Introductory Lecture: Overview of the Electron Cloud ... · Macek, M. Pivi) Snapshot of trapped...

Introductory Lecture:Overview of the Electron Cloud Effect in Particle Accelerators

Katherine C. HarkayElectron Cloud Workshop, Cornell, Oct. 8-12, 2010

Acknowledgements: ANL: Richard Rosenberg, Robert Kustom, John Galayda (now at SLAC); LBNL: Miguel Furman, Mauro Pivi (now at SLAC); LANL: Robert Macek; CERN: Frank Zimmermann; Cornell: Gerry Dugan, Mark Palmer, Jim Crittendon, many othersMany others at APS, BEPC, HCX, KEKB, PEPII, PSR, RHIC, SPS/LHC, etc

Outline

Definitions Early observations Electron cloud effects: what are they and who cares? Strategies for diagnostics, prediction, and control Summary

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K. Harkay Overview of Electron Cloud Effects in Accelerators Cornell U., October 8, 2010

What is an electron cloud?

In an atom: bound electrons

http://universe-review.ca/F15-particle.htm

In a particle accelerator: A low-energy background of free electrons that can build up in the vacuum chamber of a high-energy particle accelerator. If the cloud density becomes sufficiently large, the interaction between the cloud and the beam can seriously affect the particle beam.

3K. Harkay Overview of Electron Cloud Effects in Accelerators Cornell U., October 8, 2010

Electron clouds (EC) in particle accelerators Particle beam energy typically hundreds of million electron volts (MeV) to

several billion electron volts (GeV): electrons, positrons, protons, ions Average electron cloud energy typically 100 electron volts or less

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At Left: Electron cloud simulation with electrons and protons rendered as particles.

Illustrative purposes only –parameters do not correspond to a real machine

Acknowledgements: DOE SCIDAC Visualization and Analytics Center for Enabling Technologies


• Beam particle density: a million per cubic cm (e.g., APS)

• Average EC density can reach 1% or more of beam density

Where does the electron cloud come from?

Easy to generate an electron cloud: ubiquitous in the vacuum chamber

Mostly benign, too sparse to affect the beam In certain cases, the cloud builds up and wrecks havoc in

different ways


Multiple physics involved in EC generation & dynamics


Surface physics & chemistryA major primary source of the electron cloud are photoelectrons from synchrotron radiation scattering on the accelerator vacuum chamber walls. Ionization of residual gas another source.These electrons can be accelerated by the particle beam and collide with the chamber walls, producing secondary electrons. Beam losses on the walls an important source of secondaries in proton/ion accelerators.For particular beam distributions and chamber geometry, a resonance can occur that amplifies the cloud, somewhat like a photomultiplier tube.

Accelerator physicsBeam dynamicsSingle-particle dynamics governs the motion until space charge forces become important.Collective effectsCoupled-oscillations can occur that drive unstable beam motion, increase the beam size (“blow” up), and possibly break the beam apart.

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Synchrotron radiationLight sources typically operate with electron beams and have two different sources of synchrotron radiation:

bending magnets (BMs)

periodic arrays of magnets (wigglers or undulators)

Electron-positron particle colliders have BM radiation

Very high-energy proton accelerators have synchrotron radiation sufficient to produce photoelectrons.

Fig. courtesy D. Mills


Secondary electron emission, multipacting resonance

Fig. courtesy F. Ruggiero, G. Arduini

Beam-induced multipacting schematic using Large Hadron Collider (LHC) parameters (1011 protons per bunch, bunch spacing 25 ns).

Relativistic proton bunch

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Outline


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First experimental observations

Early observations of instabilities correlated with pressure attributed to EC in proton rings, coasting beam or single-bunch (BINP, CERN ISR, possibly others (ZGS, AGS, Orsay, Bevatron); ~1965-72)

Similar observations and systematic experiment study in LANL Proton Storage Ring (PSR) proton ring (~1988 – today)

First observations in positron rings– Multibunch (KEK PF, BEPC, CESR; ~1989-97)– Single-bunch (KEKB, PEP-II; 1999-2000)

[see V. Dudnikov, Proc. 2001 PAC, 1892; F. Zimmermann, PRST-AB 7, 124801 (2004)]

First detailed direct measurements of the EC distribution using dedicated diagnostics (retarding field analyzers (RFA))– Positron, electron ring (APS, 1997-99)– Proton ring (PSR, 2000)


APS RFA (Rosenberg)

“PSR instability” story

Evidence pointed to an electron-proton (e-p) instability , but many questions lead to skeptisicm

Experimental observations first, vertical instab. (~1988) Data consistent with e-p theory The problem: Where do all the electrons come from? Why doesn’t

threshold change with vacuum pressure variation? How do electrons survive the gap?

