Observation of Vertical Betatron
Sidebands due to Electron Clouds
at the KEKB LER

J.W. Flanagan
KEKB MAC, Feb. 2005

•Data taken over course of 2003, 2004
– Some initial observations shown at 2003 MAC•

Paper submitted to PRL July 2004
– Published 7 Feb., 2005

Introduction• Vertical beam blow-up has been observed atKEKB LER at bunch currents above athreshold of ~0.35 mA/bunch, at a spacingof 4 rf buckets (~8 ns).
• Bunch-current blowup threshold can beraised by the use of solenoids around thebeam pipe.
– Currently about 95% of drift space in LER iscovered.

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 bygated camera

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

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 bunchseparately.

• Inputs:– Feedback BPMs
• 6 mm diameter button electrodes• 2 GHz （4xfrf) detection frequency, 750 MHz bandpass– Fast PMTFourier power spectrum of BPM dataV. TuneSideband Peak

• LER 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

Spectra of Bunches 1-10
Bunch 1

Bunch 6

Bunch 2

Bunch 7

Bunch 3

Bunch 8Bunch 4

Bunch 9Bunch 5

Bunch 10Tune and sideband peaks along train

TuneSideband
PeakBunch 5
Betatron
Tune (V)Bunch• Vertical Tune andSideband Peaks
increase in tune alongtrain, saturating near
40th bunch.
• Difference between
tune and peak also
increases, thensaturates near the 40thbunch.
• If vertical tune ischanged, sideband
peak shifts by equalamount (separation
remains constant).
Horizontal tune has
no effect.

PMT setup+%/&$0&/1% $"%&'()$*+,-.$!"# $+2/34$5/(67'/(6 $$Partially block beam image withblack cardboard, and measure lightintensity of the visible part with aPMT. The PMT signal is buffered
and then recorded using a feedback
BOR digitizer/memory board.
y: 100 mV/division
x: 10 ns/divisionBeam Blow-up Measurement
4-bucket spacing, 600 bunches
4 trains, 150 bunches/train, 4 rf bucket spacing
Vertical beam size at IP (µm)

Blow-up threshold
250 mA, 0.42 mA/bunch

LER Beam Current (mA)

PMT Spectra

FB BPM Spectra

100 mA, 2.3 µm

200 mA, 2.7 µm

250 mA, 2.9 µm

Blow-up Threshold

300 mA, 3.6 µm

450 mA, 4.9 mm

1.0Tune0.5

1.0Tune0.5

Summary of BPM + PMT measurements• Sideband peak appears in both BPM and PMTmeasurements.
– Two different types of detector• Sideband peak appears in both instruments only at
and above the beam-size blow-up threshold ofbeam current– Other measurements show that the amplitude of thesideband peak at constant beam current is affected bythe strength of the solenoid field. Stronger solenoidfield  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 data
Find a bunch with characteristic
blow-up pattern, and take spectra of3 stages separately. Then average
the 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 anddamps in a burst lasting ~500 turns (5 ms).
– During this burst, beam size grows ~5% from itsalready blown-up state– Immediately after burst is complete, sideband peak isabsent, until beam size damps back down.

Effect of varying vertical feedback gain

Sideband peak amplitude does not changeEffect of varying synchrotron tune

Sideband-tune separation does not changeModel focusing wakeMode spectrum
using model wakeand airbag charge
distribution

Threshold dependence on synchrotron tuneδνs Data: 23 Dec 2003
Vc = 8 MV

25

Vc = 6 MV

Bunch

10.5

Tune

0.75 0.5

Tune0.75• 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 (measuredfrom spectral peak).
– Peak frequency bin in sideband region found, peak height averaged
for 8 MV and 6 MV subsets separately.

Sideband Peak Height Near Threshold at Different νsνs=0.0203

νs=0.0237

Peak Height (arb)

Sideband Peak Height Near Threshold at Different ξyξy =2.16
ξy = 5.16

Need to confirm

Bucket (=Bunch*4)

LER Single beam, 650 mA
2004.12.17 Study600Fill PatternBunch
00.5

Tune0.69

Summary• A betatron sideband peak has been found in the vertical tune
spectrum of positron bunches in the presence of beam-size
blow-up due to electron clouds. The sideband peak is on theupper side of the betatron peak in terms of fractional tune, firstappears early in the bunch train, and the separation between
this peak and the betatron tune peak increases going along thetrain, until it saturates at a certain point.
• The best explanation for it is that it is a signature of the headtail instability hypothesized to explain transverse beam blowup due to electron clouds.
• The presence of this sideband peak also provides a sensitive
diagnostic for the presence of electron clouds.

Topics for Further Study• Better simulation model• (Re-)Measure change in threshold asfunction of chromaticity• More sensitive measurement near thethreshold conditions• Measure at other machines– PEP-II– DAΦNE?
– …?

SPARESExtra: Sidebands in collision
Fill Pattern:
Ikeda bunch

Ikeda-1

Ikeda+1

Ikeda+2

Sidebands in collision (6.29)

Ikeda-1Ikeda bunch

Ikeda+1

Ikeda+2

Sidebands in collision
Fill Pattern:
Ikeda bunch Ohmi bunch

Ikeda-1

Ikeda+1

Ikeda+3

Observation during physics run (6.30)
Ohmi bunch (colliding bunch)Ohmi+49 bunch （non-colliding）
1.2 mA/bunch

0.8 mA/bunch←THRESHOLD→
0.6 mA/bunch

0.4 mA/bunch

Ikeda-3 (3-bucket spacing)

Ikeda-1 (4-bucket spacing)

Colliding Bunch
SpectraIkeda (2-bucket spacing)

Ikeda+1 (2-bucket spacing)

Ikeda+2 (2-bucket spacing)

Ikeda+3 (2-bucket spacing)
Ikeda+4 (4-bucket spacing)Specific Luminosity by Bunch&#!!

THRESHOLD~0.6 mA/bunch89(:+0;3<=4(7<=4>7&"!!
&!!!

By-4 bunches

%!!
$!!

?@A B
?@A C
?@
?@DCEF
?@DG

#!!
"!!
!
!'!!!!

By-2 bunches!'"!!!

!'#!!!

!'$!!!

!'%!!!

()*+,-./0+/-112.3+4567&'!!!!

&'"!!!

Summary 2• In collision, peak shows up at bunch-spacing of 2
buckets, over a threshold of ~0.6 mA/bunch.
– This is accompanied by lower specific luminosity.
– 3-bucket spacing shows a small peak, too.

• This peak is smeared out during collision.
• Could this be limiting our luminosity after all?

PMT
• フィルパッタン： ４/１５０/４ （計６００ﾊﾞﾝﾁ）• ソレノイド完全にOff• νy=0.569, νx=0.5178, νs=0.0249 (2.47 kHz)• ξy=0.7, ξx=1.1• BOR データ：•

– PMT - V– FB BPM - V– FB BPM - H
Blowup Threshold： 250 mA (0.42 mA/bunch)

LER Electron-cloud suppression solenoids

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

Detection of photoelectrons at wall

Ohnishi et al., “DETECTION OF PHOTOELECTRON CLOUD IN POSITRON RING AT KEKB,”HEAC01Fourier power spectrum of BPM data
V. TuneSideband Peak

Synchrotron Tune

100Bunch

10.5

Tune

1.0

• LER 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