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Electron-Cloud Buildup: Summary

Electron-Cloud Buildup: Summary

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Electron-Cloud Buildup: Summary

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  1. Electron-Cloud Buildup: Summary Miguel Furman LBNL ECLOUD’07 Daegu, April 9-12, 2007

  2. Disclaimer • This is not a comprehensive review of the talks • Rather, items that I found especially noticeable • My apologies if I don’t mention your contribution

  3. New and/or improved measurements • Ecloud now seen everywhere (may need searching) • FNAL Main Injector (MI) and Tevatron (R. Zwaska) • RFA measurements • CESR (M. Palmer) • HCX (M. Kireeff Covo, A. Molvik) • KEKB (J. Flanagan, M. Tobiyama, K. I. Kanazawa, S. Kato, M. Nishiwaki, M. Tobiyama, H. Jin, T. Ieiri) • SNS (S. Cousineau) • But not for nominal op. conditions • BEPC (Y. D. Liu) • RHIC (W. Fischer, SY Zhang) • e– flux correlated with P rise (P rise mostly due to e– desorption) • ANKA cold undulator, Bw=0 (S. Casalbuoni) • e– suspected but not conclusively implicated in heat load • PSR quads (R. Macek) • Swept electrons decay constant ~60-90 ms • Suspect significant primary electrons from beam scraping quads, then electrons ExB drift to neighboring field-free regions • Support from ORBIT code simul. (Y. Sato)

  4. Basics • Ecloud buildup is a local issue • Chamber properties (geometrical, electronic) are important • Seed (primary) electrons may be important, and may have strong local fluctuations • Effects on the beam (instabil., e growth) are global • In most cases, ecloud buildup dominated by SEY d(E) • Strong nonlinear amplification of e– density de if deff > 1 • Most of this summary talk is centered on the SEY theme

  5. SEY • Impressive progress at KEK (S. Kato, M. Nishiwaki) • Several materials • XPS analysis • e– bombardment at energy 5 keV, also 500 eV • All lead to dmax≈1 after e– bombardment, dose=0.1–1 C/cm2 !!! • Understanding of this conditioning mechanism: “graphitization” • Also measured conditioning due to ecloud in KEKB • similar results although dmax slightly >1 • In-situ SEY measurement essential • Would be nice to check conditioning by e– bombardment at 100 eV (this is more relevant to actual eclouds in accelerators) • Is graphitization seen eslewhere? (apparently not; M. Pivi, SLAC TiN conditioning) • Stainless steel sample measured: SS304; would be nice to check for SS316LN

  6. SEY remaining issues • Relative composition of e– emission spectrum • True secondaries dts(E), rediffused dr(E) and elastically backscattered de(E) • Desirable to measure emission spectrum and disentangle the 3 components • Complication: the measurement itself conditions the surface • How do these 3 components condition under e– bombardment? (probably not at the same rate) • Importance: dr(E) can have significant effect on ecloud buildup for large bunch spacing (eg., LHC) • Limit of d(E) when E<~10 eV: • Cimino-Collins: d(0)=1 for cold Cu (Appl. Surf. Sci. 235, 231 (2004)) • Most measurements (and buildup simul.) have d(0)=0.3-0.6 • d(0) controls the dissipation of the ecloud • d(0)=1 has significant adverse effect on buildup

  7. More on SEY • Grooved surface chambers • All indications are that effective SEY significantly lower than for flat surface • Recent PEP-II tests disappointing owing to confusion from photoelectrons • But little doubt that grooves decrease effective SEY • Clearing electrodes • Straightforward mechanism to reduce ecloud density • The devil is in the details: reliability, durability, impedance, % coverage of the circumference • Clearing electrodes based on enamel coatings (CERN) • what is enamel SEY? • The devil is in the details

  8. Simulations • Significant effort on ILC DR for some time now (M. Pivi) • Interesting that these simulations have influenced design decisions • Investigation at CESR-TA (M. Palmer) • Presently looking for 3D effects in ILC DR wigglers (C. Celata) • Self-consistency will come later, in stages • FNAL MI simulations (M. F.) point to dmax=1.3-1.5 • But direct sample measurement at SLAC says dmax=2 • Was sample contaminated in the trip from FNAL to SLAC ? • Emphasis nowadays is on ecloud effects on beam • Rightfully so • Assume de and see what it does to the beam • Fully self-consistent (“FSC”) simulation is formidable with current codes and computers • Large disparity of time scales, mixes “s” and “t” descriptions, possibly brings in large length scales (ORBIT; S. Cousineau) • Hope for FSC simulations: Lorentz boosted frame calculations (J. L. Vay)

  9. Simulation issues at high SEY • Virtual cathode formation: how much is real, how much numerical? • We do see them sometimes in simulations • Image forces of macroelectrons near wall can be too large  local multipacting in the absence of beam • Dependence of SEY on applied E-field (from sp. charge) not included • Macroelectron phase space “culling” algorithm needs improvement • No. of macroelectrons Me can grow dramatically  need to throw some away every once in a while • Correct solution: depopulate the macroelectron distribution while preserving the 6D physical e– phase space distribution • too slow (but parallelizable, so maybe OK) • Simplest solution (discard macroelectrons randomly) can lead to large numerical fluctuations owing to wide range of macroelectron charges • BTW, algorithms with fixed Me have the same problem (wide range of macroelectron charges)

  10. Puzzle • If dmax1 is so readily achieved by electron conditioning (new KEK results), why is ecloud still a significant operational problem in many machines? • I don’t think primary electrons are sufficient to explain problem (except possibly at KEKB: lots of photoelectrons, and PEP-II in the straights (?)) • Here are some possibilities: • Lots of electrons trapped in quads (PSR, KEKB, SPS,…?) • dmax is smaller than we think (1) but d(0) is higher than we think (also 1) • Problem: PSR swept electron signal decay in field-free regions points to d(E)0.5 at low E; but maybe does not contradict d(0)=1 • In fact, an upturn in d(E) as E0 might explain why signal decay is slower than exponential • Hadron storage rings are not as well conditioned as e+ storage rings because electron-wall impact energy is lower in the former than in the latter (for a given bunch intensity) • Additional reason to investigate e– dose conditioning at E<100 eV • My jet lag • Other …

  11. Acknowledgments • I am indebted to C. Celata, K. Harkay, R. Macek, M. Pivi, K. Sonnad, J.-L. Vay, R. Zwaska and other participants for discussions • I am grateful to the ECLOUD’07 local organizers especially Eun San Kim Kazuhito Ohmi Hitoshi Fukuma for such a pleasant and productive workshop!