Electron Cloud Studies at KEKB

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…

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

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.

LER Electron-cloud suppression solenoids

Fukuma et al., “Study of Vertical Beam Blow-up in KEKB LER,” HEAC01 proceedings

Blow-up measured by SR interferometer

Fukuma et al., “Study of Vertical Beam Blow-up in KEKB LER,” HEAC01 proceedings

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

Gated Camera Observations of Trailing Witness Bunch

J.W. Flanagan et al., Proc. EPAC00 (2000) 1119

Detection of photoelectrons at wall

Ohnishi et al., “DETECTION OF PHOTOELECTRON CLOUD IN POSITRON RING AT KEKB,” HEAC01

Tune Shift MeasurementsT.Ieiri, et al.,

PRST-AB 5, 094402 (2002)

Train Bunches Witness Bunches

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

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

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

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

• 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)

Analysis Procedure

Horizontal Mode Spectra

Bsol=0% Bsol=10%

Bsol=20% Bsol=100%

Horizontal: Time evolution

Bsol=0%Bsol=10%

Bsol=20% Bsol=100%

Horizontal: Effect of Chromaticity

x=-0.3 x=-0.7

Vertical Mode Spectra

Bsol=0% Bsol=10%

Bsol=20% Bsol=100%

Vertical: Time Evolution

Bsol=0% Bsol=10%

Bsol=20%Bsol=100%

Vertical: Effect of Chromaticity

y=5.2 y=4.2

• 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

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

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

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)

Model focusing wake

Mode spectrum using model wake and airbag charge distribution.

Sideband-betatron peak separation dependence on synchrotron tune reproduced.

=wR/2Q

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)

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)

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

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

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

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.

Time series data

BPM Data(Position)

PMT Data(Size)

Time Development of Instability: 512-turn slices

BPM data

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.

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.

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

Sideband Peak Height Near Threshold at Different s

s=0.0237

s=0.0203

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

Effect of varying synchrotron tune (RF voltage)

Sideband-tune separation does not changeOr does it? Hard to tell.

s

Model Spectrum

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

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

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.

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.

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?

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%

Simulated E-cloud build-up at KEKBWang et al., PRSTAB 5 124402 (2002)

Bunch: 10 20 30

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.

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

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

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.

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

ZDLM Bunch-by-Bunch Luminosity Display

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

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

Decaying Cloud, Constant Bunch Current Sideband Peak Heights

Sidebands disappear below test bunch current of ~0.4 mA

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.

Decaying Cloud, Constant Bunch Current (Second Attempt)

Ratio of specific luminosities at varying cloud density

Sideband Integrated Power (05.6.26)

DAQ Down

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)

Constant Cloud, Decaying Bunch Current Sideband Peak Heights

Sidebands disappear below test bunch current of ~0.75 mA

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.

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)

Sideband Integrated Power (Higher cloud, decaying bunch)

Difference in specific luminosities at higher cloud density (Run 1)

Difference in specific luminosities at higher cloud density (Run 2)

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?

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

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

Sideband power (Decaying Cloud, Decaying Bunch)

Specific luminosities for decaying cloud, decaying bunch

Sideband Power at low cloud density, decaying bunch

Difference in specific luminosities at low cloud density, decaying bunch

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.

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))

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

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.

Collision Offset Study

• +25 m

• 0 m

• -25 m

• -40 m

HER LERLER

HEADin

Collision

LERTAIL

inCollision

Decaying Cloud, Constant Current

LERHEAD

inCollision

LERTAIL

inCollision

HighCloud

Density

LowCloud

Density

HighCloud

Density

LowCloud

Density

Constant Cloud, Decaying Current

LERHEAD

inCollision

LERTAIL

inCollision

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

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.

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.

Still time to register!

Spares

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.