Electron Cloud Build-Up: Theory and Data

Transcription

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

2 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

3 What is the ECE (illustrated with the LHC cartoon by F. Ruggiero) 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 = m 3 (~a few nc/m) M. Furman - ECLOUD10 p. 3

4 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 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

5 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 from the vacuum chamber walls Beam is rapidly lost Or, trigger beam abort (e.g., at RHIC) M. Furman - ECLOUD10 p. 5

6 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 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

7 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 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

8 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

9 LHC : 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 <~ to avoid the problem we ll know in a couple of years, when the LHC reaches nominal intensity M. Furman - ECLOUD10 p. 9

10 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 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

11 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,...) 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

12 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 Bounces elastically Generate secondary electrons secondary electron emission: detailed model (M. Furman & M. Pivi, PRSTAB/v5/i12/e (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

13 Secondary e emission: two essential ingredients E0 E1 E2.. En Note: δ=1 means one e in, one e out (1) (2) =SEY=no. of emitted electrons per incident electron (incident energy, angle) =emitted electron energy spectrum Secondary emission is an event-by-event simulation: event=one electron-wall collision instantaneous generation of n secondaries (or absorption) detailed phenomenological model for δ(e 0,θ 0 ) and dδ/de model parameters obtained from simultaneous fits to bench measurements for δ and dδ/de for Cu, St.St., Al and TiN some parameters not well-known M. Furman - ECLOUD10 p. 13

14 Two sample measurements of SEY 2.0 Copper sample (Hilleret data) 2.0 Stainless steel sample (data R. Kirby) 1.5 fit (Furman-Pivi) measured data 1.5 measured data (R. Kirby) model fit (Furman-Pivi) E0tspk= dtspk= powts= E0ts=0 P1epk= P1einf=0.02 E0epk=0 powe=1 E0w= P1rinf=0.2 Ecr= qr= Cu E0ts=0 E0tspk=310 dtspk=1.22 powts=1.813 P1epk=0.5 P1einf=0.07 E0epk=0 powe=0.9 E0w=100 P1rinf=0.74 Ecr=40 qr= St. steel E0 [ev] E0 [ev] caveat: samples not fully conditioned! (N. Hilleret; R. Kirby) M. Furman - ECLOUD10 p. 14

15 Sample spectrum: dδ/de Three main components: elastics, rediffused, true secondaries St. St. sample, E 0 =300 ev, normal incidence, (Kirby-King, NIMPR A469, 1 (2001)) Secondary energy spectrum St. St., E0=300 ev, normal incidence true secondaries (area[0,50]=1.17) rediffused (area[50,295]=0.75) backscattered (area[295,305]=0.12) st. steel sample δ = 2.04 δ e = 6% δ r = 37% δ ts =57% δ e +δ r =43% Cu sample δ = 2.05 δ e = 1% δ r = 9% δ ts =90% δ e +δ r =10% Secondary electron energy [ev] 300 Hilleret s group CERN: Baglin et al, CERN-LHC-PR 472. Other measurements: Cimino and Collins, 2003 M. Furman - ECLOUD10 p. 15

16 Simulated movie, CESRTA field-free region, 10 bunch passages M. Furman - ECLOUD10 p. 16

17 Simulation vs. experiment at CESRTA (G. Dugan) GeV tune shift data-central density 0.75 ma/bunch POSINST simulation- Al chamber, peak SE energy 310 ev, SEY=1.8 Technique: measure bunch tune shift roughly EC density 10-bunch train, followed by a witness bunch M. Furman - ECLOUD10 p. 17

18 Simulated movie, PSR field-free region, 2 bunch passages M. Furman - ECLOUD10 p. 18

19 PSR: benchmark code POSINST Bunch length >> t a portion the EC phase space is in resonance with the bounce frequency trailing edge multipacting (Macek; Blaskiewicz, Danilov, Alexandrov, ) ED42Y electron detector signal 8µC/pulse beam 435 µa/cm 2 electron signal (δ max =2.05) measured (R. Macek) simulated (M. Pivi) M. Furman - ECLOUD10 p. 19

