Electron Cloud Build-Up: Theory and Data Miguel Furman LBNL LBNL - PowerPoint PPT Presentation

Electron Cloud Build-Up: Theory and Data Miguel Furman LBNL LBNL mafurman@lbl.gov http://mafurman.lbl.gov ECLOUD10 Workshop Cornell, 8-12 Oct, 2010 M. Furman - ECLOUD10 p. 1

Summary • What is the electron-cloud effect (ECE) • Brief history • Primary and secondary electrons • Simulations and data • Mitigation • Conclusions My apologies to the experts – this is a very basic talk Acknowledgments: I am grateful for collaboration and discussions over time with: A. Adelmann, G. Arduini, V. Baglin, S. Berg, M. Blaskiewicz, O. Brüning, Y. H. Cai, J. Calvey, F. Caspers, C. Celata, R. Cimino, R. Cohen, I. Collins, J. Crittenden, F.-J. Decker, G. Dugan, N. Eddy, A. Friedman, O. Gröbner, K. Harkay, S. Heifets, N. Hilleret, U. Iriso, J. M. Jiménez, R. Kirby, I. Kourbanis, G. Lambertson, R. Macek, A. Molvik, K. Ohmi, M. Palmer, S. Peggs, G. Penn, M. Pivi, C. Prior, A. Rossi, F. Ruggiero, G. Rumolo, D. Sagan, K. Sonnad, D. Schulte, P. Stoltz, J.-L. Vay, M. Venturini, L. Wang, S. Y. Zhang, X. Zhang, A. Zholents, F. Zimmermann, R. Zwaska,… M. Furman - ECLOUD10 p. 2

What is the ECE (illustrated with the LHC cartoon by F. Ruggiero) 25 ns 25 ns 25 ns 25 ns • Beam emits synchrotron radiation: – provides source of photo-electrons – other sources: beam-gas ionization, stray protons → wall • Photo-electrons get rattled around the chamber from multibunch passages —especially for intense positively-charged beams (e + , protons, heavy ions) • Photoelectrons yield secondary electrons – yield is determined by the secondary emission yield (SEY) function δ (E): – characterized by peak value δ max – e – reflectivity δ (0): determines survival time of e – • Typical e – densities: n e =10 10 –10 13 m –3 (~a few nC/m) M. Furman - ECLOUD10 p. 3

Consequences • Possible consequences: — single-bunch instability — multibunch instability — emittance blowup — gas desorption from chamber walls — excessive energy deposition on the chamber walls (important for superconducting machines, eg. LHC) — particle losses, interference with diagnostics,… In summary: the ECE is a consequence of the interplay between the beam In summary: the ECE is a consequence of the interplay between the beam • • and the vacuum chamber “rich physics” — many possible ingredients: bunch intensity, bunch shape, beam loss rate, fill pattern, photoelectric yield, photon reflectivity, SEY, vacuum pressure, vacuum chamber size and geometry, … The ECE is closely related to the mechanism of photo-amplifiers • * IT IS ALWAYS UNDESIRABLE IN PARTICLE ACCELERATORS * IT IS A USUALLY A PERFORMANCE-LIMITING PROBLEM * IT IS CHALLENGING TO PROPERLY QUANTIFY, PREDICT AND EXTRAPOLATE M. Furman - ECLOUD10 p. 4

More... • NOTE: if conditions are such that the bunch spacing in time is equal to the traversal time of the electrons across the chamber, you get a resonance condition • “beam-induced multipacting” (BIM) • First observed at ISR mid-70’s —Usually dramatic consequences: gas desorption —Usually dramatic consequences: gas desorption from the vacuum chamber walls —Beam is rapidly lost —Or, trigger beam abort (e.g., at RHIC) M. Furman - ECLOUD10 p. 5

Our goals… Identify the relevant variables in • each case Predict and measure • If possible, minimize the effect in the • design stages of new machines Implement mitigation mechanisms • Passive Passive • • • low-emission coatings • grooves • weak B-fields to sweep electrons Active • • Adjust the chromaticity • Feedback systems • Tailoring bunch patterns Typically, both passive and active • And wait with crossed fingers … • M. Furman - ECLOUD10 p. 6

