Electron Cloud Studies at KEKB
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Transcript of Electron Cloud Studies at KEKB
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Electron Cloud Studies at KEKB
J.W. Flanagan
2007.1.30 @ Cornell LEPP
Also: K. Ohmi, E. Benedetto, H. Fukuma, Y. Funakoshi, S. Hiramatsu, T. Ieiri, H. Ikeda, K. Kanazawa, Y.Ohnishi, K. Oide, E. Perevedentsev, M. Tobiyama, S. Uehara, S. Uno, S.S. Win, Others…
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Overview
• Brief summary of beam measurements made at KEKB, in particular as they may relate to ones which can be made at CESR– Size (Fukuma)– Tune shift (Ieiri)– Coupled-bunch mode spectra (Tobiyama)– Single-bunch head-tail sideband signal
• Focus on this – would like to try to see at CESR
– Luminosity-related measurements
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Electron cloud-induced beam blow-up
• Vertical beam blow-up has been observed at KEKB LER (positron ring) at bunch currents above a threshold of ~0.35 mA/bunch, at a spacing of 4 rf buckets (~8 ns).
• Blow-up due to photo-electrons kicked from wall by synchrotron radiation, which form clouds that interact with the positron beam.
• Bunch-current blowup threshold can be raised by the use of solenoids around the beam pipe.– Currently about 95% of drift space in LER is cover
ed.
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LER Electron-cloud suppression solenoids
Fukuma et al., “Study of Vertical Beam Blow-up in KEKB LER,” HEAC01 proceedings
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Blow-up measured by SR interferometer
Fukuma et al., “Study of Vertical Beam Blow-up in KEKB LER,” HEAC01 proceedings
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Bunch-by-bunch beam size along train as measured by gated camera
Fukuma et al., “Study of Vertical Beam Blow-up in KEKB LER,” HEAC01 proceedings
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Gated Camera Observations of Trailing Witness Bunch
J.W. Flanagan et al., Proc. EPAC00 (2000) 1119
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Detection of photoelectrons at wall
Ohnishi et al., “DETECTION OF PHOTOELECTRON CLOUD IN POSITRON RING AT KEKB,” HEAC01
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Tune Shift MeasurementsT.Ieiri, et al.,
PRST-AB 5, 094402 (2002)
Train Bunches Witness Bunches
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Tune Shift after Train
-0.015
-0.01
-0.005
0
0.005
0 10 20 30 40 50 60
Shift_Distance
dn/dc_V_380
CD
TS
(/m
A)
Distance in Bucket
w/o Solenoids
0
0.002
0.004
0.006
0.008
0.01
0.012
0.2 0.4 0.6 0.8 1 1.2
L/tune_400
Vp_808
Vp_805
Vp_802
Vp_799
Verti
cal T
une
Shift
Bunch Current (mA)
-0.001
0
0.001
0.002
0.003
0.004
0.005
0.2 0.4 0.6 0.8 1 1.2
L/tune_400
Vp_900Vp_856Vp_826
Vert
ical
Tun
e Sh
ift
Bunch Current (mA)
0
0.002
0.004
0.006
0.008
0.01
0 10 20 30 40 50 60
DistanceH1_380V1_380
Tun
e S
hift
Distance in Bucket
T.Ieiri
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Width of Tune Spectrum
Wm Wb2 Wc
2 W02
Wb: Radiation damping + Head-tail damping (Chromaticity, Impedance) + Beam-Beam + Electron CloudWc: Feedback damping + Current ripple + Nonlinear magnetic fieldW0: RBW (Resolution Band Width)
d f 3dB
Damping Rate is proportional to Frequency Width
Spectrum Width depends on various effects
Wec Wm2 Wm0
2
T.Ieiri
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Width vs Bunch Current w/o SolenoidsI_train=380 mA
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0 0.2 0.4 0.6 0.8 1 1.2
VGamma_20060224
Gamma_60Gamma_09
Gam
ma_
09
Ib_(mA)
0
0.005
0.01
0.015
0.02
0.025
0 0.2 0.4 0.6 0.8 1 1.2
VGamma_20060224
Gamma_30Gamma_60
Gam
ma_
30
Ib_(mA)
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0 0.2 0.4 0.6 0.8 1 1.2
VGamma_20060224
Gamma_60Gamma_12
Gam
ma_
12
Ib_(mA)
0
0.005
0.01
0.015
0.02
0.025
0 0.2 0.4 0.6 0.8 1 1.2
VGamma_20060224
Gamma_60Gamma_06
Gam
ma_
06
Ib_(mA)
0
0.005
0.01
0.015
0.02
0.025
0 0.2 0.4 0.6 0.8 1 1.2
VGamma_20060224
Gamma_60Gamma_03
Gam
ma_
03
Ib_(mA)
ξx=1.7ξy=4.8
電流バンチと Cloud 密度 増加で width 減少
T.Ieiri
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Beam spectrum measurements• Bunch Oscillation Recorder
– Digitizer synched to RF clock, plus 20-MByte memory.
