Recent electron cloud studies
at KEKB
J.W. Flanagan, K. Ohmi, H. Fukuma, S.
Hiramatsu, M. Tobiyama, H. Ikeda, T.
Ieiri, K. Oide, Y. Funakoshi, E.
Perevedentsev, S. Uehara, S. Uno

Introduction• Vertical betatron sidebands found at KEKB which appear
to be signatures of fast head-tail instability due toelectron 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 ofluminosity 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Beam spectrum measurements• Bunch Oscillation Recorder– Digitizer synched to RF clock, plus 20-MBytememory.
– 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• Used in initial studies, agreed with BPM data

Fourier power spectrum of BPM data
V. Tune•••Sideband PeakLER single beam, 4 trains, 100 bunches per train, 4 rf bucket spacing
Solenoids off: beam size increased from 60 µm ->283 µm at 400 mA
Vertical feedback gain lowered– This brings out the vertical tune without external excitationEffect of varying synchrotron tune (RF voltage)

ÆSideband-tune separation does not change
Or does it? Hard to tell.

ÅModel focusing wake

α=wR/2Q

ÅMode spectrum using model
wake and airbag charge distribution.
Value of Q chosen to give small νsdependence on mode separation, butother solutions possible (in fact
more common).

Effect of changing RF voltage
200 bunches/train, 4-bucket spacing, 0.6 mA/bunch, ξy = 4.27

νyTail----------Head• Vc = 6 MV
• FB gain = -18.3 dB

Tail----------Head• Vc = 8 MV
• FB gain = -14.9 dB

Sideband0.5-------------------------------------------------Tune-----------------------------------------0.7

Effect of changing RF voltage• Distance between
sideband peak andbetatron peakdecreases at lower
Vc.

• The difference inpeak separations
between 8 MV and
6 MV is ~∆νs atmaximum (towards
head of train),decreasing to ~2/3
∆νs towards back oftrain.

Note: ∆νs ~ 0.0032

Effect of changing RF voltage• Sideband onset isdelayed along train(~3 bunches).
– Confirms previousresults.

• Betatron peakgrowth is notdelayed.
– Note that it peaks
just before sideband
appearsEffect of Changing RF Voltage• Conclusion: A new measurement, with
better statistics and a larger change insynchrotron tune, finds that the separation
between the sideband peak and thebetatron peak changes by approximately
the same amount as the change in νs.
– Need for parameter tweaking in simple modelis relaxed.

Effect of Changing Chromaticity• An effect predicted by head-tail theory isthat the e-cloud density threshold for theonset of the instability should go up if thevertical chromaticity is raised.
• Original data (June 2003) showed such aneffect, but differences in beam current at
the two different chromaticities renderedinterpretation ambiguous.

Peak Height (arb)

Sideband Peak Height Near Threshold at Different ξyξy =2.16
ξy = 5.16
Bucket (=Bunch*4)

June 2003Effect of Changing Chromaticity• Data re-taken, at same beam currents(re-injecting between changes inchromaticity).
• Feedback gain was changed at eachbeam current to make νy visible. To
make sure this did not affect results,
also took data at same chromaticity
and two different feedback gains.

Sidebands at Different ξyNoiseνySideband

ξy = 1.27

ξy = 4.27

FB Gain = -13.1 dB

FB Gain = -14.9 dB

Joining/Splitting?

ξy = 6.27

ξy = 6.27

FB Gain = -14.9 dB

FB Gain = -18.3 dB

Sideband Peak Heights
ξy=1.27ξy=4.27

ξy=6.27, FB gain = -14.9dB
ξy=6.27, FB gain = -18.3 dB

Note: For KEKB, ∆ξy ~ 3 should correspond to a change in cloud-density
threshold of ~10%

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

Bunch: 10 20 30

Sideband-Betatron PeakSeparations

Effect of Changing Chromaticity• Conclusion: Threshold is found todepend on ξy, as expected under head-tailtheory. Size of threshold shift is crudely
consistent with expectation.

Effect of Changing Emittance• Experiment was done to see if the beam
blow-up and sideband-appearance
thresholds change when the initial verticalbeam size is changed.
• Vertical beam size at low current waschanged by using a dispersion bump(iSize) to change the emittance, thenbeam current ramped up.
– Beam size data taken continuously, beam
spectrum data taken at 50 mA steps.

Blowup Thresholddependence on εy(iSize Bump)

σy0= 1 µmÆ

Pattern: 4/200/4

σy0= 2.2 µmÆ

Beam-size blowup
threshold does notchange much, if at
σy0= 3.2 µmÆall.

