Plans for HG Breakdown Effort at LBNL

Transcription

1 Plans for HG Breakdown Effort at LBNL Miguel A. Furman Center for Beam Physics LBNL US Collaboration on High-Gradient Research SLAC, May 23-25, 2007 US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 1

2 Proposed Plans Plans is an overstatement We are newcomers to the HG field Our expertise is in simulated beam-plasma interactions In particular the electron-cloud effect (ECE) I am here mostly to collect information & invite commentary Interest re-awakened at LBNL: CLIC new baseline frequency=12 GHz, G=100 V/m CTF3 to be commissioned in 2008, with major testing in 2009 June workshop at CERN Two-Beam Accelerator (TBA) idea originated at LBNL (Sessler, 1982) US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 2

3 Tentative Plan Expand our tools used for ECE simulations Do systematic benchmarking calculations vs. observations Use simulations as a tool for understanding the physics Possible studies of surface and material properties (later on, if advisable): Various metals and coatings, state of conditioning Effect on electron emission/multipacting Possible partners: Tech-X Corp. (simulations -- we have an ongoing profitable collaboration) Ion Beam Technology group (LBNL --surface physics) Heavy Ion Fusion group (LBNL --simulations) But: we have no free energy at the moment We would need new funding US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 3

4 Possible codes POSINST Electron-cloud build-up code 2D Beam induced multipacting WARP/POSINST Space-charge-dominated beams+electrons+ionization 3D Self-consistent beam-environment interaction LORENTZ-HF (?) (IES Technologies) 2D and 3D versions available EM fields + ray tracing Multipacting US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 4

5 LBNL expertise in ECE simulations Code POSINST: >10 years experience (M. Furman and M. Pivi) 2D code: electron build-up due to beam-induced multipacting Successfully tested against measurements at APS (ANL) and PSR (LANL) Used to simulate many other machines, and benchmarked against other codes Features: Detailed model for secondary electron emission Allows modeling of various materials and states of conditioning Beam-electron interactions (multibunch passages, arbitrary bunch pattern) Chamber geometry (rectangular or elliptical) Space-charge forces (electron-cloud self-forces) Several sources of primary electrons POSINST needs to be augmented for HG studies: Fully 3D External time-dependent EM fields Gas desorption, ionization, surface heating, US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 5

6 LBNL expertise in ECE simulations Code WARP merged with POSINST Fully 3D, self-consistent (beam+electrons+em fields) Initially developed for space-charge-dominated beams e.g., heavy-ion beams for heavy-ion fusion applications Collaboration with HIF group, UCB (J. Verboncoeur), and TechX Features: PIC code with AMR and variable step size Arbitrary external EM fields Arbitrary chamber shape Various electron sources: SEY, ionization of gas, gas desorption, etc Excellent validation at HCX facility (LBNL) Low-energy, long-pulse K + beams Currently being augmented to simulate ILC DR e-cloud, esp. in wigglers US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 6

7 Validating POSINST at the PSR: e signal at the wall vs. time Single bunch; bunch length ~60 m Main e signal: trailing edge multipacting ED42Y electron detector signal 8μC/pulse beam 435 μa/cm 2 (simulation input) electron signal (δ max =2.05) measured (R. Macek) simulated (M. Pivi) US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 7

8 120 Validating POSINST at the APS: Time-averaged e signal at the wall vs. bunch spacing 100 APS, positron beam Detector Current vs. Bunch Spacing aver. electron-wall current [na/cm 2 ] region of BIM s B =d 2 /(r e N), b<d<a (10 bunches, 2 ma/bunch in all cases; measurements courtesy K. Harkay, ANL) beam-induced multipacting is a big effect! train of 10 bunches σ z =5 mm measured simulated bunch spacing [RF buckets] US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 8

9 Validating WARP/POSINST at HCX facility Q1 0V 0V 0V/+9kV 0V (a) Q2 (b) Q3 (c) Q4 e - WARP-3D T = 4.65 s Potential contours WARP-3D T = 4.65 s Electrons 200mA K + I (ma) 200 ma K (c) Simulation Experiment time ( s) 6. Electrons bunching Oscillations ~6 MHz signal in (C) in simulation AND experiment Beam ions hit end plate 1. Importance of secondaries - if secondary electron emission turned off: I (ma) (c) time ( s) 6. Simulation Experiment 2. run time ~3 days - without new electron mover and MR, run time would be ~1-2 months! US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 9

10 Recap We have good expertise in beam-electron-surface modeling We have strong collaborations We d like to increase theoretical understanding of HG breakdown via detailed simulations and benchmarking We need to: Exercise and extend the 3D aspects of code Add various effects: 3D EM fields, surface heating, gas desorption, Put it all together We are open to any suggestions US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 10

11 Additional material Simulated electron-cloud movies for the LHC 2D projection in an arc dipole (strong vertical B field) 3D self-consistent simulation of an entire FODO cell (with PBC s in the z dimension for simplicity) US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 11

