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Transcript of Copyright, 1996 © Dale Carnegie & Associates, Inc. Beam loss & Electron-cloud in the SNS ring:...
![Page 1: Copyright, 1996 © Dale Carnegie & Associates, Inc. Beam loss & Electron-cloud in the SNS ring: Issues and remedies Jie Wei M. Blaskiewicz, P. He, H. Hseuh,](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649e535503460f94b488f1/html5/thumbnails/1.jpg)
Copyright, 1996 © Dale Carnegie & Associates, Inc.
Beam loss & Electron-cloud in the SNS ring: Issues and remedies
Jie Wei
M. Blaskiewicz, P. He, H. Hseuh, D. Raparia, L. Wang, S.Y. Zhang (BNL)
M. Pivi, M. Furman (LBNL)
R. Macek (LANL)
BNL, December 9, 2003
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Jie Wei, Dec. 2003, BNL 2
Outline
• SNS Project Overview
• SNS Ring Vacuum Parameters
• Beam Loss and Collimation
• Electron Cloud and Mitigation
• Summary
![Page 3: Copyright, 1996 © Dale Carnegie & Associates, Inc. Beam loss & Electron-cloud in the SNS ring: Issues and remedies Jie Wei M. Blaskiewicz, P. He, H. Hseuh,](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649e535503460f94b488f1/html5/thumbnails/3.jpg)
Jie Wei, Dec. 2003, BNL 3
Spallation Neutron Source complex
• A $1.4 billion, 7-year construction project due June 2006
• Collaborated by six national laboratories, built at Oak Ridge
– Argonne, Brookhaven, Jefferson, Berkeley, Los Alamos, Oak Ridge
– At 1.4 MW beam power and 1.5x1014 particle per pulse, it will a high-power, high-intensity accelerator facility
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Jie Wei, Dec. 2003, BNL 4
Evolution of the beam-power front
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Jie Wei, Dec. 2003, BNL 5
Ring 248m in circumference
Pavg < 1x10-8 Torr
HEBT
Ring
RTBT
HEBT 220 m in length Pavg < 5x10-8 Torr
RTBT 165 m in length Pavg ~10-7 Torr
EDmp
LDmp
IDmp
Colli.
Inj.
Ext.
RF
Target
MomDmp
SCL
Vacuum system parameters
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Jie Wei, Dec. 2003, BNL 6
Vacuum requirements
• Pre-injection transport line (HEBT)– Pave < 5x10-8 Torr– Required by H- stripping due to residual gas molecules (~1/2)– Beam loss limit: < 10-6 per meter
• Accumulator Ring
– Pave < 1x10-8 Torr for design intensity of 1.5x1014 proton per pulse
– Required by residual gas ionization due to proton beam, ion desorption & pressure runaway, and ionized electron production
– Capable of high-pressure operation (10-6 Torr) for beam scrubbing
• Ring-to-Target transport (RTBT)– Near ring: Pave ~ 1x10-8 Torr, to avoid interference with ring
operation– Near target: Pave < 1x10-6 Torr, for reliable operation of Harp
profile diagnostics
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Jie Wei, Dec. 2003, BNL 7
HEBT (220m, including 3 dump lines)
P < 5x10-8 Torr
For H- stripping:
σ 1/β2 ≈ 1x10-18 cm2
(40%H2/40%H2O/20%CO)
0.3 nA/m ≈ 0.3 watts/m
@ 2 mA x 1 GeV (2 MW)
<30 mR/hr @ 1 ft
(4 hrs after 100 day operation)
Acceptable for hands-on maintenance
HEBT
LDump
SCL
MomDmp
IDump
Interface to 3 dumps (2 kW – 200 kW)
SCL, ECC/ESC, Collimators, Diag.