– Can record 4096 turns x 5120 buckets worth of data.
– Calculate Fourier power spectrum of each bunch separately.
• Inputs:– Feedback BPMs
• 6 mm diameter button electrodes
• 2 GHz ( 4xfrf) detection frequency, 750 MHz bandpass
– Fast PMT
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• Abstract : • A coupled bunch instability caused by an electron cloud has been
observed in the KEKB LER. The time evolution of the instabilities just after the turning-off the transverse bunch feedback was recorded with several weak solenoid-field conditions, which are used to suppress the vertical blowup of the beam size due to the electron cloud.
• The mode spectra and their growth rates of the coupled-bunch instabilities were compared with simulations of electrons moving in drift space, a weak solenoid field and a strong bending field.
• Mode spectra without a solenoid field support the model where the instability is dominated by the electron clouds in the drift space with a lower secondary yield of photoelectron 2,max = 1.0 rather than 1.5. With the solenoid field, the behavior of unstable modes and the growth rate with the strength of solenoid field also support the simulation with lower secondary emission yield.
Coupled-Bunch Measurements M. Tobiyama et al.,
PRST-AB 9, 012801 (2006)
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Analysis Procedure
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Horizontal Mode Spectra
Bsol=0% Bsol=10%
Bsol=20% Bsol=100%
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Horizontal: Time evolution
Bsol=0%Bsol=10%
Bsol=20% Bsol=100%
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Horizontal: Effect of Chromaticity
x=-0.3 x=-0.7
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Vertical Mode Spectra
Bsol=0% Bsol=10%
Bsol=20% Bsol=100%
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Vertical: Time Evolution
Bsol=0% Bsol=10%
Bsol=20%Bsol=100%
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Vertical: Effect of Chromaticity
y=5.2 y=4.2
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• Vertical betatron sidebands found at KEKB which appear to be signatures of fast head-tail instability due to electron clouds.– J.W. Flanagan, K. Ohmi, H. Fukuma, S. Hiramatsu, M. Tobiyama
and E. Perevedentsev, PRL 94, 054801 (2005) • Presence of sidebands also associated with loss of lumin
osity during collision.– J.W. Flanagan, K. Ohmi, H. Fukuma, S. Hiramatsu, H. Ikeda, M.
Tobiyama, S. Uehara, S. Uno, and E. Perevedentsev, Proc. PAC05, p. 680 (2005)
• Further studies have been performed:– Single beam studies:
• Varying RF voltage • Varying chromaticity• Varying initial beam size below blow-up threshold (emittance)
– In-collision studies:• Looking at specific luminosity below sideband appearance threshold• Looking at specific luminosity closer to head and tail of LER bunch
Single-bunch measurements: synchro-betatron sidebands
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Fourier power spectrum of BPM data
• LER single beam, 4 trains, 100 bunches per train, 4 rf bucket spacing• Solenoids off: beam size increased from 60 m ->283 m at 400 mA• Vertical feedback gain lowered
– This brings out the vertical tune without external excitation
V. Tune Sideband Peak
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Bunch 1
Bunch 2
Bunch 3
Bunch 4
Bunch 5
Bunch 6
Bunch 7
Bunch 8
Spectra of Bunches 1-10
Bunch 9
Bunch 10
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Synchro-betatron sideband characteristics
• Sideband appears at beam-size blow-up threshold, initially at ~+s, with separation distance from increasing as cloud density increases.