Many remained unconvinced until electron cloud measured directly with RFA, RFA sweeper (~2000)

RFA data lead to new understanding “Trailing edge multipacting” (R. Macek) Proton beam loss an important source of electrons Very low energy electrons survive the gap without beam

Story is not finished: new observations and new questions …


Outline


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Electron cloud effects: Multiple symptoms, one cause

Primary processes Beam instabilities (PSR, KEKB) Heat load (LHC, maybe ANKA)

Secondary processes Interference with standard beam diagnostics (SPS) Electron-stimulated molecular desorption, vacuum pressure rise/runaway

(RHIC, PEP-II, APS, SPS) Electron cloud trapping in magnetic fields (dipoles, quadrupoles, ion pump

fringe field, etc) (HCX, PSR, CESR)


Electron cloud effects: Who cares?

Accelerators whose performance is potentially affected: Large Hadron Collider at CERN International Linear Collider Damping Rings (ILC DR) ANKA at Karlsruhe (also APS at Argonne) Super B-Factories at KEK, Daphne Project X at Fermilab PETRA-III at DESY SNS upgrade at ORNL …

Dedicated test beds: CesrTA at Cornell for ILC DR COLDDIAG at Diamond (built at ANKA) APS, SPS (past)


Outline


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Role of EC diagnostics and simulation

Electron cloud effects are very difficult to predict: Will it be benign or will it be bad and how bad

Surface science is complex for technical materials and accelerator environment, surfaces not static

Low-energy electrons notoriously difficult to characterize –experimental uncertainties

Most advances have occurred when modeling is benchmarked against detailed measured data. Data provide realistic limits on key cloud-generation parameters for numerical modeling efforts to improve prediction capability and to guide cures Notable examples:

APS and PSR vs. POSINST HCX (at LBNL) vs. WARP/POSINST SPS (LHC) vs. ECLOUD/HEADTAIL KEKB vs. PEHT/PEHTS RHIC vs. CSEC, ECLOUD, maps

Success stories – EC cures for: LHC, SNS, JPARC, ILC, …


EC diagnostics

Electron cloud wall flux and wall-collision energy (APS, et al) Cloud density near the beam: Beam tune shift (KEKB, CesrTA) Cloud density in a local region: Transmission wave (LBNL, Cornell, CERN)


Fig. courtesy of H. Fukuma, Proc. ECLOUD’02, CERN Report No. CERN-2002-001(2002)

Fig. courtesy of F. Caspers, F. Zimmermann, Proc. 2009 PAC, Vancouver (2009).

Retarding field analyzer (RFA) (R. Rosenberg)

RFA measures distribution of EC colliding with walls, trans. eff. 50%. Energy directly related to beam interaction.

mounting on 5-m-long APS chamber, top view, showing radiation fan from downstream bending magnet. Pressure measured locally (3.5 m upstream of EA).

mounting on APS Al chamber behind vacuum penetration (42 x 21 mm half-dim.)

4.5 mm6.41.6

-300 to +60 V

+ 45V –

Multiplexer

PicoammeterRetarding Voltage

e-


[R. Rosenberg, K. Harkay, NIM-A 453, 507 (2000); K. Harkay, R. Rosenberg, PRST-AB 6, 034402 (2003)]

Time-resolved: Electron sweeper for quadrupoles (PSR)

Quadrupole pole tip

4” evacuated beam pipe

RFA Chamber

Quadrupole pole tip


Quadrupole pole tip



RFA Chamber

Schematic cross section of electron- sweeping detector for a PSR quadrupole. (Courtesy R.

Macek, M. Pivi)

Snapshot of trapped electrons in a PSR quadrupole 5 µs after passage of the beam pulse.

(Courtesy M. Pivi)

K. Harkay EC at APS Cornell, Feb 2007 19K. Harkay Overview of Electron Cloud Effects in Accelerators Cornell U., October 8, 2010

Thin RFA and shielded buttons at Cornell Electron Storage Ring Test Accelerator (CesrTA)


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Thin RFA structures were developed for use in limited aperture locations at CesrTA, for example dipole and wiggler magnet chambers (Y. Li, M. Palmer, et al.)

Prepared by M. JimenezAT Dept / Vacuum Group, ECloud’04

80 K20-30

K

Collecting plate (strips)

Cold head

Collecting stripsBeam “pipe” (< 30 K)Thermal shielding (80 K)

Experimental Set-upsCold Strip Detector (30 K)

Length of cold section = 600 mm

Prepared by M. JimenezAT Dept / Vacuum Group, ECloud’04

Motor

Moving plate

RF contacts

As seen by the beam…

Experimental Set-upsVariable Aperture Strip Detector

From 35 to 80 mm in height

COLDDIAG proposal (ANKA)

Special test chamber instrumented with RFAs and temperature sensors to be installed in-situ– Temperature -> heat load– Flux and spectrum of the low

energy electrons hitting the wall

– Pressure– Gas content

Installation planned at Diamond light source


Electron cloud modeling

First gen codes (2D analytical, PIC) developed to model EC generation and instabilities (M. Furman, K. Ohmi, F. Zimmermann at al.)

