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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
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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
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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
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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
K. Harkay Overview of Electron Cloud Effects in Accelerators Cornell U., October 8, 2010
• Beam particle density: a million per cubic cm (e.g., APS)
• Average EC density can reach 1% or more of beam density
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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
5K. Harkay Overview of Electron Cloud Effects in Accelerators Cornell U., October 8, 2010
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Multiple physics involved in EC generation & dynamics
6K. Harkay Overview of Electron Cloud Effects in Accelerators Cornell U., October 8, 2010
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
K. Harkay Overview of Electron Cloud Effects in Accelerators Cornell U., October 8, 20107
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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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K. Harkay Overview of Electron Cloud Effects in Accelerators Cornell U., October 8, 2010
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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
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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)
K. Harkay Overview of Electron Cloud Effects in Accelerators Cornell U., October 8, 201010
APS RFA (Rosenberg)
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“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 …
K. Harkay Overview of Electron Cloud Effects in Accelerators Cornell U., October 8, 201011
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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
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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)
13K. Harkay Overview of Electron Cloud Effects in Accelerators Cornell U., October 8, 2010
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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)
14K. Harkay Overview of Electron Cloud Effects in Accelerators Cornell U., October 8, 2010
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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
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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, …
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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)
K. Harkay Overview of Electron Cloud Effects in Accelerators Cornell U., October 8, 201017
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).
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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-
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[R. Rosenberg, K. Harkay, NIM-A 453, 507 (2000); K. Harkay, R. Rosenberg, PRST-AB 6, 034402 (2003)]
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Time-resolved: Electron sweeper for quadrupoles (PSR)
Quadrupole pole tip
4” evacuated beam pipe
RFA Chamber
Quadrupole pole tip
4” evacuated beam pipe
Quadrupole pole tip
4” evacuated beam pipe
4” evacuated beam pipe
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
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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.)
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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
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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
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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
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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)
K. Harkay Overview of Electron Cloud Effects in Accelerators Cornell U., October 8, 201024
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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]
K. Harkay Overview of Electron Cloud Effects in Accelerators Cornell U., October 8, 201025
CMAD, Pivi
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Strategies to control the electron cloud
Reduce primary electrons Reduce secondary emission Stabilize the unstable beam with a feedback system
K. Harkay Overview of Electron Cloud Effects in Accelerators Cornell U., October 8, 201026
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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)
K. Harkay Overview of Electron Cloud Effects in Accelerators Cornell U., October 8, 201027
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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
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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
K. Harkay Overview of Electron Cloud Effects in Accelerators Cornell U., October 8, 201029
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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
K. Harkay Overview of Electron Cloud Effects in Accelerators Cornell U., October 8, 201030
ftp://ftp.kek.jp/kek/abci/ICFA-HB2006�
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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
K. Harkay Overview of Electron Cloud Effects in Accelerators Cornell U., October 8, 201031
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