20 Simulated movie, LHC external dipole bending field High-density regions form where E w (x)=e max called stripes (F. Zimmermann) M. Furman - ECLOUD10 p. 20

21 Controlling the ECE Modify the vacuum chamber geometry (suppress both photoemission and SEY) add an antechamber (PEP-II: let photons escape) add transverse grooves (eg., LHC beam screen: suppress photoemission by ~x2) add longitudinal grooves (SLAC tests): suppress effective SEY (~x2) Modify the vacuum chamber electronic properties: low-sey coatings TiN (PEP-II, SNS) TiZrV (RHIC and LHC RT regions requires activation), Amorphous carbon coating (under tests at CERN) Note: most coatings require activation to become effective Clearing electrodes Use solenoidal B-fields (~20 G) confines electrons near the chamber, away from the beam used extensively at KEKB and PEP-II significant improvement in performance Tailor the bunch fill pattern add strategic gaps in the train Use feedback systems to actively counteract instabilities that arise M. Furman - ECLOUD10 p. 21

22 Conditioning effect of SEY The SEY usually dominates the EC build-up But, the SEY naturally decreases with electron bombardment self-conditioning effect Clearly seen in many cases Q: 1) is it fast enough? (Y) 2) does it go far enough? (N?) Copper sample: note δ(0) 1 consequences of fish hook not fully explored But known to be unfavorable because δ(0) controls the dissipation rate of the EC Evidence from PSR that δ max, but δ(0) remains constant Copper SEY (CERN) (R. Cimino and I. Collins, proc. ASTEC2003, Daresbury Jan. 03) M. Furman - ECLOUD10 p. 22

23 Conclusions The ECE is an ubiquitous phenomenon for intense beams spans broad range of charged-particle machines It is important inasmuch as it limits the machine performance Especially for high-intensity future machines It is interesting, as it involves in an essential way various areas of physics: Surface geometry and surface electronics Beam intensity and particle distribution Beam energy Residual vacuum pressure Certain magnetic features of the storage ring Simulation codes are getting better and better in their detailed modeling capabilities Enormous progress has been made since 1994 With a disproportionate credit due to CESRTA over the past ~3 years Better and more refined e detection mechanisms Simulation codes are getting better and better calibrated against measurements Phenomelogical rules of thumb are appearing that tell you when the ECE is serious But not when it s weak and safe But mysteries remain... Not a year has gone by without a couple of big surprises I encourage workshop speakers to emphasize the flies in the ointment M. Furman - ECLOUD10 p. 23

24 In closing... Thanks to our Cornell colleagues, especially to Mark Palmer, for organizing this workshop I look forward to lively and productive discussions THANK YOU FOR YOUR ATTENTION M. Furman - ECLOUD10 p. 24

25 Backup material M. Furman - ECLOUD10 p. 25

26 Secondary e emission: effective SEY if δ eff >1: N e ~exp(t/τ) EC density grows exponentially until space-charge limit close to beam neutralization level if δ eff <1: N e ~exp( t/τ) walls are net absorber of electrons EC density saturates when no. of emitted primaries=no. of absorbed e exponential decay is seen upon beam extraction What is δ eff? δ eff is a complicated function of N b, bunch fill pattern, bunch shape, vacuum chamber material, chamber geometry, δ eff is not known a priori M. Furman - ECLOUD10 p. 26

27 Conditioning effects: beam scrubbing PSR prompt e signal (BIM) is subject to conditioning: signal is stronger for st.st. than for TiN sensitive to location and N signal does not saturate as N increases up to ~8x10 13 conditioning: down by factor ~5 in sector 4 after few weeks (low current) PSR swept e signal is not: signal saturates beyond N~5x10 13 electron decay time τ 200 ns, independent of: N location conditioning state st. st. or TiN Tentative conclusion: beam scrubbing conditions δ max but leaves δ(0) unchanged M. Furman - ECLOUD10 p. 27

