1. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 Frank Zimmermann  introduction  electron build up  pressure rise  heat load & scrubbing  instabilities  incoherent effects  simulation needs Overview of LHC Electron-Cloud Effects & Present Understanding

2. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 introduction CERN ISR (70s) & KEK PF (late 80s) experience → 1997: 1st LHC ECLOUD simulation, crash program 1999: e- cloud seen with LHC beam in SPS, PS & even PS- SPS transfer line 1999: e- cloud at both B factories ~2002: e- cloud at RHIC & Tevatron → observed in all proton rings with LHC-like parameters (though for 1/5 LHC bunch charge or 10x bunch spacing) 2004: DAFNE, 2006: CESR ?: SNS & J-PARC truly astonishing if this problem will not occur in LHC

3. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 blue: e-cloud effect observed red: planned accelerators

4. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 e - build up

5. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 schematic of e- cloud build up in LHC arc beam pipe, due to photoemission and secondary emission [F. Ruggiero]

6. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 LHC strategy against electron cloud 1) warm sections (20% of circumference) coated by TiZrV getter developed at CERN; low secondary emission; if cloud occurs, ionization by electrons (high cross section ~400 Mbarn) aids in pumping & pressure will even improve 2) outer wall of beam screen (at 4-20 K, inside 1.9-K cold bore) will have a sawtooth surface (30  m over 500  m) to reduce photon reflectivity to ~2% so that photoelectrons are only emitted from outer wall & confined by dipole field 3) pumping slots in beam screen are shielded to prevent electron impact on cold magnet bore 4) rely on surface conditioning (‘scrubbing’); commissioning strategy; as a last resort doubling or tripling bunch spacing suppresses e-cloud heat load unique vacuum system!

7. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 R. Cimino, I. Collins, 2003; CERN-AB-2004-012 probability of elastic electron reflection seems to approach 1 for zero incident energy and is independent of  * max yield

8. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 data from SLAC: R.E. Kirby, F.K. King, “Secondary Emission Yields from PEP-II Accelerator Materials”, NIM A 469, 2001 dependence of secondary emission yield on impact angle  Copper - different surface finish and surface chemistry - large variation in behavior, CERN data not available model

9. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 [M. Furman, 1997] Present Model of Secondary Emission Yield secondary electrons consist of true secondaries and elastically reflected; since 2003 we assume that elastic reflection is independent of  (no data) [Kirby, 2001; Henrist, 2002; Furman, 1997] true secondaries: elastic reflection: [Cimino, Collins, et al., 2003] this quantum-mechanical formula fits the data well for E 0 ~150 eV M. Furman includes rediffused electrons and finds that they increase the heat load by 100%

10. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 R=1, Illustration of present secondary-yield model

11. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 e- cloud diagnostics @ SPS Collecting strips Beam “pipe” (< 30 K) Thermal shielding (80 K) cold strip detector Motor Moving plate RF contact s variable aperture strip detector shielded pick ups quadrupole strip detectorCOLDEX + WAMPAC1-4 + pick-up calor. + SD1-2 + RGAs… J.M.Jimenez, V. Baglin, N. Hilleret et al. in-situ  max

12. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 benchmarking ECLOUD code with SPS measurements surface conditions (  max, R) and detector properties are uncertain constrain parameters by benchmarking multiple measurements change distance between trains & use relative measurements two different bunch train spacings two different pressures (40 ntorr and 4 ntorr) Daniel Schulte ECLOUD simulation

13. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 three curves intersect at  max =1.35, R=0.3; flux at later times (  =0.3 mA)  max =1.2 was reached flux: (1) ratio 1&2 trains, (2) two spacings, (3) absolute Daniel Schulte note: results sensitive to pressure, chamber geometry, etc., variation:  max ~1.4-1.3 R~0.1-0.7 ECLOUD simulation

14. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 pressure rise

15. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 vacuum pressure rise warm cold measured e-flux pressure rise observations RHIC SPS dipole field no field TEVATRON threshold ~4x10 10 ppb vacuum increase in most straights

16. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 e- flux@wall vs. intensity, 25 ns spacing, ‘best’ model R=0.5 calculation for 1 batch  max =1.7  max =1.5  max =1.3  max =1.1 vacuum pressure with electron cloud 17 hr running at 3 mA/m gives CO pressure corresponding to 100-hr beam lifetime (N. Hilleret, LHC MAC December 2004) ECLOUD simulation

17. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 desorption yield strongly bound molecules, varies with e- dose!, cleaning rate is a function of material, cleanliness, temperature “recycling desorption yield”, varies with surface coverage, pressure, sticking coefficient usually BS pumping speed hole pumping surface coverage e- flux Vincent Baglin in equilibrium: Vincent Baglin; see W. Turner, PAC93

18. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 AT-VAC (V.Baglin, N.H.) has simulated the LHC pressure evolution. According to Noel’s lab measurement, for E> 30 eV, the e- recycling yield is large. Therefore, under electron bombardment the BS will have a bare surface without any monolayers. Monolayers will be only on the cold bore. 1 monolayer = 1E15 molecules/cm2 N. Hilleret, LHC MAC Dec 2004 Information from Vincent Baglin pressure effects 1 min 17 hours for 2E16 e/m/s i.e. 3 mA/m  max ~1.3 1 hour

19. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 Pressure increase due to e-cloud. Level is a linear function of the electron flux. It depends only on the electron dose for 2E16 e/m/s i.e. 3 mA/m 1 min 17 hours (assuming 2 stripes of 3 mm each) N. Hilleret, LHC MAC Dec 2004 Information from Vincent Baglin e- flux dose 100-hr lifetime H 2 100-hr lifetime CO 2  and cleaning rate a depend on the e- energy; if the energy decreases from 300 eV down to 100 eV, the eta decreases by a factor 3, similarly, the cleaning rate decrease as well. V.B. expects the pressure of Noel's plot will be about the same for 300 eV or 100 eV. 1 hour shortest lifetime ~ 10 hr

20. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 heat load

21. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 arc heat load vs. intensity, 25 ns spacing, ‘best’ model calculation for 1 train R=0.5 computational challenge! higher heat load for quadrupoles in 2 nd train under study  max =1.7  max =1.5  max =1.3  max =1.1  max =1.3-1.4 suffices BS cooling capacity injection low luminosity high luminosity ECLOUD simulation

22. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 heat load in COLDEX (prototype LHC vacuum chamber in the SPS) heat load - constant !? (possibly consistent with conditioned state) threshold at ~7x10 10 p/bunch V. Baglin favored interpretation: very fast conditioning (?) estimated SEY simulated heat load

23. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 is “scrubbing” needed in LHC?  still lacking experimental data, e.g., on  max  uncertainty in heat load prediction of factor ~2  also incomplete understanding of scrubbing (COLDEX data vs. prediction, RHIC, DAFNE)  if  max ~1.3 reached in commissioning, no scrubbing is needed for heat load and fast instabilities  pressure should be ok too according to N. Hilleret  one concern: long-term emittance growth and poor lifetime (observed in SPS after scrubbing)  we still believe we need to prepare a scrubbing strategy in case it turns out to be necessary to go to  max ~1.3 (e.g., tailor train spacings & train lengths at nominal bunch intensity)

24. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006  max =1.7  max =1.5  max =1.3  max =1.1 nominal N b stability limit at injection the challenge is to decrease  max to 1.3 with a stable beam nominal filling pattern top energy ECLOUD simulation

25. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 instabilities

26. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 INP Novosibirsk, 1965 Argonne ZGS,1965 BNL AGS, 1965 Bevatron, 1971ISR, ~1972PSR, 1988 AGS Booster, 1998/99 KEKB, 2000 CERN SPS, 2000

27. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 coupled-bunch instability extrapolating instability threshold from SPS to LHC electrons protons SPS: 26 GeV/c,  ~40 m; LHC: 450 GeV/c,  ~100 m → CBI is ~7 times weaker in LHC

28. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 single-bunch “TMC” instability fast  growth above e- density threshold; slower  growth below    = 1 x 10 11 m -3    = 2 x 10 11 m -3    = 3 x 10 11 m -3 “Transverse Mode Coupling Instability (TMCI)” for e- cloud (  >  thresh ) Long term emittance growth (  <  thresh ) E. Benedetto LHC, Q’=0, at injection

29. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 estimate of threshold density pinch enhancement assume only vertical pinch second term is much larger → synchrotron tune changes if  z and  || are held constant → TMCI e- threshold density

30. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 attempt at extrapolating TMCI threshold from SPS to LHC using analytical estimate SPS: C~6900 m,  z ~0.3 m,  ~40 m, 26 GeV/c,  c ~1.8x10 -3 LHC: C~26700 m,  z ~0.011 m,  ~100 m, 450 GeV/c,  c ~3.2x10 -4 → threshold LHC ~ 1/3 threshold SPS → threshold LHC ~ 1/2 threshold SPS without pinch enhancement factor H:

31. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 simulated emittance growth vs. electron density no field dipole no field dipole SPS 26 GeV/c LHC 450 GeV/c rise time ~1/s threshold LHC ~ SPS E. Benedetto fast growth slow growth HEADTAIL simulations

32. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 incoherent effects

33. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 KEKB: Emittance increase with current below threshold & reduced luminosity w/o instability sidebands RHIC: transverse instabilities, emittance growth, and beam loss, especially near  t SPS: Poor beam lifetime & bunch-length shrinking after scrubbing TEVATRON: Fast beam emittance growth and short beam lifetime observed simultaneously with the ECE pressure rise. But little coherent motion seen on Schottky monitor. Longitudinal quadrupole oscillation. Experimental Indications

34. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 KEKB e+ beam blow up, 2000 (H. Fukuma, et al.) threshold of fast vertical blow up slow growth below threshold? beam current IP spot size

35. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 Fourier power spectrum of KEKB BPM data LER single beam, 4 trains, 100 bunches per train, 4 rf bucket spacing Solenoids off: beam size increased from 60  m ->283  m at 400 mA No excitation V. TuneSideband Peak J. Flanagan et al.

36. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 KEKB Sidebands and Spec. Lum. Sidebands disappear at around a bunch current of 0.8 mA. Specific luminosity of 2- bucket and 4-bucket spacing bunches do not merge at that point, however. –Possible indication of the presence of an incoherent component below the sideband threshold (non- linear focusing by cloud leading to non-Gaussian beam tails, e.g.) Sideband Peak Height Specific Luminosity 4-bucket spacing 2-bucket spacing Sideband Threshold J. Flanagan et al.

37. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 evolution of longitudinal profile during beam loss near   t RHIC beam loss at transition J. Wei

38. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 TEVATRON emittance growth >34  mm mrad/hr (> 100% hr); beam lifetime ~24 hr (normally ~1000 hr) vertical emittance vs. timevertical Schottky power vs. time Schottky power -12 dBm (normal instability signals between 0 and 10 dBm) X.L. Zhang et al.

39. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 poor lifetime in the SPS after scrubbing Poor beam lifetime with LHC beam in the SPS on August 13, 2003 (can it be explained by electron cloud?) Courtesy G.Arduini at 26 GeV/c lifetime 10-20 minutes, decreasing along bunch train not a problem per se in SPS, but it would be in LHC at injection origin not understood

40. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 poor lifetime in the SPS after scrubbing, cont’d two nominal batches at 26 GeV/c, 225 ns spacing between batches; bunch intensity in storee-cloud on shielded pick up J.M. Laurent, J.M. Jimenez, ~2002 E. Shaposhnikova et al., 11/11/04 both patterns are similar and show similar dependence on batch spacing

41. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 typical “TMCI” instability threshold at injection e- central density vs. N b, 25 ns spacing R=0.5 calculation for 1 train  max =1.7  max =1.5  max =1.3  max =1.1 challenge: how to go from  max =1.7 to 1.3? scrubbing should be done at nominal N b (stripes) ECLOUD simulation

42. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 simulated e- density evolution during a bunch passage in an LHC field-free region on log scale E. Benedetto HEADTAIL code bunch tail

43. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 e- density on horizontal axis at different time steps during a bunch passage, for the LHC E. Benedetto high local density, high tune shift, varying with x,y,z

44. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 average e- density inside circle of variable radius E. Benedetto HEADTAIL code bunch tail high local density, high tune shift, varying with x,y,z  rotation in e- phase space

45. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 tune footprint obtained by tracking through a frozen e- potential at z=+2  z by a frequency-map analysis of HEADTAIL simulation E. Benedetto, Y. Papaphilippou

46. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 e- distribution in dipole measured by SPS strip detector approximation for HEADTAIL code E. Benedetto

47. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 e- density evolution in a dipole field SPS LHC x y E. Benedetto

48. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 evolution of on-axis e- density for the SPS   E. Benedetto

49. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 simulated emittance growth for 1 and 10 e-beam Interaction points per turn with & w/o synchrotron motion E. Benedetto HEADTAIL code  =2x10 11 m -3

50. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 horizontal invariant of a proton vs. turn number G. Franchetti, E. Benedetto HEADTAIL code two mechanisms: resonance crossing and trapping → halo growth linear motion may become unstable → core growth TsTs

51. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 incoherent emittance growth due to e- cloud simulated either by HEADTAIL (weak-strong mode) or by analytical field model G. Franchetti, E. Benedetto → emittance growth is not a numerical artifact → analytical model allows accessing longer time scale

52. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 vertical phase space and frequency spectrum of particle motion at different z positions along the bunch E. Benedetto, G. Franchetti single interaction point  =10 14 m -3 linear instability, hyperbolic fix point chaotic motion

53. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 simulation needs

54. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 e-cloud build up code e-cloud SB/ CB instability code self-consistent code optics code e.g., MADX beam sizes apertures, B fields, … cloud density, local growth rates, around the ring or ‘from DR to IP’ (M. Pivi), “ECLOUD TWISS TABLE”, incl. 3D e- motion wake/impedance code, e.g., HFSS, MAFIA, GdfidL E(x,t), B(x,t) ‘ecloud wake’, generalized impedance ion code vacuum code ionization E(x,t) ion desorption, ionization electron desorption, scrubbing, “e- pumping” regular instability code beam- beam code s.c. code ee beam motion, losses future ‘complete’ e-cloud simulation? CARE-HHH-2004

55. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 summary & conclusions  simulations based on SPS benchmarking lead to optimistic heat load prediction;  max ~1.3 sufficient to reach nominal & ultimate (  max ~1.3 was obtained in SPS after ~1-2 days at 25-ns spacing)  fast instabilities also under control for  max ~1.3  ~5x10 11 m -3, but slow  growth <1%/s !?!  uncertainties: (1) LHC vacuum chamber is different from SPS; COLDEX either shows no conditioning or it conditions too fast to notice (2) RHIC, Tevatron & KEKB experience (3) poor lifetime in SPS resembling e-cloud build up pattern (4) dynamic vacuum & detector background in LHC  incoherent slow emittance growth remains concern we identified two mechanisms causing halo or core blow up: periodic crossing of resonance or unstable region may explain

56. Frank Zimmermann, LHC Electron Cloud, GSI Meeting 30.03.2006 thanks to Gianluigi Arduini, Vincent Baglin, Giulia Bellodi, Elena Benedetto, Giuliano Franchetti, Noel Hilleret, Bernard Jeanneret, Miguel Jimenez, Laurent Tavian, Kazuhito Ohmi, Francesco Ruggiero, Giovanni Rumolo, Daniel Schulte, Elena Shaposhnikova, Jie Wei, and Xiaolong Zhang for important contributions & discussions & help