Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

SINGLE BUNCH PURITY
DURING SPRING-8 STORAGE RING TOP-UP OPERATION
K. Tamura, T. Aoki, JASRI/SPring-8, 1-1-1 Kouto, Mikazuki, Sayo, Hyogo, 679-5198, Japan
Abstract
Top-up operation of the SPring-8 storage ring has been
started since May 2004. Top-up injection is repeatedevery minute at several-bunch mode operation to keep thestored beam current at 99mA. To supply a stable andhighly purified pulse light, it is an important subject tokeep bunch purity less than 10-9 level during the top-up
operation. A gated photon counting system for single
bunch purity measurement that utilizes a fast light shutter
system and current status of the bunch purity during thetop-up operation are presented.

eliminates unwanted grid emission current from anelectron gun. When it operates successfully, the linac will
be able to supply a highly purified single bunch beam
into the booster synchrotron.
Measurement of the bunch purity of the SPring-8
storage ring has been done by a gated photon countingsystem, which operates in the visible light range.
Performance of the bunch purity monitor of the SPring-8
will be explained in section 2. In section 3, present statusof the bunch purity during the top-up operation will beshown.

2 BUNCH PURITY MONITOR

1 INTORODUCTION
To make use of pulse characteristics of synchrotron
radiation, the SPring-8 storage ring has been operated atmany kinds of filling modes, in which several singlets or
trains of bunches are located at equal intervals. At thesefilling modes, so-called several-bunch modes, some timeresolved experiments demand the bunch impurity of lessthan 10-9. Top-up operation has been started since May
2004 [1]. Very low current of 30~40 µA is injected every
minute, and the stored beam current is kept at 99mA. To
satisfy the demands of the bunch purity during the top-up
operation, we need a reliable bunched beam cleaningsystem that can supply stably the purified single bunch
beam for a long period and the bunch purity monitor witha high sensitivity.
At the SPring-8, a highly purified single bunch beam
has been produced with an RF-knockout (RF-KO) systemin the booster synchrotron and injected into the storage
ring [2]. With the RF-KO, the impurity of less than 10-9
has been constantly achieved [3]. In addition to the RFKO, development of a beam deflector system to improvethe bunch purity is underway [4]. The beam deflector

The bunch purity monitor has been installed at theaccelerator diagnosis beamline #1 (BL38B2), which has abending magnet light source. The monitor consists of afast light shutter system that operates in visible lightregion and a photon counting system that uses a microchannel-plate type photo-multiplier tube (MCP-PMT) asa photon detector [5].
Although the photon counting method is known as themethod with an excellent dynamic range, it takes a longmeasuring time to realize its dynamic range. Furthermore,even if we have taken enough measuring time, thesensitivity of the impurity is limited to around 10-6 level
because of noises due to the structure of the MCP-PMT.
In order to realize the sensitivity of the impurity of lessthan 10-9 in shorter time, we have developed a lightshutter system that can pick out a particular light pulse
from a synchrotron light pulse train. When we adjust thetiming of the shutter to be opened to a satellite bunch
adjacent to the main bunch, we can decrease the contrast
between the light pulse from the satellite bunch and thatfrom the main bunch.

HVPulser
Polarizer 1 (V)Polarizer 2 (H)Mirror 1

Pockels Cell 1
SR (white)

Output

MCPPMT

Pockels Cell 2

ND

Polarizer 4 (V)Polarizer 3 (H)50 OhmFigure 1: Schematic drawing of the SPring-8 bunch purity monitor.
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Mirror 2

Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

counting method. Counting rate of each bucket
corresponds to the intensity of light pulse. In this case,counting rates of the satellite bunches in +1st and -1stbuckets were 0.09 and 0.06 cps, respectively. Since thelight shutter was kept closed in this measurement, theshutter did not modify the intensities of light pulses. The
contrasts between light pulse from main bunch and thosefrom the satellite bunches are equal to the bunch
impurities. Counting rate of the main bunch was 3.2 kcps,
and the bunch impurities of +1st and -1st bucket were 3×10-5 and 2×10-5, respectively.
6105(a) Light Shutter CLOSED
1000sec

