DETECTION OF PHOTOELECTRON CLOUD IN POSITRON RING ATKEKBY. Ohnishi∗ , T. Murakami. K. Kanazawa, M. Tanaka, M. Tejima
High Energy Accelerator Research Organization(KEK), Tsukuba, JapanAbstract
We have developed photoelectron detector and observed
photoelectrons in the positron ring at KEKB. The detector consists of a suppressor that is mesh grid to apply abias voltage followed by a pick-up collector within a special vacuum chamber. The suppressor and collector areshielded by a double mesh grid which is grounded against
the beam. Photoelectron effect causes beam instability or
beam-size blow up in the positron ring is very serious forthe high-current collider for the aim to high luminosity. We
measured the amount of photoelectron and their energy forvarious beam current and bunch-filling patterns.

1INTRODUCTIONKEKB is an asymmetric-energy, double-ring, electronpositron collider[1]. The beam energy is 3.5 GeV forpositron(LER) and 8.0 GeV for electron(HER) ring. The
storage rings intersect at a single interaction point(I.P) toproduce collisions. The requirement of the peak luminosity is 1034 cm−2 s−1 for observation of CP violation at highenergy physics experiment. In order to achieve such highluminosity operations, large beam current and low β optics are needed. Recently, we have observed vertical beam
blow up, when large beam current is stored in the positronring[2]. Beam blow up is very serious for the high luminosity machine because the luminosity will be saturated byincreasing beam size even though higher beam current isstored. A model of photoelectron cloud is proposed forthe reason of the beam blow up[3]. The beam blow up isexplained as a single-bunch instability of a positron bunch
due to a large number of photoelectrons. Thus, the behavior of photoelectrons is very interested at high beam current
and high luminosity machine, KEKB or PEP II.
In order to detect photoelectrons, a special vacuum
chamber to which photoelectron detector attached wasbuild and installed into the positron ring at KEKB.

2 PHOTOELECTRON DETECTOR
The photoelectron detector consists of three mesh grids infront of a collector which is a pick-up electrode for photoelectrons. Outer two mesh grids are grounded to shield thecollector from the beam, and the inner mesh grid is a suppressor to which a bias voltage can be applied. The shapeof the collector is a disk of 26 mm diameter and the distance to the center of the vacuum chamber is 76 mm. A∗ Email:yukiyoshi.onishi@kek.jp

bias voltage can be also applied to the collector to increase
the detection efficiency of photoelectrons, however the collector bias voltage was set to be zero during the experiment
except for measurement of photoelectron energy. Figure
1 shows the cross sectional view of the photoelectron detector. The photoelectron detector is attached within thespecial vacuum chamber whose diameter is 94 mm. The
location of the photoelectron detector is 1.5 m downstreamof a bending magnet. Therefore, photoelectrons caused bysynchrotron light should be observed by the detector.
The schematic diagram of the readout electronics system is shown in Fig. 2. We used a current amplifier withthe decay constant of ∼50 nsec and the rise time is lessthan a few nsec. The gain of the amplifier is 0.5 mV/pC.
We put the amplifier and power supply just below the vacuum chamber, the cable length from the collector is about
50 cm, to reduce noise and to avoid decrease of the signal
as possible. The amplifier and the power supply are protected by lead sheet(1mm thickness) against the radiation.
The power supply is cooled by air using a fan to keep thetemperature constant. The output signals of the amplifier
are sent to the scope(Tektronix TDS380P). The drifting ofamplifier gain is estimated less than 2% for long-term operation.

3 MEASUREMENT OF
PHOTOELECTRON
Figure 3 shows typical output voltages from the collector
as a function of time. Two samples are shown in Fig. 3;
the upper shows 4 trains, 64 bunches in each train, 4 bucketspacing(4/64/4) and single train, 1153 bunches in a train, 4bucket spacing(1/1153/4) for the bottom figure. The beam
current was 180 mA for 4/64/4 and 660 mA for 1/1153/4
filling pattern, respectively. The revolution time is 10 µsec
and one bucket spacing is 2 nsec. The ground level is 0mV. Peak voltages and train gaps are clearly seen for 4/64/4
and a large abort gap(∼1 µsec) for 1/1153/4 filling pattern.
Therefore, the negative output voltages should correspondto the photoelectrons. Figure 4 shows peak to peak of theoutput voltages as a function of beam current for 4/64/4 and1/1153/4 filling pattern. This measurement was done by thesingle shot mode without average. The fluctuation of themeasurement is about 5 mV at the peak to peak. The peakto-peak output voltage is found to be nearly linear to thepositron beam current. The photoelectron current throughthe detector can be estimated by 6.7 µA at 500 mA positron
beam current in the case of 1/1153/4 filling pattern.
Solenoid field can be provided in the vacuum chamber

Vout (mV)y

4 trains/64 bunches/4 bucket spacing

0-10-20x
synchrotron light10

train gap

-30

e+-40

Vacuumchamber

positron beame- (photoelectron)bunch train (512 nsec)-50-60

0

123

4

5

6

7

8

910

Vout (mV)time (µsec)

e-10

1 train/4 bucket spacing

abort gap(1µsec)

0-10-20

mesh-shielde-suppressor-30-40-50

Vs-60

collector

012345678910time (µsec)

φ26mm

Vout

Vcdistance btw center of vacuum
chamber and collector: 76mm

Figure 1: Cross sectional view of photoelectron detector.
The VS and VC means a suppressor bias voltage and a collector bias voltage, respectively.