ECC
ESC
Colli
Pre-injection transport (HEBT)
(Courtesy H. Hseuh et al)
![Page 8: Copyright, 1996 © Dale Carnegie & Associates, Inc. Beam loss & Electron-cloud in the SNS ring: Issues and remedies Jie Wei M. Blaskiewicz, P. He, H. Hseuh,](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649e535503460f94b488f1/html5/thumbnails/8.jpg)
Jie Wei, Dec. 2003, BNL 8
4 Arcs x 34m, 4 SS x 28m
isolatable by all-metal gate valves
Pavg <1x10-8 Torr
σi ≈ 6x10-19 cm2 (res. gas ionization)
for 40%H2/40%H2O/20%CO
~3x10-3 ionization/p.msec smaller than
other source of e-
TiN Coating of all chambers w/ ~100nm to reduce SEY to ≤ 1.9
Conductive coating of inj. kicker ceramic chambers (~0.04 Ω)
TiN coating of ext. kicker ferrites
Arc
Inj.
Collim.
RF+Diag.
Ext.
Arc
Arc
Arc
Accumulator ring
(Courtesy H. Hseuh et al)
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Jie Wei, Dec. 2003, BNL 9
EDmp
Target
RTBT
Kickers + Lambertso
n
Locating leaks in Target interface (Q26-Q30-target window)
No significant pressure profile due to large conductance (36cm Φ)
Locate leaks w/ remote He manifolds + RGA?
Leak check in RTBT/Target interface
(Courtesy H. Hseuh et al)
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Jie Wei, Dec. 2003, BNL 10
Primary concern: uncontrolled beam loss • Minimize uncontrolled beam loss for hands-on maintenance
– 1 Watt / meter loss of beam power– 1 mSv / hour (100 mrem/hour) activation level
• 1 W/m loss in linac; 10-3 loss in ring; >90% cleaning efficiency
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Jie Wei, Dec. 2003, BNL 11
Uncontrolled loss during normal operation
0
0.5
1
1.5
2
2.5
3
0 100 200 300 400 500 600 700 800
Length [m]
Beam
loss
[W
/m]
SCL
DTL
CCLRING
FE
HEBTRTBT
High rad areas
Predicted SNS loss distribution
Mechanism Location Fraction Power [W/m]
Ring beam halo collimator 1.9x10-3 2,000
Excited H0 at foil collimator 1.3x10-5 20
Energy straggling at foil collimator 3x10-6 4.5
H- magnetic stripping injection dipole 1.3x10-7 0.3
Nuclear scattering at foil injection foil 3.7x10-5 2.5
Collimator inefficiency all ring 10-4 0.9
(N. Catalan-Lasheras et al)
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Jie Wei, Dec. 2003, BNL 12
Source of beam loss• High radio-activation at injection, extraction,collection
– AGS: up to 10 rem/hour at localized area
• High beam loss– FNAL Booster (30 - 40%): ramp tracking, debunching-
recapturing, transition, aperture!– AGS/Booster (20 – 30%): pushing record intensity– ISIS (~15%): injection capture, initial ramp– PSR (0.3% Full energy accumulation): injection loss
(1) space-charge tune shift (0.25 or larger) & resonance crossing (2) limited geometric/momentum acceptance (3) premature H- and H0 stripping and injection-foil scattering (4) errors in the magnetic field and alignment (5) instabilities (e.g., electron-cloud instability) (6) accidental beam loss (e.g., malfunction of the ion source/linac & misfiring of ring extraction kickers) (7) beam-halo loss during fast extraction.