12 LBNL expertise in ECE simulations (contd.) POSINST based on a detailed secondary emission model M.Furman and M. Pivi, PRST-AB 5, (2002) Phenomenological model for the SEY δ(e 0,θ 0 ) and spectrum dδ /de Model parameters obtained from fits of δ(e 0,θ 0 ) and dδ /de to data Simulation: use a probabilistic technique to generate n electrons consistent with δ(e 0,θ 0 ) and dδ /de (E=secondary e energy) E0 E1 E2 En This is an event-by-event simulation.. event = e -wall collision sample δ(e 0,θ 0 ) and dδ /de US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 12

13 LBNL expertise in ECE simulations (contd.) Application to the LHC Main issue: power deposition by electrons on the chamber Cryogenic system needs to dissipate it! Extensive effort at CERN and elsewhere since 1997 Lessons learned: Complicated system, many parameters (chamber geometry, electronic properties of surface, beam intensity, beam time structure, conditioning effects, ) Vary a few parameters at a time until you understand the various pieces of physics need for methodical simulation approach and systematic benchmarking Incident-angle dependence of δ(e 0,θ 0 ) is essential Emission electron spectrum dδ /de is essential (significant role of rediffused electrons) Beam-induced multipacting is important even if not perfectly resonant Surface conditioning is well supported by observations fairly well characterized, if not fully understood Current wisdom: δ max needs to be <~1.3 at nominal beam specs. US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 13

14 Sample simulated movie of electron cloud LHC arc dipole, bunch spacing=25 ns, N b =10 11 p/bunch, E=7 TeV Dipole field B y =8.4 T Video decompressor Video decompressor bunches # 1-11 bunches # [movie credits: Vernon Chaplin (summer student, 2005)] US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 14

15 WARP/POSINST LHC simulation movie courtesy Jean Luc Vay, HIF group One FODO cell in the arcs (~100 m long) Self-consistent, 3D Animation decompressor US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 15

16 WARP+POSINST code structure WARP ion PIC, I/O, field solve f beam, Φ, geom. f b,wall f b,wall n b, v b Reflected ions f b,wall ions emisssion from walls gas module gas transport ambient wall electron source volumetric (ionization) electron source ioniz. charge exch. Φ n e electron dynamics (full orbit; interpolated drift) sinks Key: operational; implemented, testing; partially implemented; active offline development US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 16

17 The HCX driver for HIF E~1-1.8 MeV K + ions N~10 13 /pulse T~4 μsec ~11 m US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 17

18 HCX instrumentation to carry out electron cloud (and gas desorption) experiments Slits & Faraday Cup Electrostatic transport QI7-10 D2 e - clearing Electrodes (+9 kv) MA1 MA2 MA3 MA4 D-end a b c e - suppressor (-10 kv) Optical Diagnostics Magnetic transport 1 MeV, 0.18 A, t 5 μs, 6x10 12 K + /pulse Slits, Faraday cup, Optical Diag. Diagnostics Inside beam tube: capacitive monitors, e probes, tiltable target beam Gas-Electron Source Diagnostic (GESD) US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 18

19 What is the ECE? Step 1: beam produces primary electrons Photoelectrons, ionization of residual gas, stray beam particles striking the chamber, Step 2: electrons get rattled around the chamber Amplification by secondary electron emission Particularly intense for positively-charged beams Possible consequences: dipole 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, The ECE is a consequence of the interplay between the beam and the vacuum chamber beam intensity, bunch shape, fill pattern, photoelectric yield, photon reflectivity, secondary emission yield (SEY), vac. chamber size and geometry, US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 19

20 Importance PEP-II and KEKB: controlling the EC was essential to achieve luminosity performance ECE limits performance of PSR at high current RHIC: vacuum pressure instability a high current Possibly serious in future machines: LHC: potentially large energy deposition from electrons need to dissipate it otherwise, less-than-nominal performance ILC DR s: potential for instability and/or emittance growth main concern: wiggler regions MI upgrade: N b x5; recently begun to investigate US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 20

21 Observations ECE has been observed at many machines: PF, PEP-II, KEKB, BEPC, PS, SPS, APS, PSR, RHIC, Tevatron, MI, SNS undesirable effects on performance, and/or dedicated experiments Old effects: two-stream instabilities (BINP, mid 60 s) beam-induced multipacting (ISR, mid 70 s) multibunch effect pressure rise instability trailing-edge multipacting (PSR, since mid 80 s) single-long-bunch effect beam loss and instability US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 21

22 Controlling the ECE Add weak solenoidal fields (~20 G) confines electrons near the chamber, away from the beam used in PEP-II and KEKB RHIC tests Tailor the bunch fill pattern (gaps in train) used at PEP-II for a while, before solenoids Modify vacuum chamber geometry antechamber (eg., PEP-II) antigrazing ridges (tests at RHIC) grooves (LHC arcs; tests at SLAC) Lower the SEY coatings (TiN, TiZrV, ) PEP-II, LHC, SNS, RHIC, conditioning US High-Gradient Collab. Mtg. SLAC, May M. Furman, p. 22