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

Transcription

1 Introductory Lecture: Overview of the Electron Cloud Effect in Particle Accelerators Katherine C. Harkay Electron 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 others Many others at APS, BEPC, HCX, KEKB, PEPII, PSR, RHIC, SPS/LHC, etc

2 Outline Definitions Early observations Electron cloud effects: what are they and who cares? Strategies for diagnostics, prediction, and control Summary 2

3 What is an electron cloud? In an atom: bound electrons 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. 3

4 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 Beam particle density: a million per cubic cm (e.g., APS) Average EC density can reach 1% or more of beam density 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,

5 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 5

6 Multiple physics involved in EC generation & dynamics Surface physics & chemistry A 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 physics Beam dynamics Single-particle dynamics governs the motion until space charge forces become important. Collective effects Coupled-oscillations can occur that drive unstable beam motion, increase the beam size ( blow up), and possibly break the beam apart. 6

7 Synchrotron radiation Light 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 Fig. courtesy D. Mills Very high-energy proton accelerators have synchrotron radiation sufficient to produce photoelectrons. 7

8 Secondary electron emission, multipacting resonance Relativistic proton bunch Beam-induced multipacting schematic using Large Hadron Collider (LHC) parameters (10 11 protons per bunch, bunch spacing 25 ns). Fig. courtesy F. Ruggiero, G. Arduini 8

9 Outline Definitions Early observations Electron cloud effects: what are they and who cares? Strategies for diagnostics, prediction, and control Summary 9

10 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); ~ ) 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; ~ ) Single-bunch (KEKB, PEP-II; ) [see V. Dudnikov, Proc PAC, 1892; F. Zimmermann, PRST-AB 7, (2004)] First detailed direct measurements of the EC distribution using dedicated diagnostics (retarding field analyzers (RFA)) Positron, electron ring (APS, ) Proton ring (PSR, 2000) APS RFA (Rosenberg) 10

11 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 11

12 Outline Definitions Early observations Electron cloud effects: what are they and who cares? Strategies for diagnostics, prediction, and control Summary 12

13 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) 13

14 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) 14

15 Outline Definitions Early observations Electron cloud effects: what are they and who cares? Strategies for diagnostics, prediction, and control Summary 15

16 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 cloudgeneration 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, 16

17 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) Fig. courtesy of F. Caspers, F. Zimmermann, Proc PAC, Vancouver (2009). 17

18 Retarding field analyzer (RFA) (R. Rosenberg) RFA measures distribution of EC colliding with walls, trans. eff. 50%. Energy directly related to beam interaction. [R. Rosenberg, K. Harkay, NIM-A 453, 507 (2000); K. Harkay, R. Rosenberg, PRST-AB 6, (2003)] mm e- + 45V Multiplexer mounting on APS Al chamber behind vacuum penetration (42 x 21 mm half-dim.) Retarding Voltage -300 to +60 V Picoammeter 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). 18

19 Time-resolved: Electron sweeper for quadrupoles (PSR) Quadrupole pole tip 4 evacuated beam pipe RFA Chamber Snapshot of trapped electrons in a PSR quadrupole 5 µs after passage of the beam pulse. (Courtesy M. Pivi) Schematic cross section of electron- sweeping detector for a PSR quadrupole. (Courtesy R. Macek, M. Pivi) K. Harkay EC at APS Cornell, Feb

20 Thin RFA and shielded buttons at Cornell Electron Storage Ring Test Accelerator (CesrTA) 2 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.) 20

21 Experimental Set-ups Cold Strip Detector (30 K) Length of cold section = 600 mm Cold head Collecting strips Beam pipe (< 30 K) Thermal shielding (80 K) 80 K K Collecting plate (strips) Prepared by M. Jimenez AT Dept / Vacuum Group, ECloud 04

22 Experimental Set-ups Variable Aperture Strip Detector Motor Moving plate RF contacts From 35 to 80 mm in height As seen by the beam Prepared by M. Jimenez AT Dept / Vacuum Group, ECloud 04

23 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 23

24 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, (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) 24

25 CMAD, Pivi Courtesy A. Adelmann et al., ECLOUD04 Extensive benchmarking study launched , spearheaded by F. Zimmermann Std. params for single-bunch instab: Build-up, thresh. vary by [E.Benedetto et al., Proc EPAC, 2502] [see also HB2006, benchmark session] 25

26 Strategies to control the electron cloud Reduce primary electrons Reduce secondary emission Stabilize the unstable beam with a feedback system 26

27 Grooves, antigrazing surfaces (collimation) Int l R&D Effort (SLAC, KEK, CERN, LANL, Frascati): M. Pivi et al., Proc PAC, 24; G. Stupakov, ECLOUD04 RHIC: S.Y. Zhang et al., PRST-AB 8, (2005) 27

28 TiN, NEG coatings, surface roughness Pressure-Rise Workshop (2003) Electron emis. 1 MeV K+ ions vs. dust/bead blasting; ion range must be << roughness, PRST-AB 7, (2004) 28

29 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: 1 st phase: growth s -1 FB damping rate: s -1 2 nd phase: e-p growth s -1 Beam in gap believed responsible 29

30 References and workshops Review 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) ICFA BD Newsletter No. 33, Apr. 2004: special edition on Electron Cloud Effects in Accelerators Workshops, past: Multibunch Instabilities Workshop, KEK, 1997 KEK Proc Two-Stream ICFA Mini Workshop, Santa Fe, Two-Stream Workshop, KEK, ECLOUD02, CERN, Pressure Rise Workshop, RHIC/BNL, Dec ICFA ECLOUD04, Napa, CA, Apr 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

31 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 31