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Jie Wei, Dec. 2003, BNL 13
Low-loss design philosophy
• Localize beam loss to shielded area – 2-stage collimation: HEBT, Ring, RTBT– 3-step beam-gap chopping/cleaning: LEBT, MEBT, Ring
• A low-loss design– Matching between linac structures; space charge effects– Resonance minimization; Magnet field compensation &
correction– Proper lattice design with adequate aperture & acceptance– Injection painting; Injection & space-charge optimization– Impedance (extraction kicker) & instability control (e-p)
• Flexibility: – Adjustable energy (+/- 5%), Variable tunes (H 1 unit, V 3
units), flexible 3-D injection painting; adjustable collimation; foil interchange
• Accident prevention: – Design redundancy: immune to front end, linac & kicker errors
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Jie Wei, Dec. 2003, BNL 14
Normal & fault condition protection
• Linac halo: adjustable foil scraper in HEBT
• Linac energy tail: scraping at high-dispersion location in HEBT
• Linac gap residual: beam-in-gap kicker or momentum collection during initial ramping
• Linac malfunction: scraper in HEBT
• Ring halo: two-stage collimation
SNS ring and transport
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Jie Wei, Dec. 2003, BNL 15
Beam-loss localization Ring primary scraper
• “Sacrifice” collimation region for the rest
• Two-stage system, efficiency above 90%
• Needs a large vacuum-pipe aperture and a long straight section
collimator in HEBT
(Courtesy H. Ludewig et al)
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Jie Wei, Dec. 2003, BNL 16
Secondary collector design• Length enough to stop primary protons (~ 1 m for 1
GeV beam)
• Layered structure (stainless steel particle bed in borated water, stainless steel blocks) to shield the secondary (neutron, )
• Fixed, enclosing elliptical-shaped wall for operational reliability
• Double-wall Inconel filled with He gas for leak detection
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Jie Wei, Dec. 2003, BNL 17
Ring Lattice FODO arcs & doublet straights
• Matched, hybrid lattice– FODO arc:
easy-to-implement correction system, moderate magnet strength
– Doublet straight: long, uninterrupted straight
» Improved collimation efficiency
» Robust injection
• Zero-dispersion injection– Independent painting in
the transverse & longitudinal directions
![Page 18: Copyright, 1996 © Dale Carnegie & Associates, Inc. Beam loss & Electron-cloud in the SNS ring: Issues and remedies Jie Wei M. Blaskiewicz, P. He, H. Hseuh,](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649e535503460f94b488f1/html5/thumbnails/18.jpg)
Jie Wei, Dec. 2003, BNL 18
Remote handling
Remote vacuum clamp
• Overhead, around-the-ring crane• Quick handling fixtures incorporated
into shielding/absorber design• Remote vacuum clamps; remote
water fittings• Passive dump window & change
mechanism
Collimator remote water fitting
HEBT collimator & shielding
(Courtesy G. Murdoch et al)
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Jie Wei, Dec. 2003, BNL 19
Quick disconnect flanges, clamps & seals
250mm to 360mm Φw/Helicoflex Seal· Diamond gasket w/ internal springs· Flange has O-ring groove · Low sealing torque 20 ft-lbs· ~ 6mm gap for seal insertion· Easy assembly by one person· ~ 4 minute assembly time· Light weight Al chain
250mm to 360mm Φ w/ Al diamond· Seal w/ knife edge, Al Chain ~ 8 lbs· Low sealing torque 22 ft-lbs· ~ 6mm gap for seal insertion· Moderately difficult for one person· ~ 5 minute assembly time · Flange surface & seal may be damaged
<250mm Φ w/ CFX Flanges/ Chain· Durable Cu seal, SS Chain· No knife edge on seal or flanges· Medium sealing torque 62 ft-lbs· ~ 3mm gap for seal insertion· ~ 5 minute assembly by one person· Moderately heavy chain ~ 22 lbs
Gaols: Reliable, ease of assembly, light weight, low torque, cost
(Courtesy H. Hseuh et al)
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Jie Wei, Dec. 2003, BNL 20