Brief history: BCE and CE • BCE: effect first seen many years ago in proton storage rings: — two-stream instabilities (in space-charge compensated coasting beams) BINP, mid 60’s: G. I. Budker, V. G. Dudnikov, … • • ISR, early 70’s: E. Keil, B. Zotter, H. G. Hereward,… • Bevatron (LBL), early 70’s: H. Grunder, G. Lambertson… — beam-induced multipacting (ISR, mid 70’s, bunched beams) • O. Gröbner, ICHEA 1977 • multibunch effect; pressure rise instability — High-intensity instability at PSR (LANL), since mid 80’s — High-intensity instability at PSR (LANL), since mid 80’s • single-long-bunch effect • Fairly conclusively identified as an electron effect in 1991 (D. Neuffer, E. Colton, R. Macek et al.) • CE: started in early 90’s, KEK Photon Factory: — M. Izawa, Y. Sato and T. Toyomasu, PRL 74 , 5044 (1995) • First observation of instability sensitivity to beam-charge sign in a lepton ring • Electrons in the chamber were immediately suspected Quick decision to add an antechamber to the PEP-II e + ring chamber • • Caveat: an electron-beam interaction had been previously observed at CESR (J. Rogers et al; “anomalous antidamping”) M. Furman - ECLOUD10 p. 7

ECE at KEK Photon Factory Izawa, Sato & Toyomasu, PRL 74, 5044 (1995) Qualitative difference in coherent spectrum of e + vs. e – multibunch beams • under otherwise identical conditions: electron beam spectrum positron beam spectrum Fast multibunch instability for e + beam: — insensitive to “clearing gap” — sensitive to bunch spacing — electrons in the chamber were immediately suspected — first simulations: K. Ohmi, PRL 75 , 1526 (1995); “photoelectron instability” (PEI) — immediate concern for the B factories’ design M. Furman - ECLOUD10 p. 8

LHC • 1995-96: concerns that electrons would spoil LHC vacuum (based on ISR experience, O. Gröbner) • Early 1997: first simulations by F. Zimmermann that included photoelectrons showed a significant ECE — first proton machine with significant synchrotron radiation: critical energy of photon spectrum: intensity: photons/proton/bend — main concern: excessive power deposition — initial estimates: ~a few W/m, vs. 0.5 W/m cryo capacity — “LHC crash programme” started 1997 by F. Ruggiero — big simulation effort, along with measurements — conclusion: main sensitivity is SEY — current consensus: peak SEY must be <~ 1.1–1.3 to avoid the problem — we’ll know in a couple of years, when the LHC reaches nominal intensity M. Furman - ECLOUD10 p. 9

Importance of the EC • ECE has been observed at many other machines: — PEP-II, KEKB, BEPC, PS, SPS, APS, RHIC, Tevatron, MI, SNS, CESRTA … — diminished performance and/or — dedicated experiments • PEP-II and KEKB: — controlling the EC was essential to achieve and exceed luminosity goals —Antechamber: lets ~99% of photons escape — TiN coating at PEP-II: suppresses SEY —Solenoidal B-fields, B~20 G (at both machines) trap electrons near chamber surface —Solenoidal B-fields, B~20 G (at both machines) trap electrons near chamber surface —Complicated beam fill patterns were used for a while • PSR: high-current instability, beam loss − Decision to coat SNS vacuum chamber with TiN • RHIC: fast vacuum pressure rise instability at high current forces beam dump (in some fill patterns) − Not any more (TiZrV coatings suppress SEY) • Concern for future machines (LHC, ILC DR’s, MI upgrade,…) CESRTA is most significant, dedicated, systematic program to understand the ECE in e + e – rings • • Funding started ~3 yrs ago • Great progress! ECLOUD10 workshop rightfully sited at Cornell M. Furman - ECLOUD10 p. 10

Simulations of the ECE • Ideally, a single description of the combined beam+EC dynamics • Such “self-consistent codes” are maturing, but not yet ready for regular, steady use • Complicated dynamics, many variables, some more relevant than other • Slow • So, there are 2 kinds of codes typically in use: 1. Build-up codes: simulate the development of the EC by the action of a given, prescribed beam (ECLOUD, POSINST, PEI,...) prescribed beam (ECLOUD, POSINST, PEI,...) • This is the subject of this talk 2. Beam dynamics codes: simulate the dynamics of the beam by te action of a given, prescribed EC (WARP, CLOUDLAND, PEHTS, HEADTAIL,...) Typically, both approaches are good approximations (“1 st -order” approximations) • M. Furman - ECLOUD10 p. 11

Code “POSINST” features (M. Furman and M. Pivi) • Electrons are dynamical • represented by macroparticles • Beam is not dynamical • represented by a prescribed function of time and space • A simulated photoelectron is generated on the chamber surface • It is then “tracked” (F=ma) under the action of the beam • When it strikes the chamber wall, there is a probabilistic process: • Absorbed • Absorbed • Bounces elastically • Generate secondary electrons • secondary electron emission: detailed model (M. Furman & M. Pivi, PRSTAB/v5/i12/e124404 (2003)) • field-free region, dipole field, solenoidal field, others… • round or elliptical vacuum chamber geometry (with a possible antechamber) • perfect-conductor BCs (surface charges included) •EC density reaches saturation, one way or the other M. Furman - ECLOUD10 p. 12