Detailed semi-empirical secondary electron emission model developed [M.A. Furman, M.T.F. Pivi, PRST-AB 5, 124404 (2002)]

Second gen codes (2-3D) developed for more realistic modeling for positron, proton, heavy ion beams (Friedman et al., Mori et al)

Accelerator lattice included Model diagnostics themselves (J. Calvey et al)


Courtesy A. Adelmann et al., ECLOUD04

Extensive benchmarking study launched 2002-2004, spearheaded by F. ZimmermannStd. params for single-bunch instab: Build-up, thresh. vary by 3-100[E.Benedetto et al., Proc. 2004 EPAC, 2502] [see also HB2006, benchmark session]


CMAD, Pivi

Strategies to control the electron cloud

Reduce primary electrons Reduce secondary emission Stabilize the unstable beam with a feedback system


Int’l R&D Effort (SLAC, KEK, CERN, LANL, Frascati): M. Pivi et al., Proc. 2005 PAC, 24; G. Stupakov, ECLOUD04

RHIC: S.Y. Zhang et al., PRST-AB 8, 123201 (2005)

Grooves, antigrazing surfaces (collimation)


TiN, NEG coatings, surface roughness

, PRST-AB 7, 093201 (2004)

Pressure-Rise Workshop (2003)

Electron emis. 1 MeV K+ ions vs. dust/bead blasting;ion range must be

Prototype fast beam feedback for e-p

PSR/LANL, SNS/ORNL, LBNL, IU, SLAC collaboration [see R. Macek, Proc. HB2006; C. Deibele THPCH13]

Grow-damp-grow measurements:1st phase: growth 1.03×104 s-1

FB damping rate: 1.75×104 s-1

2nd phase: e-p growth 3.35×104 s-1

Beam in gap believed responsible


References and workshopsReview talks at Accelerator Conferences: J.T. Rogers (PAC97), F. Ruggiero (EPAC98),

K. Harkay (PAC99), F. Zimmermann, K. Harkay (PAC01), G. Arduini, F. Zimmermann (EPAC02), M. Furman, M. Blaskiewicz (PAC03), M. Pivi, L. Wang (PAC05), K. Harkay (EPAC06) http://www.jacow.org

ICFA BD Newsletter No. 33, Apr. 2004: special edition on Electron Cloud Effects in Accelerators http://www-bd.fnal.gov/icfabd

Workshops, past: Multibunch Instabilities Workshop, KEK, 1997 KEK Proc. 97-17 Two-Stream ICFA Mini Workshop, Santa Fe, 2000

http://www.aps.anl.gov/conferences/icfa/two-stream.html Two-Stream Workshop, KEK, 2001 http://conference.kek.jp/two-stream/ ECLOUD02, CERN, 2002 http://slap.cern.ch/collective/ecloud02/ Pressure Rise Workshop, RHIC/BNL, Dec. 2003

http://www.agsrhichome.bnl.gov/AP/PressureRise/Page1.htm ICFA ECLOUD04, Napa, CA, Apr. 2004 http://www.cern.ch/icfa-ecloud04/ ICFA High Brightness Hadron Beams, KEK/JAEA, May 2006

ftp://ftp.kek.jp/kek/abci/ICFA-HB2006 ECLOUD07, Daegu, S. Korea, Apr. 2007; ICFA ECLOUD10, Cornell, Oct. 2010


ftp://ftp.kek.jp/kek/abci/ICFA-HB2006�

Summary

Electron cloud effects important in high performance rings; continue to surprise us

Understanding built from across scientific disciplines: beam physics, surface physics & chemistry, and plasma physics

Much progress on cures Surface science is complex: primary, secondary effects Benchmarking of models against measured data is critical to advance

understanding Modeling effort driving towards massively parallel 3D Much work has been done, many important results Following introductory lectures go into much more detail


Introductory Lecture:�Overview of the Electron Cloud Effect in Particle AcceleratorsOutlineWhat is an electron cloud?Electron clouds (EC) in particle acceleratorsWhere does the electron cloud come from?Multiple physics involved in EC generation & dynamics Slide Number 7Secondary electron emission, multipacting resonanceOutlineFirst experimental observations“PSR instability” storyOutlineElectron cloud effects: Multiple symptoms, one causeElectron cloud effects: Who cares?OutlineRole of EC diagnostics and simulationEC diagnosticsRetarding field analyzer (RFA) (R. Rosenberg)Time-resolved: Electron sweeper for quadrupoles (PSR)Thin RFA and shielded buttons at Cornell Electron Storage Ring Test Accelerator (CesrTA)Experimental Set-ups�Cold Strip Detector (30 K)Experimental Set-ups�Variable Aperture Strip DetectorCOLDDIAG proposal (ANKA)Electron cloud modelingSlide Number 25Strategies to control the electron cloudGrooves, antigrazing surfaces (collimation)TiN, NEG coatings, surface roughnessPrototype fast beam feedback for e-pReferences and workshopsSummary

Introductory Lecture: Overview of the Electron Cloud ... · Macek, M. Pivi) Snapshot of trapped...

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