Major sources of electron cloud• Proton beam loss, especially
at a shallow grazing angle
• Stripped & scattered electrons
• Gas ionization
• Beam-driven multipacting(courtesy P. Thieberger et al)
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Jie Wei, Dec. 2003, BNL 21
Electrons from collimator surface
– Designed to absorb 2 – 10 kW (0.1% -- 0.5%) proton beam loss– Possible saw-tooth surface complicated by proton stopping distance
Rely on two-stage collimation for a large impact distance Use clearing solenoids
(courtesy H. Ludewig, N. Simos)
![Page 22: Copyright, 1996 © Dale Carnegie & Associates, Inc. Beam loss & Electron-cloud in the SNS ring: Issues and remedies Jie Wei M. Blaskiewicz, P. He, H. Hseuh,](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649e535503460f94b488f1/html5/thumbnails/22.jpg)
Jie Wei, Dec. 2003, BNL 22
Stripped electrons (Meng, Jackson, Brodowski, Lee, Abell …)
(injection chicane #2)
simulation
measurement
– 2 kW of stripped electrons must be properly collected
– Carbon-Carbon block on water-cooled Cu plate; reduced backscattering
– Window/video monitor– Tapered magnet ends– Dedicated clearing electrode (10
kV)
![Page 23: Copyright, 1996 © Dale Carnegie & Associates, Inc. Beam loss & Electron-cloud in the SNS ring: Issues and remedies Jie Wei M. Blaskiewicz, P. He, H. Hseuh,](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649e535503460f94b488f1/html5/thumbnails/23.jpg)
Jie Wei, Dec. 2003, BNL 23
Ionization and desorption
• Electron production due to ionization is proportional to vacuum pressure, average beam current, and ionization cross section
– Molecular density m=3.3x1022 m-3 at 300 K
– Ionization cross section ion=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
e
PI
dtds
d ionme
2
![Page 24: Copyright, 1996 © Dale Carnegie & Associates, Inc. Beam loss & Electron-cloud in the SNS ring: Issues and remedies Jie Wei M. Blaskiewicz, P. He, H. Hseuh,](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649e535503460f94b488f1/html5/thumbnails/24.jpg)
Jie Wei, Dec. 2003, BNL 24
Beam-induced multipacting
Captured electron
Long proton bunch (~170m)
Secondary electrons
Tertiary electrons….
Vacuum Chamber Wall
Lost proton
net
en
erg
y g
ain
proton-electron yield
e -
Head of proton bunch:captures electrons
Tail of proton bunch:repels electrons
Repelled electron
electron-electron yield
(R. Macek, D. Danilov, M. Furman, M. Pivi …)
![Page 25: Copyright, 1996 © Dale Carnegie & Associates, Inc. Beam loss & Electron-cloud in the SNS ring: Issues and remedies Jie Wei M. Blaskiewicz, P. He, H. Hseuh,](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649e535503460f94b488f1/html5/thumbnails/25.jpg)
Jie Wei, Dec. 2003, BNL 25
Effects of electron clouds in SNS Ring• 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
![Page 26: Copyright, 1996 © Dale Carnegie & Associates, Inc. Beam loss & Electron-cloud in the SNS ring: Issues and remedies Jie Wei M. Blaskiewicz, P. He, H. Hseuh,](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649e535503460f94b488f1/html5/thumbnails/26.jpg)
Jie Wei, Dec. 2003, BNL 26
Tune-shift & the Pacman effect• Electron neutralization
causes positive tune shifts on trailing-edge particles
• For given space-charge tune spread at injection (typically 0.2), the electron tune-shift if enhanced by factor 2 -- may be important at a high injection energy
• May keep on losing trailing-edge particle upon resonance crossing – the Pacman effect
• Require detailed evaluation of neutralization level in proton beam
(courtesy A. Fedotov)
imagee
yxyxyxf
pscyx A
B
RNrf
2,
2,
0,
11
2
![Page 27: Copyright, 1996 © Dale Carnegie & Associates, Inc. Beam loss & Electron-cloud in the SNS ring: Issues and remedies Jie Wei M. Blaskiewicz, P. He, H. Hseuh,](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649e535503460f94b488f1/html5/thumbnails/27.jpg)
Jie Wei, Dec. 2003, BNL 27
Simulation of electron production
• SNS: electron-cloud tune-shift ~ 0.4 e,peak: +0.04 (~ 0.4?)
