The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Measurement of Single Bunch Impurity in SPring-S Storage Ring using a Light Shutter System.
Kazuhiro TAMURAThe Institute of Physical and Chemical Research (RIKEN)
1-1-1, Koto, Mikazuki, Sayo, Hyogo 679-5148, Japan
Abstract
A light shutter system which operates in the visible lightregion has been developed. The light shutter has the rise/falltime of less than 1 ns, flat top of 1.1 ns and extinction ratioof larger than 200. These values are enough to pick out thelight pulse from a particular bunch in a bunch train at theSPring-8 storage ring. By applying the light shutter to a
photon counting system for the measurement of the bunch
impurity, the detection rate of photons from satellite
bunches was increased more than by the two order. A
satellite bunch of the impurity of 10· 9 level was successfullydetected.

principle of a light shutter system is briefly explained.
Experimental setup is shown in section 3. Characterizationof the light shutter and the application of it to the bunch
impurity measurement are described in section 4 and 5.
Related parameters of the SPring-8 storage ring are listed inTable 1.
2 Light Shutter System
The light shutter makes use of the linear electroopticaleffect of birefringent crystals, known as Pockels effect. The
effect results in rotating the plane of polarization of thelinearly polarized light propagating through a crystal as a
response to an applied voltage.

1 Introduction
When the SPring-8 storage ring is operated in the single- or
the several-bunch mode, some time-resolved experiments
demand the bunch impurity of less than 1o- 6. The bunch
impurity is defined as the ratio of the number of electrons ina satellite bunch to that in the main bunch. In an electron
storage ring, synchrotron radiation (SR) is emitted as lightpulses. The contrast between the light pulse from a satellite
bunch and that from the main bunch is proportional to thebunch impurity. We can obtain the bunch impurity bymeasuring the contrast of the light pulses. A photon
counting method is widely used for the bunch impurity
measurement [ 1]. In the method, SR is attenuated anddetected as a photon. The detection timing of the photonwith respect to the timing signal which synchronized to thebunch revolution is analyzed in digitally. An excellent
dynamic range can be easily obtained when a large numberof events are collected. This method, however, takes a longmeasuring time to obtain the contrast of the light pulses oflarger than 106 because it collects the events one by one.
This situation will be eased by use of a light shutter [2]which cuts the light pulse from the main bunch but passes
the light pulse from the satellite bunch. The light shutter
decreases the contrast of the light pulses and enables us tomeasure the bunch impurity quickly or precisely.
Table 1: Main parameters of the SPring-8 storage ring.
EnergyEGeV
8Circumference
c1436
mRadio frequency
508.58
MHzfrrHarmonic numberh2436
Revolution freguenc~
209
kHz~~'~We have developed a light shutter system which consistsof a fast Pockels cell, two polarizers and a high voltage
pulser. We measured the characteristics of the light shutter,such as the time profile and extinction ratio, and applied itto the bunch impurity measurement. In section 2, thePolarizer 1Pockels cell

Polarizer 2

Fig. 1 Conceptual drawing of the light shutter system.
The light shutter system consists of two polarizers
whose polarization angles are perpendicular to each otherand a fast Pockels cell placed between the polarizer [3]. As
shown in Fig. 1, when a certain voltage is applied to the cell,
the cell rotates the plane of polarization of the incident light.
The transmitted light from the cell can pass through thesecond polarizer, i.e., the shutter is opened. While no
voltage is applied, the plane of polarization of the incident
light is unchanged and the light can not pass through thesecond polarizer, i.e., the shutter is closed. We can operate
this system as a light shutter by applying high voltage
square pulses to the cell.
We have used a two-crystal type Pockels cell (Fastpulse
Technology, N1 072FW). The material of the crystals is
DKDP (KD 2P0 4). The pulser (Kentech Instruments, HMPI
Pulse Generator) can operate up to 220 kHz which covers
the revolution frequency of the SPring-8 storage ring. The
height and rise/fall time of the output pulses are 650V andless than 1ns, respectively.
The performance of the light shutter is dominated by thetime response and extinction ratio. In order to transmit thehigh voltage pulses with first rise/fall time, the cell 1072 is
designed to match the 50-0hm coaxial line system. The
extinction ratio is defined as the contrast between theintensity of the transmitted light with the light shutter
opened and that with the shutter closed. Precise alignmentof the cell with respect to the path of the incident light isnecessary to obtain high extinction ratio. The cell is alignedwith a precise three-axial rotational stage which the cell ismounted on.