28 BIM in the APS: benchmark code POSINST APS, positron beam Detector Current vs. Bunch Spacing aver. electron-wall current [na/cm 2 ] (10 bunches, 2 ma/bunch in all cases; measurements courtesy K. Harkay, ANL) region of BIM s B =d 2 /(r e N), b<d<a measured measured simulated Simulated (code POSINST) bunch spacing s B [RF buckets] e + beam, 10-bunch train, field-free region (Furman, Pivi, Harkay, Rosenberg, PAC01) M. Furman - ECLOUD10 p. 28

29 Lowering the SEY Low-SEY coatings - TiN (used in PEP-II, SNS; tested at PSR) - TiZrV: studied at CERN fully suppresses multipacting after activation (SPS tests) used in RHIC warm sections ( works better than solenoids ) will be used in LHC warm straights drawback: cannot be used in cold regions (needs activation ~ C) SEY decreases with e bombardment: scrubbing self-conditioning effect SPS ECE studies: ~5 years of dedicated EC studies with dedicated instrumentation scrubbing very efficient; favorable effects seen in: vacuum pressure in-situ SEY measurements electron flux at wall M. Furman - ECLOUD10 p. 29

30 Results for e line density vs. t (one turn) M. Furman - ECLOUD10 p. 30

31 MI: sample time-averaged EC density 2 MI_1p3_6_spc1-K 1.2x y [cm] cm** x [cm] M. Furman - ECLOUD10 p. 31

32 Conclusions for FNAL MI There seems to be a critical value N b ~1.25x10 11 at which the EC grows exponentially and reaches saturation ( beam neutralization level) within ~110 ns this assumes a specific model for the SEY, and δ max =1.3 also assumes a drift section of the MI What to do next: vary δ max; find N b as a function of δ max look at different models of SEY look at magnetic sections (dipoles, quads) vary s b (?) study effects of EC on beam this is outside the scope of POSINST For a full 3D self-consistent simulation, see seminar by Jean-Luc Vay next week here (almost ready for quantitative predictions) M. Furman - ECLOUD10 p. 32

33 MI: preliminary results for a drift section Choose E=8 GeV (f=1.2% of beam lost during t inj =0.4 s): (assumes η eff =100 e/p, from PSR experience) Assume T=305 K, P=20 ntorr, σ i =2 Mbarns: Assume δ max =1.3, model K (from fits to old St.St. SLAC data; see PRSTAB/v5/i12/e (2003)) M. Furman - ECLOUD10 p. 33

34 Calculated azimuthal distribution of photons (from G. Dugan) P=0.75 P=0.5 P=0.25 P=±1 P=0 P=-0.75 P=-0.5 P=-0.25 vac. chamber cross section x-axis: P= scaled perimeter, from -1 to 1 M. Furman - ECLOUD10 p. 34

35 EC formation: seed or primary electrons Three main primary electron processes: photoelectrons residual gas ionization beam-particle losses Instead of use = no. of e generated per proton per meter of beam traversal (units m 1 ) N b = bunch population Y eff = eff. quantum efficiency (e yield per γ) σ i = ioniz. cross-section per beam particle P = vac. pressure, T = temperature η eff = eff. e yield per proton-wall collision n pl = beam particle loss rate per unit length per beam particle M. Furman - ECLOUD10 p. 35

36 LHC EC power deposition (F. Zimmermann - ECLOUD 02) Sensitive to model for secondary emission (peak SEY, spectrum, fraction of elastics/rediffused/true secondaries) M. Furman - ECLOUD10 p. 36

37 EC dissipation after beam extraction simplest analysis beam has been extracted, or gap between bunches field-free region, or constant B field assume monoenergetic blob of electrons neglect space-charge forces 2b N N and τ = dissipation time simulations show that this formula works to within ~20% If not monoenergetic and not along a straight line, then where K=f(angles) M. Furman - ECLOUD10 p. 37