σ∗y0 = 1 µm
300 mA

350 mA

400 mA450 mA

0.5---------------Tune-----------------0.7 0.5---------------Tune-----------------0.7

σ∗y0 = 2.2 µm
300 mA

350 mA

400 mA450 mA

0.5---------------Tune-----------------0.7 0.5---------------Tune-----------------0.7

σ∗y0 = 3.2 µm
300 mA

350 mA

400 mA450 mA

0.5---------------Tune-----------------0.7 0.5---------------Tune-----------------0.7

Sideband growth at different σ∗y0

Integrated beam spectrum oversideband regionEffect of Changing Emittance• Conclusion: Threshold is found not todepend on initial vertical beam size.

Sidebands in Collision

Spectral Power (arb)Spectral Power (arb)• Sidebands present in collision, but smeared out comparedto their appearance in non-colliding bunches.
• =>Strategy: Measure spectrum of non-colliding bunch, andspecific luminosity of colliding bunch under sameconditions.
νxνy0.01
Sidebanda) Non-colliding bunch

0.001
0.5 0.520.540.560.58 0.6 0.620.640.660.68 0.7
Fractional Tune
0.01b) Colliding bunchνy Sideband

0.001
0.5 0.520.540.560.58 0.6 0.620.640.660.68 0.7
Fractional Tune

Decaying Cloud, Constant Bunch Current
StudyDecaying
Test Bunch

Fill Pattern:

Buckets:

43

4

3

Regular physics pattern bunches
Average spacing: 3.5 buckets
Bunch current: Constant 1.2 mA(using continuous injection)

2ConstantObserver Bunch

23PAC05

Spectral Power (arb)

Decaying Cloud, Constant Bunch Current
Sideband Peak Heights
Sideband Height (Non-colliding bunch)
0.006
0.005
0.004
0.003
0.002
0.001
000.2
0.4
0.6
0.8
1
1.2
Current of preceeding bunch (mA)

Sidebands disappear below test bunch current of ~0.4 mA
PAC05

Decaying Cloud, Constant Bunch Current
Specific Luminosity– Consistent withsideband behavior,and explanation thatloss of specificluminosity is due toelectron cloudinstability.

0.0025Specific Luminosity (arb)• Specific luminosity ofobserver bunch islower than that ofregular bunches
above 0.4 mA, but isnearly the same
below 0.4 mA.

0.002

0.00150.001

0.00052-Bucket Observer Bunch
3-Bucket Bunches (avg.)
4-Bucket Bunches (avg.)

00

0.2

0.4

0.6

0.8

1

1.2

Current of decaying test bunch (mA)

PAC05

Constant Cloud, Decaying Bunch Current
StudyDecaying
Test Bunch

Fill Pattern:

Buckets:

434

3

Regular physics pattern bunches
Average spacing: 3.5 buckets
Bunch current: 1.2 mA constant(using continuous injection)

223

PAC05

Spectral Power (arb)Constant Cloud, Decaying Bunch Current
Sideband Peak Heights
Sideband Height (Non-colliding bunch)
0.006
0.005
0.004
0.003
0.002
0.001
00

0.2

0.4

0.6

0.8

11.2

Bunch current (mA)

Sidebands disappear below test bunch current of ~0.75 mA
PAC05

Constant Cloud, Decaying Bunch Current
Specific Luminosity••

Specific luminosity ofobserver bunch is lower
than that of regularbunches above 0.75 mA,
but is nearly the same
below 0.75 mA.
Again, consistent withsideband behavior, andexplanation that loss of
specific luminosity is due
to electron cloud instability.
Also consistent with streak
camera observations ofvertical bunch size: bunchlarger above ~0.8 mA.
– H. Ikeda et al., PAC05
poster RPAT052.

0.007
Specific Luminosity (arb)•2-Bucket Bunch
3-Bucket Bunch
4-Bucket Bunch

0.006
0.005
0.004
0.003
0.002
0.001
00

0.2

0.4
0.6
0.8
Bunch Current (mA)

1

1.2

PAC05

Constant Cloud, Decaying Bunch Current
Study II
Decaying
Test Bunch

Fill Pattern:

Buckets:

4343

Regular physics pattern bunches
Average spacing: 3.5 buckets
Bunch current: 1.2 mA constant(using continuous injection)

22

3

Sidebands and Spec. Lum.
Sideband Peak Height•

Sidebands disappear at around abunch current of 0.8 mA.

•Specific luminosity of 2-bucket
and 4-bucket spacing bunches
do not merge at that point,
however.
– Possible that sidebands continue,
but below noise level.
– OR, possible indication of thepresence of an incoherent
component below the sideband
threshold (non-linear focusing by
cloud leading to non-Gaussian
beam tails, e.g.)

