Ring Beam Dynamics Progress Report...4 Managed by UT-Battelle for the U.S. Department of Energy...
Transcript of Ring Beam Dynamics Progress Report...4 Managed by UT-Battelle for the U.S. Department of Energy...
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Ring Beam
Dynamics Progress
Report
Sarah M. Cousineau
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Outline
1. Lattice and optics
– Twiss beta measurement
– Chromaticity measurement
2. High Intensity Effects
– Space charge
• dilution, broadening
• intensity dependent transverse coupling
– e-p instabilities
• threshold studies
• longitudinal shape studies
• production beam observations
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Beta Function Measurement
• Previous MIA based approach showed significant beta beating and large difference from design in certain regions.
• Used trim quads to do an independent measurement of beta.
4Q
kl
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Measurement of Ring Beta Function
• Deviation from design <= 15%.
• There is a discrepancy with MIA based measurement.
0
5
10
15
20
25
30
35
0 50 100 150 200 250
Bet
a (m
)
s (m)
Horizontal Beta
Model
Measured
0
5
10
15
0 50 100 150 200 250
Bet
a (m
)
s (m)
Vertical Beta Model
Measured
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Chromaticity
• Chromaticity was measured in the range of natural to zero.
• From coasting beam theory:
f
f0
p
p0
m Qf
f
f0
p
p0
m Qf
Measure independently.
Measured
via BTF
m = Mode
= Slip Factor (-0.2173)
Courtesy R. Hardin
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Chromaticity Measurement
• Measurements agree well with MAD predictions.
Courtesy R. Hardin, T. Pelaia
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Part II: Collective Effects
• Presently the ring intensity is not limited by collective effects. However, collective effects strongly influence the final particle distribution.
Begin hollow
End flat
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Working Tune Range
• Working point has varied within 0.05 range
on lower side of design.
• Broadening seen as we drive bare tune to 6.1
Design tune is
(Qx=6.23, Qy=6.20).
Vertical
• Incoherent tune shift down to integer for
working tunes.
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High Intensity Profile Dilution
• Significant profile dilution by 1e13 ppp.
• Diluted profile is more ideal for target, e.g. painting scheme is appropriate.
• No discernable tails.
Measured Profiles (March 2011, WS20)
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Emittance vs. Intensity
• rms emittance is not well correlated with profile dilution.
Horizontal Emittance vs. Intensity
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Intensity Dependent Transverse Coupling
• We observe an intensity dependent transverse coupling that effects profile shapes.
• It compromises our ability to control beam shape on target.
vertical smaller
vertical larger
Width (mm)
Vertical Profile
R. Potts, graduate student
vertical smaller
vertical larger
Horizontal Profile
Width (mm)
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Transverse Coupling and Emittance
April 2009 Production Beam
July 2009 High Intensity
Run
March 2011 Production Beam
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Recent Work on Instabilities
• Instabilities observed in the SNS ring:1) Extraction kicker transverse instability, 2) e-p, and 3) resistive wall.
• Recent instability work has focused on e-p. At SNS it does not have a clear-cut parameter dependence. Case by case variation seen for:
– Intensity threshold
– Dependence on 1st and 2nd harmonic RF
– Leading plane (horizontal or vertical)
– Trailing or leading edge instability.
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Sum
Difference
Leading versus Trailing Edge e-P
• e-P is sometimes observed on the leading edge of beam, and sometimes on the trailing edge. Sometimes both.
Distance [m]
Sum
Difference
BP
M S
ignal
Z. Liu, PhD thesis
head tail
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e-P vs. 1st
Harmonic RF
• The instability is not consistent with Landau damping laws.
Z. Liu, PhD thesis
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Effect of Bunch Shape on Instability
Change phase of 2nd
harmonic buncher
• The instability can be suppressed by creating a flatter profile.
Lower 1st harmonic
buncher by 4 kV
Bunch Shape Bunch Shape
Z. Liu, PhD thesis
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e-p Activity During Production
• Trace e-p activity sometimes observed during operation.
~90 MHz
oscillation
Long trailing edge.
100 MHz signal. < 0.5 mm
880 kW production beam on 03/17/2011
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Summary of Collective Effects
Phenomenon
Observed
During
Production?
Impact on Production Poses Problem
for 3 MW
Production
Space charge
profile broadening
Yes Makes a flatter beam on
target.
No
Transverse
coupling
Sometimes Loss of independent
control of planes.
???
Broadening due to
resonance
No ---- ???
e-p Instability Yes (trace) No impact. ???
Extraction kicker
transverse
instability
No ---- No
Resistive wall
instability
No ---- No
0
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Looking Forward
1. Work to resolve Twiss beta measurement method discrepancy. Data taken on 12-20-2011.
2. Initiate in depth study on intensity dependent transverse beam coupling. Graduate student project (R. Potts).
3. Continue to understand e-P parameter dependence.
4. Understand additional observations not included in this presentation: tune splitting in BTF measurements, benchmark discrepancies, disappearance of tune split dependence in transverse coupling…
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Auxiliary Slides
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Beta Functions: MIA vs Quad Tune Method
1. Observe a high degree of non-symmetric beta beat, large deviation from model.
Level depends on scale factor choice.
2. Ratio between straight section beta and arc beta is too low, seems unphysical.
Scale factor choice #2 – match to BPM B02
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Case 2: March 2011
Threshold
Threshold ??
Case 1: May 2010
• For some cases the presence of coupling depends only on intensity, and for
others it depends on intensity and tune split.
• The two cases above have different machine configurations, specifically the
closed orbit in the injection region.
More on Coupling