pivi, Kato, Nishiwaka)– Strong recontamination effects not observed at KEK

(compared to SLAC)

– Let’s not forget that grazing incidence max is higher, and higher energy part can be enhanced

Grooves result (Pivi) Photoelectrons can dominate in wigglers (Celata,

ECL2)

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What do we need to pay more attention to?

PE dominate after SEs cured (i.e. after max < 1)

– Photoelectron mitigation? How to lower reflectivity?– Maybe we can benefit from high-brightness

photocathode electron gun community to better understand emission process

– Can other PEs dominate?

How does quad-trapped EC interact with beam?

Studies of EC dynamics in quads in short-bunch e+ rings: same phenomenon as PSR?

Transition crossing, bunch length effects (RHIC, FNAL MI)

Electron-ion (background) interaction; virtual “cathode” at wall (Furman)

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Final thoughts

It would be interesting if someone critically reviewed clearing electrode experience in depth (ion/EC clearing)

It would be interesting if someone summarized EC density thresholds for different instability effects at different machines and relate to machine and surface parameters

Need to model multiple elements (quads, ejection, wiggler) rather than one at a time (Macek et al)

Future opportunities for machine studies: CesrTA, BEPC2

K. Harkay ECLOUD07 Daegu, Korea, Apr 2007

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Final thoughts

Cloud density: definition, need to be specific– Long p+ bunch, line density– Short e+ bunch, avg volume density, whole cross

section vs. only within radius r Bombardment rate unit – C/cm2 proposed by Furman Other useful figures-of-merit? Beam neutralization?

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Final thoughts – x vs. y

Monday’s discussion:– X-y coupling; horizontal modulations may drive EC

into vertical plane; may be important in colliders. Think about pumping EC out of collider interaction region (longitudinal potential well) (Heifets)

– Trapping elsewhere? (Macek) grad. B vs. EB drift

– Resonance effects in periodic lattice? (Celata) E-p FB (Macek)

– only 15-30% improvement in threshold– 2nd growth phase observed: EC in gap?– Appearance of H instab at PSR and SNS