104count

10

3

102

10

1

100

10-1

01

relative bucket number
6105(b) Light Shutter OPENED
50sec/satellite bunch10410countBasic components of the light shutter are twopolarizers whose polarization angles are perpendicular toeach other, a fast Pockels cell placed between thepolarizers, and a high voltage pulser that drives thePockels cell. Two-set of the light shutter are used for theSPring-8 bunch purity monitor as shown in Fig. 1.
Performance of the light shutter is expressed by theextinction ratio and the time response. The extinction
ratio is defined as the contrast between the intensity oftransmitted light with the light shutter opened and thatwith the shutter closed. Transmission efficiency of thelight shutter depends on the wavelength of the light andthe applied voltage to the Pockels cell. Since we use
white light in visible region, we are concerned about thetotal efficiency of transmission. In our case, the light
shutter shows the maximum transmission efficiency at theapplied voltage of around 1.5 kV, and the extinction ratio
becomes maximum value. As reported before [5], theextinction ratio of a light shutter system is about 200. To
improve the extinction ratio, we have arranged two light
shutter systems in tandem as shown in Fig. 1. The time
response of a high voltage pulser and the characteristicsof the transmission line of high voltage pulses dominate
the time response of the shutter. We have used the pulser
with a pulse height of 1.5 kV, rise/fall time of 1 ns, pulse
width of 6~10 ns (variable). The pulser can operate at 209
kHz, which is equal to the revolution frequency of theSPring-8 storage ring. The Pockels cell (FastpulseTechnology, N1072) is designed to match the 50 Ω
coaxial line system.
2.1 Extinction Ratio of Light Shutter

310210The extinction ratio is defined as the intensity ratio oftransmitted light with the shutter opened with respect toleaked light with the shutter closed. Using thesynchrotron radiation, we determined the extinction ratio
as follows.
At the SPring-8, a 1 ns beam from the linac has been
purified by the RF-KO of the booster synchrotron. When
we turn off the RF-KO, we can store a single bunch beamaccompanied with satellite bunches due to the gridemission of the electron gun. Adjusting the operational
timing of the light shutter to one of the satellite bunches,
we can determine the extinction ratio by comparing theintensities of the light pulse from the satellite bunch at thetime of opening and closing the shutter.
Turning off the RF-KO, we stored a single bunch beamof 1.1 mA. Figure 2 (a) is the time spectrum of lightpulses measured with the light shutter closed (measuringtime: 1000 s). Horizontal axis shows the bucket number,
0 means the bucket where the main bunch is stored, -1
and +1 represent the buckets that locate before andbehind the 0th bucket, respectively. The RF acceleration
frequency of the SPring-8 storage ring is 508.58 MHz,and an interval of buckets corresponds to about 2 ns.
Vertical axis shows the number of counts by the photon

110010-101

relative bucket number

Figure 2: Time spectra of the light pulses measured (a)with the light shutter closed and (b) with the light shutter
opened.
Figure 2 (b) is the time spectrum with the light shutter
opened successively measured after the measurement of
the spectrum in Fig. 2 (a). The spectrum was obtained by
combining two spectra measured with adjusting theoperational timing of the light shutter to +1st or -1stbucket. Measuring time of each spectrum was 50 s. As adata of the main bunch in Fig. 2 (b), we displayed thedata of the main bunch measured with the light shutter
opened at +1st bucket. Opening the light shutter at +1stor –1st bucket, counting rates of the satellite bunches in+1st and -1st bucket are increased up to 11 and 7 kcps,
respectively, which are greater than that of the mainbunch.
The magnifications of counting rates show that theextinction ratio of the order of 105 has been achieved.
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Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)610500sec/satellite bunch
5104count10

310-112-10<2*10-92.5 10-92 10-9

1 1004/6/1604/6/1504/6/1404/6/1304/6/1204/6/1104/6/10-10

5 10Date

Figure 4: Growth of the bunch impurity during the top-up
operation.
The bunch impurity increased from 9×10-10 up to 2×10-9 during one week top-up operation. Since the bunch
current of the isolated bunch is 1.5 mA (4.5 ×1010
electrons), the number of electrons at +1st bucket
increased from 40 up to 90. In this period, the number oftop-up injections and total injected current into thisisolated bunch was 480 and 15.5 mA, respectively.