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The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

3 Experimental Setup

We installed the light shutter system into the photon
counting system in the temporal photon beam extraction
line where the visible-light component in SR from abending magnet have been used for the beam diagnostics.
Not only the bunch impurity measurement, but the verticalbeam size measurement with a Young's interferometer [4]and the bunch length measurement with a streak camera
system have been also performed there.

which is shorter than a bunch· spacing of the SPring-8
storage ring (about 2ns). The ringing of the profile whichappears after closing the shutter is due to the undershoot of
the high voltage pulses.

508.58MHz,---------~RF~--------~·8-6-202468time (ns)Fig. 3 Time profile of the light shutter.

A Block diagram of the photon counting system isshown in Fig.2 together with the light shutter system. SR isreduced by ND filters down to the level of one photon
detection per hundred revolutions of a bunch and detectedby a micro-channel-plate type photomultiplier tube (MCPPMT, Hamamatsu Photonics, R3809U-52). The pulses from
the MCP-PMT are input to a fast timing discriminator(EG&G ORTEC, 9307). The discriminator produces thetiming pulses which synchronize the detection timing ofphotons. The time intervals between the pulses output fromthe discriminator and the timing signals synchronized to thebunch revolution are converted to the pulse heights by atime-to-amplitude converter (TAC, EG&G ORTEC 567).
The distribution of pulse heights is analyzed with a multichannel analyzer (SEIKO EG&G MCA7700-0IO).
In order to synchronize the operation of the light shutter
with the bunch revolution, the high voltage pulser is
e~ternally triggered by the timing signal generated using thesignal of RF system of the SPring-8 storage ring. The output
timing of the pulses, i.e., the timing of the light shutter to beopened, can be adjusted by changing the delay of theexternal trigger pulse.
4 Characterization of Light Shutter System

The time response and extinction ratio of the light shutter
were measured with the photon counting system. We usedthe visible part (wavelength range 400-650 nm) in SR for
the measurement of the time profile. Changing the trigger
timing of the light shutter to be opened by 250 ps step, we
measured the change of the intensity of the light pulse froma particular bunch in a bunch train. Figure 3 shows the time
profile of the light shutter. The ordinate is expressed by theextinction ratio. The light shutter opens/closes within 1 ns

We also investigated the dependence of the extinction
ratio on the wavelength range of the incident light. We
measured the extinction ratio at the wavelength of 441 nm,
488 nm and 532 nm (bandwidth 10 nm in FWHM). The
extinction ratio became worse as the wavelength became
shorter (620 @532 nm -> 156 @441 nm). This is due to theeffect of insufficient collimation of the incident light. If thepath of the incident light is not parallel to the optical axis of
the crystal in the cell, the plane of polarization will besomewhat changed even if no voltage is applied to the cell.
This effect gets severer and makes the extinction ratio worse
as the wavefength became shorter. Typically, the extinction
ratio for the wavelength range of 400-650 nm is larger than200. As summarized in Tab1e.2, the performance of the light
shutter is enough for extraction of the light pulse from aparticular bunch in a bunch train.
Table 2: Main parameters of the light shutter.
ns0.9
Rise time (10-90 %)ns
1.1
Flat top
ns0.8
Fall time (90-10 %)> 200
Extinction ratio
kHz209Repetition frequency
5 Application to Bunch Impurity Measurement
5.1 Performance of Bunch Impurity Measurement System
The Performance of the bunch impurity measurement
system with the light shutter was confirmed as follows. We
stored the single bunch beam the purity of which was madeworse than that of usual single bunch beam of the SPring-8
storage ring by the two or three order. We measured thedetection rates of photons from four satellite bunches before
and behind the main bunch with the light shutter and those
without the shutter. Since the light shutter opens for only
one bucket in a revolution period of the electron beam, thedetection rate was measured by adjusting the timing of theshutter to be opened to one of four satellite bunches. Thesame measurement was repeated for each satellite bunch.