SidebandThreshold

Specific Luminosity
4-bucket spacing2-bucket spacing

Collision Offset Study• Background: We get the best luminositywith a non-zero horizontal offset at theinteraction point.
• Question: Could this be due to electron
cloud blow-up in the tail of the LERbunch?
• Measured: Specific luminosity of highcloud (2-bucket spacing) and low-cloud (4bucket spacing) bunches at several
collision offsets.

Collision Offset Study• +25 µm

HERLERLERHEADin
Collision• 0 µm• -25 µm
• -40 µm

LERTAILinCollisionDecaying Cloud, Constant Current
Normalized spec. lum. (arb)

2-bucket spacing
4-bucket spacing

1.4
1.2
10.8
0.6
0.4
0.2

Offset 0 um

Normalized spec. lum. (arb)

LER
HEAD
inCollision

Offset +25 um

1.2
10.8
0.6
0.4
0.2
00
0.2

0.4

0.6

0.8

1

1.2

0.8
0.6

Low
Cloud
Density
0.2

0.8

1

Offset -40 um

100.6

Offset -25 um

1.2

00.4

Bunch Current (previous) (mA)

2-bucket spacing
4-bucket spacing

0.2

0.2

Bunch Current (previous) (mA)

1.4

0.4

0

1.4

0.4

0.6

0.8

LERHigh
TAILCloudinDensity
11.2
1.4 Collision

Bunch Current (previous) (mA)

Normalized spec. lum. (arb)0Normalized spec. lum. (arb)

2-bucket spacing
4-bucket spacing

1.4

1.2

1.4

2-bucket spacing
4-bucket spacing

1.4
1.2
1
0.8
0.6

Low
Cloud
Density

0.4
0.2
00

0.2

High
Cloud
Density

0.4
0.6
0.8
1
1.2
Bunch Current (previous) (mA)

1.4

Constant Cloud, Decaying Current
Normalized spec. lum. (arb)

2.5

2-bucket spacing
4-bucket spacing

21.5
10.5

Offset 0 um
2.5Normalized spec. lum. (arb)

LER
HEAD
in
Collision

Offset +25 um

2
1.5
1
0.5
00
0.2

0.4

0.6

0.8

11.2

0.4

0.6

0.8

1

Bunch Current (self) (mA)

Offset -25 um

Offset -40 um
2.5

2-bucket spacing
4-bucket spacing

2
1.5
1
0.5
0
0.2

0.2

Bunch Current (self) (mA)

2.5

00

1.4

0.4

0.6

0.8

1

Bunch Current (self) (mA)

1.2

LER
TAIL
in
1.4 Collision

Normalized spec. lum. (arb)

0Normalized spec. lum. (arb)

2-bucket spacing
4-bucket spacing

1.2

1.4

1.2

1.42-bucket spacing
4-bucket spacing

21.5
1
0.5
00

0.2

0.4
0.6
0.8
1Bunch Current (self) (mA)Collision Offset Study• Conclusion:– Under high-cloud conditions, specificluminosity appears to be lower towards thetail of the LER bunch than towards the head.
– Under low-cloud conditions, there is no
specific luminosity difference between thehead and tail of the LER bunch.

Miscellaneous Topics

Sideband Dependence on ν_s(Vc)νySideband

Vc = 4 MEVTail--------------Head？？！！

0.5------------------------------------------------Tune----------------------------------------0.75
ÆDec. 2005 Study: Try to reproduce spectrum from June 2005, andtake data with streak camera and longitudinal BOR in addition.

Results: Cannot reproduce!
But sideband splits…

Tail-------------Head

Vc = 4 MV

Tail-------------Head

Vc = 8 MV

0.5-------------------------------------------------Tune-----------------------------------------0.7

…And reunifies (blurs) if verticalchromaticity lowered

Tail-------------Head

Vc = 4 MV
ξy = 2.2

Tail-------------Head

Vc = 4 MV
ξy = 4.2

0.5-------------------------------------------------Tune-----------------------------------------0.7

This also happens back at 8MV, thoughchromaticity threshold for splitting is higher

Tail-------------Head

Vc = 8 MV
ξy = 4.2

Tail-------------Head

Vc = 8 MV
ξy = 6.2

0.5-------------------------------------------------Tune-----------------------------------------0.7

And sometimes (almost) disappears

FB Gain = -19.3 dB

Tail-------------Head

Vc = 8 MV
ξy = 6.2
FB Gain = -14.6 dB

Tail-------------Head

Vc = 8 MV
ξy = 6.2

0.5-------------------------------------------------Tune-----------------------------------------0.7

When sidebands disappear(in circle)

Goal 2: Hunt for left sideband via v. tuneνy = 0.5678
νy = 0.5581
νy = 0.5421
νy = 0.5356
νy = 0.5506
0.5-------------------------------------------------Tune-----------------------------------------0.7

When sidebands disappear(in circle)

Miscellaneous Results• Failed to reproduce strange behavior at Vc=4MV– But found sideband splitting at higher chromaticity– Also found sidebands disappear when betatron amplitudelarge (FB gain low)
• Not clearly seen before• Betatron oscillations interfere with cloud pinching?
• Something else going on?