• Sideband peak moves with betatron peak when betatron tune is changed.
• Sideband separation from changes with change in s. (Flanagan et al., EPAC06)
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Model focusing wake
Mode spectrum using model wake and airbag charge distribution.
Sideband-betatron peak separation dependence on synchrotron tune reproduced.
=wR/2Q
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Simulations of electron cloud induced head-tail instabilityE. Benedetto, K. Ohmi
Tail of train
Head of train
Betatron sideband
Head-tail regime
Incoherent regime
Simulation (PEHTS)
(HEADTAIL gives similar results)
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Feedback does not suppress the sideband
• Bunch by bunch feedback suppresses only betatron amplitude.
Sideband signal is Integrated over the train
Betatron sideband
Simulation (PEHTS)
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PMT Memory (BOR)
Beam Image
Black Cardboard
Partially block beam image with black cardboard, and measure lightintensity of the visible part with aPMT. The PMT signal is bufferedand then recorded using a feedbackBOR digitizer/memory board.
PMT setup
y: 100 mV/divisionx: 10 ns/division
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LER Beam Current (mA)
Vertical beam
size at IP
(m
)
Beam Blow-up Measurement4-bucket spacing, 600 bunches
4 trains, 150 bunches/train, 4 rf bucket spacing
Blow-up threshold250 mA, 0.42 mA/bunch
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PMT Spectra FB BPM Spectra
450 mA, 4.9 mm
300 mA, 3.6 m
250 mA, 2.9 m
200 mA, 2.7 m
100 mA, 2.3 m
Blow-up Threshold
1.0 Tune 0.5 1.0 Tune 0.5
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Summary of BPM + PMT measurements
• Sideband peak appears in both BPM and PMT measurements.– Two different types of detector
• Sideband peak appears in both instruments only at and above the beam-size blow-up threshold of beam current– Other measurements show that the amplitude of
the sideband peak at constant beam current is affected by the strength of the solenoid field. Stronger solenoid field smaller sideband peak.
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Time series data
BPM Data(Position)
PMT Data(Size)
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Time Development of Instability: 512-turn slices
BPM data
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Blow-up Pattern Analysis: PMT dataFind a bunch with characteristic blow-up pattern, and take spectra of3 stages separately. Then averagethe 3 spectra over all bunches thathave this pattern.
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Summary of time domain data
• Sideband oscillation has a burst-like structure.– Sideband peak is present at a low level, then grows
and damps in a burst lasting ~500 turns (5 ms).– During this burst, beam size grows ~5% from its
already blown-up state– Immediately after burst is complete, sideband peak is
absent, until beam size damps back down.
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s Data: 23 Dec 2003
• Data taken at 8 MV and 6 MV.– Sequence: 8 MV×2, 6 MV×2, 8 MV×2, 6 MV×1– Synch. Tune = 0.0237 @ 8 MeV, 0.0203 @ 6 MeV (measure
d from spectral peak).– Peak frequency bin in sideband region found, peak height av
eraged for 8 MV and 6 MV subsets separately.
Vc = 8 MV Vc = 6 MV
Bunch
25
10.5 Tune 0.75 0.5 Tune 0.75
Threshold dependence on synchrotron tune
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Sideband Peak Height Near Threshold at Different s
s=0.0237
s=0.0203
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Effect of changing RF voltage200 bunches/train, 4-bucket spacing, 0.6 mA/bunch, y = 4.27
• Vc = 6 MV• FB gain = -18.3 dB
0.5-------------------------------------------------Tune-----------------------------------------0.7
Sidebandνy
Tail----------H
ead
• Vc = 8 MV• FB gain = -14.9 dB
Tail----------H
ead
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Effect of varying synchrotron tune (RF voltage)
Sideband-tune separation does not changeOr does it? Hard to tell.
