Transcript BEAM STUDIES AT THE SNS LINAC Yan Zhang

BEAM STUDIES AT THE SNS LINAC
Yan Zhang
On behalf of the SNS team
42nd ICFA, HB2008, Nashville, USA, August 25-29, 2008
Managed by UT-Battelle
for the Department of Energy
Outline
 Introduction to the SNS Linac
 Longitudinal Beam Dynamics Studies
 Transverse Beam Dynamics Studies
 Unsolved Puzzles
 Summary
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The SNS Linac
To Ring
402.5 MHz
RFQ
MEBT
805 MHz
DTL
CCL
SCL, ß=0.61
HEBT
SCL, ß=0.81
Linac dump
Injector
2.5 MeV
86.8 MeV
186 MeV
b=0.55
391 MeV
b=0.71
1 GeV
b=0.87
 Length ~260 m, 96 independently phased RF cavity/tanks
 Normal conducting linac from the H- ion source to 186 MeV
 Superconducting linac from 186 MeV to 1 GeV
 Beam commissioning of the SCL began in August 2005
 Achieved the design repetition rate 60 Hz, maximum beam
energy 1.01 GeV, peak beam current 40 mA, pulse length 1 ms,
beam power on the mercury target 520 kW.
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Longitudinal Lattice
RMS phase in the warm linac, linear map and zero current
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Y. Zhang, S. Henderson, this proceedings.
Phase oscillation in one of the SCL commissioning lattice
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Longitudinal beam emittance increase in
the SCL commissioning lattice
0.6
x
baseline
y
z
0.5
emit. (mm*mrad)
x
commission
y
z
0.4
0.3
0.2
0
50
100
z (m)
150
IMPACT, beam current 20 mA
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200
Y. Zhang, et. al., NIM B 261 (2007)
Beam phase oscillation and damping are sensitive to RF errors
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SCL phase oscillation measured in February 2006
First cavity gradient 10% & 15% reduction, model & measurement
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A. V. Aleksandrov, et. al., EPAC2008
RF Shaker, warm linac model prediction and BPM measurement
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SCL phase oscillation, dominated by random errors (~2
phase, ~2% amplitude) and likely caused by RF driftings
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Longitudinal Beam Emittance Measurement
A. V. Feschenko, et. al., PAC2007
Beam phase profile measured with a BSM in CCL1
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Y. Zhang, et. al., submitted to PRST-AB
Model predicted the SCL
longitudinal acceptance
BCM measured acceptance
SCL longitudinal acceptance measurement with BCM and BLMs
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Bunch shape
measurement
Energy profile
measurement
Bunch shape and beam energy profile measured with BCM
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Longitudinal beam contours at the SCL entrance (Cav_01b)
RMS emittance: 2.9 MeV*deg ~ 2.2 times the nominal design
Cav_01a reference phase set at -85, -90 and -95 degree
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12/23/2007
Phase tails
> 40 deg
06/15/2008
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12/23/2007
Energy tails
> 3 MeV
06/15/2008
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Transverse Beam Dynamics
Beam emittance and twiss parameter measurement at MEBT
Multiple wire scans measure beam profiles, and fit the
measured beam RMS size with the accelerator models
•On-line model in the XAL is based on TRACE3D.
•Transverse – longitudinal phase space coupling.
•Higher order effects: emittance growth in RF gaps,
chromatic aberrations in “short” quadrupoles.
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Y. Zhang, J. Qiang, this proceedings
MEBT beam parameter measurement using the IMPACT model
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Beam Trajectory Correction
T. Pelaia, et. al., ICALEPCS 2007
Warm Linac
SCL
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A. Shishlo, A. V. Aleksandrov, EPAC2008
x
Before quads shaker
Y
x
After quads shaker model based alignment
Y
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Unsolved Puzzles
Linac residual activations after neutron productions
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SCL Longitudinal Acceptance and …
SCL Acceptance, MEBT Particles and CCL Particles
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Longitudinal emittance in the warm linac
The nc. linac could be a halo filter (scrapper) if no error existed
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RFQ Beam Tails plus Linac RF Errors ?
Beam loss in the linac, No Error, RF Error, MEBT RBs gradients
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Beam Loss In the SNS Linac (500 kW)
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Partially chopped beams ?
CCL1 BPM amplitude and phase measurement for 4 mini-pulse
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Increase the SCL acceptance ?
s = -35 (design: -20), sacrifices ~100 MeV output energy
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Summary
 Beam dynamics studies at the SNS linac are performed with
several conventional and newly developed techniques.
 Beam longitudinal tails/halo shown in the measurements, but
the exact cause of the longitudinal halo is not fully understood.
 Simulation study of halo in the linac does not have a complete
picture, might need 3D particle tracking from the ion source.
 Characterize beam halo in the order of 10-5 to 10-4 and reduce
the fractional beam loss to below 10-4 in the SCL is a challenge,
existing problems should be fixed first. E.g., MEBT rebuncher,
LEBT and MEBT beam chopper, performance of the RFQ, and
transverse beam matching through the entire linac correctly…
 Beam loss in the linac especially in the SC linac is one of the
major concerns to further ramp up the beam power, a factor of
two to three beam loss reduction is needed. Suggestions and
ideas are welcome.
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