Input
To scope
100 m corrugated tube

Band width 50 MHz

Figure 2: Schematic diagram of readout electronics.

Figure 3: Waveforms from the scope showing the pick-up
voltage as a function of time. The revolution time is 10µsec. Upper figure is obtained from 4/64/4 filling pattern,bottom from 1/1153/4 filling pattern.
Peak to peak of Vout (mV)

Amp

60

50

4 trains/64 bunces/4 bucket spacing4030
1 train/4 bucket spacing
20

1000100

200

300

400

500

600

700

800

Positron Beam Current (mA)that the photoelectron detector equipped. If the polarity of
the current applied to the solenoid coil changes, the drifting
path of the photoelectrons also changes. The photoelectrons emitted from the wall the synchrotron light hits movetoward the positron beam and change the direction via theLorentz force. Figure 5 shows the effect of the solenoid
field in the vacuum chamber. The peak-to-peak voltages
changes when the polarity of the solenoid is changed andthe solenoid off. This observation becomes another reason
to show the detected signals caused by the photoelectrons.
Figure 6 shows the photoelectron energy distribution.
With changing suppressor bias voltage, we measured thepeak-to-peak voltages normalized by those of zero suppressor bias voltages. The collector bias voltage was set to be
2 V to make the measurement stable. Beam current waschanged from 620 mA to 530 mA during this measurement. The filling pattern is 1/1153/4. The upper shows
ratio of the photoelectron intensity for zero suppressor bias

Figure 4: Peak to peak of the output voltage as a function ofbeam current. The filling patterns of 4/64/4 and 1/1153/4
are shown in the figure.
voltage as a function of the suppressor bias voltage. Thebottom shows the differential of the ratio(upper) shown in
Fig. 6. The amount of photoelectrons is rapidly decreased
below the energy of 20 eV. Significant large excess can befound around 35 eV for the energy distribution. This behavior might be determined by the radius of the vacuum
chamber and the acceleration of photoelectrons by positron
beams[4].

4SUMMARY

We have developed photoelectron detector and readout
electronics. Signals from photoelectrons were observed

Peak to peak of Vout (mV)24solenoid polarity(-)

20
16

solenoid off

128

solenoid polarity(+)
4246

248

250

252

254

256

258

260

downstream of the bending magnet. We also measured energy distribution of the photoelectrons and observed large
excess around 35 eV.
Time structure of the photoelectrons such as build up anddecay along bunches is under study. The comparison withtheoretical simulations[3] should be done and will be reported soon.

Beam Current (mA)
Polarity(-)

5 ACKNOWLEDGEMENTS

Polarity(+)
ev

INB

OUTINvOUT

We would like to thank the KEKB staff for their help withthis experiment.

Be

6
Photoelectron
Detector

REFERENCES[1] KEKB Design Report, KEK Report 95-7.
[2] H. Fukuma et al., Proc. of EPAC 2000 (2000).

Ratio of photoelectrons (R)Figure 5: Effects of solenoid field in the vacuum chamber.
Synchrotron light emitted from beam through the bending
magnet hits the outer wall of vacuum chamber. Peak-topeak output voltages of negative and positive polarities ofsolenoid filed, and no solenoid filed are shown.
Beam current: 620-530 mA
1Vc = 2V
0.8
0.6
0.4
0.2
0-300

-250-200-150-100-50

0dR/dE (1/eV)

Vs (V)

-2

10-3

10050

100

150200250

300Energy (eV)Figure 6: Photoelectron energy spectra obtained by applying bias voltages to the suppressor. The filling pattern is
1/1153/4 and beam current changes from 620 to 530 mA
during measurement. Upper figure shows ratio of the photoelectron intensity as a function of the suppressor bias
voltage and bottom shows the differential of the upper.

in the scope with single shot and waveforms of the output voltages were measured. In order to justify the photoelectron measurement, we measured the peak-to-peak output voltages with changing the polarity of the solenoid
field in the vacuum chamber of the photoelectron detector.
Peak-to-peak output voltages correspond to the photoelectrons were measured. The peak-to-peak output voltage wasfound to be nearly linear to the positron beam current. This
result shows effect of secondary electrons can be negligible
at the location of the photoelectron detector, namely 1.5 m[3] F. Zimmermann, CERN-SL-2000-017(AP).
[4] K. Ohmi, private communication.