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s
Model Spectrum
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Effect of changing RF voltage
Separation is found to be close to s towards the head of the train, and it decreases going towards the back of the train, where the cloud density is higher.
s
s
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Effect of changing RF voltage
• Sideband onset is delayed along train (~3 bunches).– Confirms previous re
sults.
• Betatron peak growth is not delayed.– Note that it peaks ju
st before sideband appears
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Effect of Changing s (RF Voltage)
• Conclusion: – Threshold and separation between betatron p
eak and sideband peak are found to depend on s, in agreement with model.
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Effect of Changing Chromaticity
• An effect predicted by head-tail theory is that the e-cloud density threshold for the onset of the instability should go up if the vertical chromaticity is raised.
• Feedback gain was changed at each beam current to make y visible. To make sure this did not affect results, also took data at same chromaticity and two different feedback gains.
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Sidebands at Different y
y = 1.27
FB Gain = -14.9 dB
y = 4.27
y = 6.27 y = 6.27
FB Gain = -14.9 dB
FB Gain = -18.3 dB
FB Gain = -13.1 dB
y SidebandNoise
Joining/Splitting?
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Sideband Peak Heights
y=4.27
y=1.27
y=6.27, FB gain = -18.3 dB
y=6.27, FB gain = -14.9dB
Note: For KEKB, y ~ 3 should correspond to a change in cloud-density threshold of ~10%
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Simulated E-cloud build-up at KEKBWang et al., PRSTAB 5 124402 (2002)
Bunch: 10 20 30
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Effect of Changing Chromaticity
• The lower the chromaticity, the earlier in the train the sideband appears. (No change is seen for two different feedback gains at y=6.3, as expected.) Raising y from 1.3 to 4.3 pushes the onset of the instability back ~10 bunches along the train, as does further increasing y from 4.3 to 6.3. From simulations of electron cloud build-up (L.F. Wang, et al., PRSTAB 5 124402 (2002)), these would correspond do changes in the electron cloud density of ~20-40%.
• In numerical simulations (K. Ohmi, Proc. 2001 PAC, Chicago, p. 1895 (2001)), changing y from 0 to 12 raises the threshold by a factor of 2, from 5x1011 electrons/m3 to 1x1012 electrons/m3. Scaling from this, each change in y used in the machine study would be expected to change the threshold by ~20.– Basic agreement between simulations and experimental res
ults.
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Effect of Forced Excitation
• Experiment performed on a non-colliding bunch during physics operation, with the cloud-suppression solenoids on. A test bunch was inserted between two bunches (high cloud region).
• Test bunch was excited near the vertical tune. Bunch-by-bunch feedback was turned off for this bunch.
• As the excitation amplitude increased, in addition to the betatron peak amplitude increasing, the sideband amplitude decreased, dropping below detectable levels at the highest excitation amplitude.
• Mechanism under investigation
xy
Side-band
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Sidebands in Collision • How do sidebands behave in collision?
– They are present, but smeared out compared to their appearance in non-colliding bunches.
y
y
xSideband
Sideband
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Specific Luminosity Studies• Electron cloud instability in a bunch depends on
– Cloud density encountered by bunch– Bunch current of the bunch itself (must be sufficient to pinch the cloud)
• But this may be a weak dependence.
• Two studies reported on at PAC05:– Constant cloud, decaying bunch current.
• Inject extra test bunch into regular physics pattern, let it decay from 1.2 mA to zero.
• Space between test bunch and preceeding bunch is 2 rf buckets (~4 ns).– Decaying cloud, constant bunch current.
• Inject extra test bunch, let it decay, and look at observer bunch behind it.• Space between observer bunch and test bunch preceeding it is again 2 rf b
uckets (~4 ns).
• In both cases, have one colliding test/observer bunch and one non-colliding test/observer bunch.
• Compare spectrum of non-colliding bunch, and specific luminosity of colliding bunch.