38 EC dissipation in PSR after beam extraction Sweeping e detector measures electrons in the bulk τ 200 ns δ eff 0.5 if E = 2 4 ev since δ eff δ(0), you infer δ(0) well supported by simulations (see next slide) (measurements by Macek and Browman) (PAC03, paper RPPB035) M. Furman - ECLOUD10 p. 38

39 EC dissipation after beam extraction: PSR simulation 1000 EC line density vs. time (field-free region) PSR simulation field-free section, N=5e13 p loss rate=4e-6/m, yield=100 e/p PSRdissip3 NB: primary e rate is 100 x nominal 100 aver. neutralization level line density [nc/m] 10 1 EC line density beam line density slope = 200 ns exponential decay (slope=2e-07 s) input SEY: δ max = 1.7 δ(0) = x10-6 time [s] M. Furman - ECLOUD10 p. 39

40 MI: beam neutralization factor vs. N b M. Furman - ECLOUD10 p. 40

41 Sensitivity to relative ratios of δ e, δ r and δ ts : case of LHC power deposition vs. time (LHC arc dipole) 800 LHC arc dipole simulation: electron-cloud power deposition photoelectrons: outer edge only n' e(γ) =6.3e-4 e/m, δ max = aver. power deposition on [W/m] beam signal (arb. units) Copper Stainless steel Copper, true sec. only Aver. power deposition in 0.5<t<1.2 µs copper: 11 W/m st. st.: 152 W/m copper, TS only: 2.1 W/m. δ e +δ r = 43% δ e +δ r = 0 δ e +δ r = 10% x10-6 time_sm [s] x10-6 time_sm [s] M. Furman - ECLOUD10 p. 41

42 EC in the LHC (contd.) Later in 1997 it became apparent, both from CERN and LBNL simulations, that the main concern for the LHC is the energy deposition by the electrons on the vacuum chamber screen LHC is first storage ring ever in which this is a potential problem Initial estimates for heat load were ~several W/m Exceeds the available cooling capacity of the LHC cryogenic system. Cryogenic system was designed before the effect was discovered At face value, would have to cut N b or increase s b by factors of ~a few to accommodate heat load operational limitation! This was the motivation of the Electron-Cloud Crash Program at CERN And of the LARP involvement in LHC EC research M. Furman - ECLOUD10 p. 42

43 More history: EC in the LHC : concerns from the EC on LHC vacuum by O. Gröbner based on ISR experience Early 1997: first simulations by F. Zimmermann that included photoelectrons showed a significant ECE; concern about electron energy deposition LHC is the 1st proton machine in which synchrotron radiation is significant: critical energy of photon spectrum: intensity: photons/proton/bend lots of photoelectrons! at 7 TeV The ECE in the LHC is dominated by secondary electron emission, not by the photoelectrons M. Furman - ECLOUD10 p. 43

44 EC formation: beam-induced multipacting (BIM) γ or p e e e e train of short bunches, each of charge Q=NZe, separated by s b t = e chamber traversal time b = chamber radius (or half-height if rectangular) The parameter defines 3 regimes: If G = 1 and δ eff > 1, EC can grow dramatically (O. Gröbner, ISR; 1977) M. Furman - ECLOUD10 p. 44

45 PSR Layout PSR Layout ROED1 Merging Dipole StripperFoil H- Beam Matching Section Final Bend Extraction Line Skew Quad C Magnets BumpMagnets H-/H0 DumpLine Circumference = 90m Beam energy = 798 MeV Revolution frequency =2.8 MHz Bunch length ~ 250 ns (~63 m) Accumulation time ~ 750 ms ~2000 turns ED92 ED02 ED52 ED /17/00 RJM_ICANS-XV.ppt M. Furman - ECLOUD10 p. 45

46 PSR instability (R. Macek) BPM V signal CM42 (4.2 µc) (Circulating Beam Current) (200 µs/div) Growth time ~ 75 µs or ~200 turns High frequency ~ MHz Controlled primarily by rf buncher voltage M. Furman - ECLOUD10 p. 46

47 SPS spectrum (K. Cornelis, ECLOUD02) M. Furman - ECLOUD10 p. 47