• Failed to find left sideband– But sideband disappeared at very low νy, and reappeared
when νy was raised again.
– Looking at beam size, beam size was a bit larger whensideband had disappeared. (So no help for luminosity…?)

Extra: extra sidebands?
νySideband
Tail----------HeadVc = 8 MEVHello?

Tail----------Head

Vc = 6 MEV

0.5------------------------------------------------Tune----------------------------------------0.75

Sideband Splits off from Betatron Line?
From Quadropole Solenoid study (to be reported on in detail by H. Fukuma)

2-bucket spacing

3-bucket spacing

Final Summary• Found sideband-betatron separation dependence on νs.
• Found sideband appearance threshold dependence on ξy.
– Sizes of above 2 effects seem consistent with head-tail theory.

• Found no apparent threshold dependence on σy0.
– Under study• Found cases where specific luminosity remains suppressed
even below sideband threshold– Possible indication of incoherent effects• Found specific luminosity is lower at tail of LER bunch thanat head, when electron cloud density is high.
– Possible indication of blow-up towards tail of bunch• Found cases where sidebands disappear or almost
disappear– Conditions similar to those where luminosity is highest (low νy, V. FBgain).

• Found examples where sideband line seems to have split off
from the betatron line near the head of the train.
– Never clearly observed before

SparesMeasurements at PEP-II
With U. Wienands, D. Teytelman• What would be nice to do at PEP-II:– Ideally, turn off solenoids• create e-clouds, and look at beam spectra.
• Don’t necessarily need to turn off all solenoids. Perhaps alimited region, to minimize impact on orbit, would be sufficient.

– Alternatively, create localized e-cloud regions alongthe train.
• Increase bunch currents of a set of bunches at end of train.
• Pack a few extra bunches into train, at by-1 spacing.
• Can be done more-or-less parasitically– Note: may even already have clouds• May be worth creating and looking at some pilot bunches inthe LER with the current fill pattern.

Study Fill Patterns at PEP-II
Fill Pattern 1 (least disruptive):

HER
End of Train

LER

Buckets:

222

2

2

2

Create a few non-colliding bunches at the end of the train,
examine transverse beam spectrum. If instability signal appears,
try letting non-colliding LER bunches decay.
TRIED: No signal found at 1 mA/bunch. May retry with higher
pilot bunch currents, and longer pilot bunch train.

Study Pattern 1: By-2 Pilot Bunches

Study Pattern 1: By-2 Pilot Bunches

Sample spectrum at PEPII(GAGE abort log)

Bunch
0.58 Fractional Tune

0.59

Study Fill Patterns at PEP-II
Fill Pattern 2 (tricky, but still parasitic with physics running):

End of Train

LER

Buckets:

211

1

11

1

1

11

1

Same as Pattern 1, but add some extra bunches between existingbunches to increase electron cloud density.
TRIED: Strong synchrotron harmonics appear all over the spectrum.
May retry with longitudinal feedback turned off in pilot section of fill.

Study Pattern 2: By-1 Pilot Bunches

Study Fill Patterns at PEP-II
Fill Pattern 3 (dedicated machine time needed):

HER
End of Train

LER

Buckets:

222

2

2

2

LER single-beam, or at least not in collision.
Turn solenoids off, correct steering, fill to enoughcurrent to create clouds, take beam position spectra
while injecting. Monitor beam size in parallel.
Could be done in parallel with other activities in the HER.
Not yet tried.

Can we understood this behavior?
• Threshold
ρe,th =2γν s ωeσ z c

Q=min(Qnl, ωeσz/c)

3KQr0 β L

λ p re c 2
ωe =σ y (σ x + σ y )• ωe of numerator is cancelled by ωe in Q soperfectly?
Future problem 3• Maybe true, since ωeσz/c=2.5 for KEKB.
K. Ohmi, Snowmass 2005

Threshold and Chromaticity
Note: For KEKB, ∆ξy ~ 3 should give achange in cloud-density threshold of~10%, if ρthresh α (ωe-ξω0/α)/ωe