<7*10104 SUMMARY

110010-9

1.5 1004/6/9

We stored a single bunch beam of 1 mA with the RFKO, and measured the time spectrum of the beam withthe light shutter opened at +1 or -1st bucket. Figure 3shows the result. The time spectrum is also combined
spectrum of two measurements (measuring time: 500s per
satellite bunch). Counting rate of the main bunch was 2.7kcps.
We could not detect any photons at -1st bucket. This
corresponds to the impurity of less than 2×10-11 ascompared with main bunch of 1mA. We can conclude
that no electron exists at -1st bucket because the numberof electrons in main bunch is 3×1010. On the other hand,
there appeared about hundred events at +1st bucket. We
could not have distinguished the events whether they
originated from electrons or noises of the measurement
system yet. Supposing these events are not the noises but
the photons from the electrons, the bunch impurity at +1stbucket is 7×10-10. This corresponds to the existence ofabout twenty electrons at most. The sensitivities of themonitor are 2×10-11 or less at -1st bucket, and 7×10-10 at+1st bucket.

the sensitivity of the impurity measurement has been
limited to 7×10-10 at +1st bucket. We supposed all
events at +1st bucket were originated from theelectrons, and expressed the upper-limit of theimpurity in Fig 4. Error bars represent the statisticalerror.

Impurity

2.2 Performance of Bunch Purity Monitor-101

relative bucket number

Figure 3: Time spectrum of the single bunch beam
injected with the RF-KO of the booster synchrotron.

The SPring-8 bunch purity monitor that utilizes the fast
light shutter system with the extinction ratio of 105 has
been in operation. The sensitivities of the monitor are 2×10-11 or less at -1st bucket, and 7×10-10 at +1st bucket.
The bunch impurity during the top-up operation wasmeasured for one week and small degradation of thebunch impurity of the order of 10-9 was observed.

3 BUNCH PURITY DURING TOP-UP
OPERATION
Since May 2004, top-up operation of the SPring-8
has been started. In the top-up operation at severalbunch modes, a purified single bunch beam with asmall current of 30~40 µA is injected every minute. To
verify the performance of the top-up operation, weobserved time variation of bunch impurity.
From 9th to 16th June, the SPring-8 was operated
with the filling mode of 2/21-filling + 18 isolated
bunches. Thanks to the top-up operation, bunch currentof every isolated bunch was kept at about 1.5mA. We
continuously measured the impurity at +1st bucket justbehind a particular isolated bunch, and found that
small growth of the impurity occurred. The result isshown in Fig 4. As mentioned before, we have noteliminated the effect of noise of measuring system, andACKNOWLEDGEMENTThe authors wish to express their appreciation to Dr. N.
Kumagai, Dr. H. Ohkuma, Dr. S. Takano, Dr. H. Tanaka
and H. Yonehara of JASRI for their valuable discussions.

REFERENCES[1] H. Tanaka et al., “Top-up Operation at SPring-8 –Towards Maximizing the Potential of a 3rdGeneration Light Source ”, presented at EPAC’04.
[2] H. Suzuki et al., Nucl. Instr. And Meth. A, 444 (2000)
515-533.
[3] T. Aoki et al., PAC’03, Portland, May 2003, p.2551.
[4] T. Kobayashi et al., “ Dark Current Reduction System
for SPring-8 Linac”, presented at EPAC’04.
[5] K. Tamura, the 12th Symp. on Acc. Sci. and Tech.,Wako, Saitama, Japan, 1999, pp.546-548.
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