(courtesy M. Pivi, M. Furman)
within vacuum pipewithin vacuum pipe within beamwithin beam
![Page 28: Copyright, 1996 © Dale Carnegie & Associates, Inc. Beam loss & Electron-cloud in the SNS ring: Issues and remedies Jie Wei M. Blaskiewicz, P. He, H. Hseuh,](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649e535503460f94b488f1/html5/thumbnails/28.jpg)
Jie Wei, Dec. 2003, BNL 28
Mitigation measures• Suppress electron production
– Tapered magnets for electron collection near injection foil; back-scattering prevention
– TiN coated vacuum chamber to reduce multipacting– Striped coating of extraction kicker ferrite (TiN)– Beam-in-gap kicker to keep a clean beam gap (10-4) – Good vacuum (5x10-9 Torr or better)– ports screening, step tapering; BPMs as clearing electrodes– Install electron detectors around the ring– Two-stage collimation; winding solenoids in the straight
section
• Enhance Landau damping– Large momentum acceptance with sextupole families; high
RF voltage; momentum painting– Inductive inserts to compensate space charge– Reserve space for possible wide band damper system
![Page 29: Copyright, 1996 © Dale Carnegie & Associates, Inc. Beam loss & Electron-cloud in the SNS ring: Issues and remedies Jie Wei M. Blaskiewicz, P. He, H. Hseuh,](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649e535503460f94b488f1/html5/thumbnails/29.jpg)
Jie Wei, Dec. 2003, BNL 29
Goal: low SEY, good adhesion and low outgassing Q
Use Magnetron DC with permanent magnets
higher sputtering rate due to dense plasma
Bake & coat @ 250 C to ~ 100 nm of TiN
to minimize impurity & improve adhesion
Need uniform gas flow along the length
to get correct stoichiometry (0.95 – 1.03)
Surface coating
• SEY of SS > 2.5• TiN coated at low pressure ~ 1.9 – 2.2• TiN coated at high pressure ~ 1.5 – 1.8
SEY of SNS coating samples measured by CERN & KEK
(Hseuh, He, Todd, Hilleret, Sato …)
magnets w/ spacersTi
cathode
N2 distribution line
![Page 30: Copyright, 1996 © Dale Carnegie & Associates, Inc. Beam loss & Electron-cloud in the SNS ring: Issues and remedies Jie Wei M. Blaskiewicz, P. He, H. Hseuh,](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649e535503460f94b488f1/html5/thumbnails/30.jpg)
Jie Wei, Dec. 2003, BNL 30
Electron-confining solenoids
• Using solenoids to reduce electron multipacting in straight sections (collimation section)
• Efficiency studied
-100 -80 -60 -40 -20 0 20 40 60 80 100-100
-80
-60
-40
-20
0
20
40
60
80
100
X [mm]Y
[mm
]
0 10 20 30 40 5010
-3
10-2
10-1
100
101
102
Bz [Gauss]
Ele
ctr
on
de
nsity [n
C/m
]
Opposite Polarity Configuration
Equal Polarity Configuration
(courtesy L. Wang)
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Jie Wei, Dec. 2003, BNL 31
Clearing electrodes
• Under strong beam potential (~10 kV), how does weak clearing field perform?
• Floating-ground BPM serve as clearing electrodes (up to +/- 1 kV)
• Dedicated electrode (10 kV) at injection
0 1000 2000 3000 4000 5000 60000
2
4
6
8
10
12
14
16
18
20
Clearing Voltage [V]
Eele
ctr
on L
ine D
ensity (
nC
/m)
Average densityDensity within beam
(courtesy L. Wang, P. Cameron)
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Jie Wei, Dec. 2003, BNL 32
Acknowledgements
• SNS colleagues
• A. Aleksandrov, J. Brodowski, P. Cameron, N. Catalan-Lasheras, S. Cousineau, V. Danilov, D. Davino, A. Fedotov, S. Henderson, N. Hilleret, Y.Y. Lee, H. Ludewig, W. Meng, M. Plum, F. Ruggiero, N. Simos, P. Thieberger, F. Zimmermann
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Jie Wei, Dec. 2003, BNL 33
Summary
• In a high-intensity ring like SNS, beam loss is of primary concern. Multi-location, two-stage beam collimation plays a crucial role
• Electron cloud is one of the intensity limiting mechanisms at the SNS ring. Mitigation measures like surface coating, solenoid confinement, and electrode clearing are expected to be highly effective