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Zero-Degree Luminosity Monitor (ZDLM)S. Uehara and S. Uno (Belle)
• Set just downstream of quadrupole QC2RP near interaction point.
• Detects recoil electrons emitted to the zero-degree angle and deflected to the outside by Q magnetic fields
• Pb Glass 50 x 70 x 190 mm & a PMT– Record Hit Timings– Monitor Rates
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ZDLM Bunch-by-Bunch Luminosity Display
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Decaying Cloud, Constant Bunch Current Study
DecayingTest Bunch
Fill Pattern:
2 2 333 44Buckets:
Regular physics pattern bunchesAverage spacing: 3.5 buckets
Bunch current: Constant 1.2 mA(using continuous injection)
ConstantObserver Bunch
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Sideband Peak Tune
• Tune of sideband peak drifts down as cloud decays.
• Sideband peak tune constant if cloud constant, even if bunch current decays.
Decaying CloudConstant Bunch
Constant Cloud,Decaying Bunch
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Decaying Cloud, Constant Bunch Current Sideband Peak Heights
Sidebands disappear below test bunch current of ~0.4 mA
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Decaying Cloud, Constant Bunch Current Specific Luminosity
• Specific luminosity of observer bunch is lower than that of regular bunches above 0.4 mA, but is the same below 0.4 mA.– Consistent with sideba
nd behavior, and explanation that loss of specific luminosity is due to electron cloud instability.
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Decaying Cloud, Constant Bunch Current (Second Attempt)
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Ratio of specific luminosities at varying cloud density
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Sideband Integrated Power (05.6.26)
DAQ Down
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Constant Cloud, Decaying Bunch Current Study
DecayingTest Bunch
Fill Pattern:
2 2 333 44Buckets:
Regular physics pattern bunchesAverage spacing: 3.5 buckets
Bunch current: 1.2 mA constant(using continuous injection)
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Constant Cloud, Decaying Bunch Current Sideband Peak Heights
Sidebands disappear below test bunch current of ~0.75 mA
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Constant Cloud, Decaying Bunch Current Specific Luminosity
• Specific luminosity of observer bunch is lower than that of regular bunches above 0.75 mA, but is the same below 0.75 mA.
• Again, consistent with sideband behavior, and explanation that loss of specific luminosity is due to electron cloud instability.
• Also consistent with streak camera observations of vertical bunch size: bunch larger above ~0.8 mA.– H. Ikeda et al., PAC05 po
ster RPAT052.
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Constant Cloud, Decaying Bunch Current Study II (Higher cloud density)
DecayingTest Bunch
Fill Pattern:
2 2 333 44Buckets:
Regular physics pattern bunchesAverage spacing: 3.5 buckets
Bunch current: 1.2 mA constant(using continuous injection)
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Sideband Integrated Power (Higher cloud, decaying bunch)
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Difference in specific luminosities at higher cloud density (Run 1)
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Difference in specific luminosities at higher cloud density (Run 2)
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Higher cloud density, decaying current
• Sidebands disappear at around a bunch current of 0.8 mA.
• Specific luminosity of high-cloud and low-cloud bunches do not merge at that point, however.– Possible that sidebands continue, but below noise level.– Or possible indication of the presence of an incoherent c
omponent below the sideband threshold (non-linear focusing by cloud leading to non-Gaussian beam tails, e.g.)
– Alternatively, simple tune shift due to focusing effect of cloud?
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Dependence of Lum. On Tune
• Luminosity drops about 1% for a change of 0.001 in tune.
• Tune of high-cloud bunch is 0.01 greater than that of no-cloud bunch, while luminosity is 20% lower.