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The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

By comparing the detection rate with the light shutter to thatwithout the shutter, we determined the magnification of thedetection rate of photons.
Figure 4 (a) is the time spectrum of the light pulses
measured without the light shutter (measuring time: 1000s).
We recognize three satellite bunches in -2nd, -1st and +1st
buckets but not in +2nd bucket. The sensitivity of theimpurity measurement at +2nd bucket is limited to 7 X 10· 7(statistical error of 99% level) by the noises due to thestructure of the MCP-PMT. Figure 4 (b) is the time
spectrum successively measured with the light shutter after
the measurement of the spectrum in Fig. 4 (a). The spectrum
was obtained by combining four spectra. Each spectrum
was measured for one of four satellite bunches selected by
the light shutter. The measuring time was 250s per satellite
bunch. The data of the main bunch is expressed by theaverage of four measurements. It is found that the contrast
between the light pulses is weakened more than by the twoorder. We confirmed that the detection rate of photons isenhanced by about 200 by the light shutter.

satellite bunches in ±1st buckets correspond to 6 - 7 X 1o-x.
In -2nd bucket, we recognize a satellite bunch of theimpurity of 7 X 1o- 9 .
In +2nd bucket, we can not find a satellite bunch atmore than noise level of 5 X 1o- 9 (statistical error of 99%level). It is consistent with the sensitivity without the shutter
mentioned in sub-section 5.1. Therefore, we can evaluate asatellite bunch of the impurity of above 4 X 1o- 9 with respect
to the main bunch of lmA for measuring time of 1000s.
10'JO'

with the light shutter· 500sec/satellite bunch ...

105

E104

0

10'::::

u102

JO'to"
10 7 ,-------.------,-------.------.~(a) without the light shutter
10 6 JOOOsec

-2!

-1

0

2bucketFig. 5 Time spectrum of the light pulses measured with thelight shutter.

6 Summary-2-I

0In order to improve the sensitivity of the impurity
measurement, we developed the light shutter system which
opens/closes within the bunch spacing and operates up to220 kHz. In the measurement of the single bunch impurity,
the light shutter well emphasized the intensity of the lightpulses from the satellite bunches by about 200. We detecteda satellite bunch of the impurity of 1o- 9 level successfully.
By use of the light shutter system, the sensitivity of theimpurity measurement has been improved to 4 X 1o- 9 formeasuring time of 1000 s.

2bucket106(b) with the light shutter
250sec/setellite bunch ·E::::Acknowledgement

0u-2-I

02bucketFig. 4 Time spectra of the light pulses measured (a) without
the light shutter and (b) with the light shutter.

5.2 Bunch Impurity Measurement
We stored the single bunch beam with the usual purity andevaluated the sensitivity of the measurement of the single
bunch impurity with the light shutter. The time spectrum of
the light pulses measured with the shutter is shown in Fig. 5.
It is also the combined spectrum of four measurements(measuring time: 500s per satellite bunch). Considering themagnification of the detection rate, the impurities of two

The author wish to express his appreciation to Dr. H.
Kitamura of RIKEN for his guidance and encouragement.
He would also like to show appreciation to Dr. N. Kumagai,Dr. H. Ohkuma and Dr. S. Takano of SPring-S for theirvaluable suggestion and useful discussion. He is alsoindebted to Dr. M. Masaki, Dr. M. Shoji, Mr. M. Oishi ofSPring-S and Dr. H. Suzuki of JAERI for their support tothe experiments of the impurity measurement.

References[ 1] T. Obina eta!., Nucl. Instr. and Meth. A354 (1995) 204.
[2] K. Tamura and S. Takano, Annual Report of SPring-S,
1994, p. 23S.
[3] A. Yariv, QUANTUM ELECTRONICS, third ED.,JOHN WILEY & SONS, Chap. 14.
[4] M. Masaki eta!, presented in this proceedings.

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