Electron-cloud Effects in High-intensity Proton Accelerators - CERN

Electron-cloud Effects in High-intensity Proton Accelerators Jie Wei Brookhaven National Laboratory R. Macek Los Alamos National Laboratory CERN, April 2002

Copyright, 1996 © Dale Carnegie & Associates, Inc.

Acknowledgements • PSR colleagues – A. Browman, D. Fitzgerald, R. McCrady, T. Spickermann, T. S. Wang

• SNS colleagues – A. Aleksandrov, M. Blaskiewicz, J. Brodowski, P. Cameron, V. Danilov, D. Davino, A. Fedotov, M. Furman, S. Henderson, H. Hseuh, Y.Y. Lee, H. Ludewig, W. Meng, S. Peggs, M. Pivi, D. Raparia, P. Thieberger, S.Y. Zhang

Jie Wei, April 2002, CERN

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Outline • Phenomena • Electron generation – Injection; collimation; gas ionization; multipacting

• Electron concentration & neutralization – Trailing-edge multipacting and tune shifts – The Packman effect

• Electron-proton instabilities – Scaling law – Approaches

• Preventive measures – Minimization of electron production – Enhancement of Landau damping

• Summary Jie Wei, April 2002, CERN

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Electron-cloud phenomena: RHIC Ramp 1797 11/19/01

110-bh, 7.5e8/bh , 39 bunches injected

55-bh, 9e8/bh

Intensity Pressure rise at Bo11

• Pressure rise & interlock when the bunch spacing is halved • Total intensity reaching only 60% before pressure rise occurs (courtesy RHIC crew; S.Y. Zhang) Jie Wei, April 2002, CERN

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Electron-cloud phenomena: AGS Booster

• Vertical instability occurs only when the beam is debunched • 10% slow loss in 1 ms, then 60% fast loss in tens of µs (courtesy M. Blaskiewicz) Jie Wei, April 2002, CERN

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Electron-cloud phenomena: PSR Instability signals BPM ∆V signal CM42 (4.2 µC) (Circulating Beam Current)

Control by rf buncher voltage

Threshold Intensity ( µ C/pulse)

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• Growth time ~ 75 µs or ~200 turns • High frequency ~ 70 – 200 MHz • Controlled primarily by rf buncher voltage • Requires electron neutralization of ~ 1% (from centroid model)

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4

Historical data for good tunes

2

0 0

5

10

15

rf Buncher Voltage (kV)


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Bk86, p98

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Effects of electron clouds • Electron neutralization, tune shifts, and resonance crossing • Transverse (horizontal or vertical) instability • Associated emittance growth and beam loss • Vacuum pressure rise • Heating & damage of vacuum chamber • Interferences with diagnostics system


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Electron generation • PSR e-flux measure – High electron concentration near injection & extraction – Electron wallstriking level to be distinguished from electron neutralization level in the bunch

• Major sources of electrons in a high-intensity ring – – – –

From stripping foil (H-, H0) Proton striking collimators or aperture bottleneck Gas ionization, ion desorption, electron desorption Beam-induced electron multipacting


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Electron generation: H- (H0) injection • Stripping-foil generated electrons – Stripped electrons » 100% (H0) – 200% (H-); same γ and emittance as injecting beam » Intense, localized high-energy electrons – Foil-secondary and knock-on electrons » Low one-pass yield » Proportional to number of foil traversal (SNS: average 6 hits) – Thermionic electrons

• Back-scattered electrons PSR H0 injection with average 300 foil traversal

(courtesy M. Plum)


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Damages due to stripped electrons • PSR – 80 W beam in a small area – Needs watercooled Cu collector

• SNS – 3 kW beam in a [cm2] area – Needs special collector material » Heat resistant » Shock resistant » Light density for reduced back-scattering

PSR burn due to uncollected, stripped electrons


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Collection of stripped electrons (SNS) Tapered magnet pole Vacuum Chamber Wall Injecting H- beam

Stripped proton beam Stripping foil

e-

Stripped electrons

Electron collector Water-cooled copper block

(courtesy Y. Y. Lee, W. Meng, J. Brodowski) Jie Wei, April 2002, CERN

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Collector material test

• SNS electron collection

(courtesy J. Brodowski, C. J. Liaw)


– Special magnetic field to guide stripped electrons – Collector of carbon-carbon block on water-cooled Cu plate – Tested with touch-gun and electron welder – Confirmed with computer modeling of stress, heating, fatigue under pulsed mode 12

Electrons from collimator surface (courtesy H. Ludewig, N. Simos)

– – – – –

Designed to absorb 2 – 10 kW (0.1% -- 0.5%) proton beam loss Beam halo at large grazing angle enhances electron production Possible saw-tooth surface complicated by proton stopping distance Possible magnetized surface proposed at LANL Rely on two-stage collimation for a large impact distance


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BNL’s Tandem study on grazing angle

(courtesy P. Thieberger et al)

Measured at BNL’s Tandem for 3 ion species – Confirmed dependence on grazing angle (0 – 89o) – Confirmed benefit of sawtoothed surface – Energy dependence not verified; expecting peaked at Emax~0.7 MeV


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Ionization and desorption • Electron production due to ionization is proportional to vacuum pressure, average beam current, and ionization cross section

d 2λe ρ m βIσ ion P = dtds e

– Molecular density ρm=3.3x1022 m-3 at 300 K – Ionization cross section σion=2 Mbarn = 2x10-22 m2 – Pressure P [Torr]

• Rate of ion or electron desorption is proportional to the number of ion or electron hitting surface – resulting in pressure run-away


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Beam-induced electron multipacting • Multi-bunch multipacting – Sensitive to bunch spacing and parameter configuration (e.g., O. Grobner, 1998)

– Short-bunch regime

–

Rl ≡

bβ p

2π σ s βp

b ≥ βe

Rl ≈ 1

RHIC:

2π σ s β e

βe ≈

1 2

2e ∆V p mec 2

SNS:

∆Vp ≈

I peak Z 0 2πβ p

Rl ≈ 0.015

• Single-bunch regime “trailing-edge” multipacting (e.g., R. Macek, 1999; V. Danilov, 1999) – Insensitive to bunch spacing – PSR and SNS b 2π σ s – Long-bunch regime