• Hard to trust extrapolation
During study
Tuning after study
LER Vertical Tune
Spe
cific
Lum
inos
ityS
peci
fic L
umin
osity
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Decaying Cloud, Decaying Bunch Current Study
DecayingTest Bunch
Fill Pattern:
2 2 333 44Buckets:
Regular physics pattern bunchesAverage spacing: 3.5 buckets
Bunch current: 1.2 mA constant(using continuous injection)
DecayingLead Bunch
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Sideband power (Decaying Cloud, Decaying Bunch)
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Specific luminosities for decaying cloud, decaying bunch
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Sideband Power at low cloud density, decaying bunch
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Difference in specific luminosities at low cloud density, decaying bunch
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Summary of Luminosity Studies
• For high bunch-current beams and a varying cloud density, the luminosity loss accompanies the appearance of betatron sidebands, indicating the onset of head-tail instability.
• For high cloud density and varying bunch currents, even low-current bunches have a similar luminosity loss as high-current bunches.– Sidebands not visible at low currents: signal presen
t but below noise threshold? (Measured signal proportional to square of bunch current.)
– Or else indication of incoherent effects below head-tail instability threshold, as predicted by simulation.
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Collision Offset Study
• Background: We get the best luminosity with a non-zero horizontal offset at the interaction point.
• Question: Could this be due to electron cloud blow-up in the tail of the LER bunch?– Indications are yes (Ieiri et al., PRSTAB 8, 12
4401 (2005))
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Collision Offset StudyT. Ieiri et al., PRSTAB 8, 12
4401 (2005)
• Measurements of total luminosity, beam-beam kick, and beam-beam tune shift made at different horizontal collision offsets showed lower luminosity at tail of bunch than at head of bunch, attributed to effects of electron cloud on positron beam.
• Measurements made with all bunches at same current:– High-cloud, high “observer”
bunch current– Los-cloud, low “observer” bunch
current
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Follow-up Collision Offset Study
• Use bunch-by-bunch luminosity monitor.
• Vary cloud density by letting pilot bunch in front of observer bunch decay.
• Vary observer bunch current while keeping pilot bunch current constant.
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Collision Offset Study
• +25 m
• 0 m
• -25 m
• -40 m
HER LERLER
HEADin
Collision
LERTAIL
inCollision
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Decaying Cloud, Constant Current
LERHEAD
inCollision
LERTAIL
inCollision
HighCloud
Density
LowCloud
Density
HighCloud
Density
LowCloud
Density
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Constant Cloud, Decaying Current
LERHEAD
inCollision
LERTAIL
inCollision
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Implications• Tune shifts due to cloud should
increase towards the tail of the bunch due to pinching.
• As the bunch current decreases, the pinching effect should become weaker.
• This would imply that the luminosity difference between the head and tail of the bunch should decrease as the bunch current decreases.
• The fact that this does not happen suggests that simple tune shift is not the dominant mechanism for luminosity loss under high-cloud, low bunch current conditions.
e+ bunche- cloud
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Collision Offset Study• Conclusion:
– Under high-cloud conditions, specific luminosity appears to be lower towards the tail of the LER bunch than towards the head.
– Under low-cloud conditions, there is no specific luminosity difference between the head and tail of the LER bunch.
• Agrees with Ieiri’s results
– In addition, we find that under high-cloud but low-bunch-current conditions, the luminosity difference between the head and the tail of the bunch is the same as for a high-current bunch.
• This suggests that the mechanism of luminosity loss for high-cloud/low-current conditions is not primarily due to simple tune shift. If it were, then the difference between the head and tail of the bunch would be expected to decrease as the bunch current decreases, due reduced cloud pinching.
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Summary
• Electron cloud-related beam dynamics observables include beam size, tune shift, tune spread, and coupled-bunch effects, which all react to the presence of solenoids.
• Above the blow-up threshold, single-bunch synchro-betatron sidebands indicative of head-tail instability appear. Below threshold, possible indications of incoherent effects as well.
• Much to measure.
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Still time to register!
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Spares
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Effect of Changing Emittance
At a low current (200 mA) below the blow-up threshold, the vertical emittance of the beam was adjusted via dispersion bumps that are used for luminosity tuning. The beam size was set to 1, 2.3, and 3.2 m (as expressed at the interaction point) in successive runs, and at each initial beam size the beam current was then ramped up to 600 mA while recording beam sizes and spectra.
The instability threshold does not depend on initial beam size.
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