The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

STUDY OF THE PHOTON BEAM STABILITY IN
COMPACT STORAGE RING HiSOR
I. Sakai, M. Arita, K. Goto, T. Nakahara* and T. Hori
Hiroshima Synchrotron Radiation Center , Hiroshima University, 2-313 Kagamiyama, HigashiHiroshima, 739-8526, Japan*Graduate School of Science, Hiroshima University,1-3-1 Kagamiyama, Higashi-Hiroshima, 7398526, Japan

Abstract
The rf cavity of the HiSOR is known to have the higherorder mode (HOM) which induces a longitudinal coupled
bunch instability (LCBI). We have found that thequadrupole bunch oscillation due to the LCBI contributes
to a considerable increase in the beam life time. However,there remains the question that the LCBI has an influenceon the photon beam stability. The photon bean position
and flux measurements were made to see how it depends
on the LCBI and the results showed an effect of the LCBI
should not be neglected. For the measurement of thephoton beam position, different types of monitors wereindependently used to mutually examine the error
included in the measured value.

INTRODUCTION
The photon beam stability is a important demand forsynchrotron radiation experiments. The photon beam
position monitoring is made in many synchrotron
radiation facilities[1-3]. In the HiSOR ring the beamdiagnostic beamline (BL8) has been constructed and a
preliminary studies for the photon beam stability weremade by the conventional two-wire monitor [4,5]. Inaddition, an alternative method has been developed by ourgroup and used independently to mutually examine theerror included in the measured value.
The compact electron storage ring HiSOR,
manufactured by Sumitomo heavy industries [6], wasinstalled at Hiroshima University in 1997 and it has been
operated successfully as a synchrotron radiation source
for the research and the education since then [7-10]. It isof a racetrack type with one rf accelerating cavity system(frf = 190.244 MHz) and two undulators at straight section.
The normal operating ring energy is 0.7GeV and theinitial stored beam current is 200 mA in the orbit of 0.8 m
in the radius. The other basic machine parameters aredescribed in ref. 8.
The rf cavity of the HiSOR is known to have the higher
order mode (HOM) which induces a longitudinal coupled
bunch instability (LCBI) [11]. When the quadrupole
bunch oscillation have appeared over several hours in an
operation, the beam life time has founded to beappreciable increased. However, photon beam flux
fluctuation has been observed at almost all the beamlines.
There are possibility that the electron beam instability due
to the excitation of the longitudinal HOM has an
influence of the photon beam stability. The photon beam

position and flux measurements were made to see how it
depends on the electron beam instability.

EXPERIMENTAL PROCEDURES
Photon beam stability measurements
Two kind of monitor can be used independently in theBL8. The first is the conventional two-wire monitor [4]. It
was placed at the position of 6300 mm from the source
point. In the analysis, the photon beam position measured
P can be expressed in a simple form
(I − I d )(1),
P = Cη = C u(I u + I d )where C is the coefficient to be calibrated, η is theresponse of the monitor, Iu and Id are the currents
generated from upper and lower wire, respectively. The
second is the two beam type monitor using verticalpolarized quasi-monochromatic beams. Figure 1 shows
schematically the structure of this monitor.
DI uSour cepoi nt( I u- I d) / ( I u+I d)

ABCDIdEFigure 1: Schematic structure of the polarized two-beammonitorA) Cylindrical lens; B) Band-pass filter; C) Slit; D)
Photodiode.
The cylindrical lens is used to focus the photon beam in
the horizontal direction. The band-pass filter cantered at
650 nm (λ=10 nm) is inserted in the beam path andpolarization filter is used to exclude the verticallypolarized component of the photon beam. The slit
aperture is rectangular with 1 mm (vertical) × 1.8 mm(horizontal). The photodiode (IRD UVG-20BNC) is put
at the position 8515 mm from the source point. Thecurrents from the photodiodes are measured byelectrometer (ADVANTEST R8240). The photon beam
position in the vertical direction can be calculated fromthe data referring to the eq. (1). The response η calculated
against the beam displacement is shown in Fig. 2. It
shows a linear relation within ±1mm.
On the other hand, the flux fluctuation of the incident
beam was recorded in the VUV Beamline 9 (BL9). The

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

flux monitoring device has been put on the focusing
mirror, the photoemission current generated can bemeasured in unit of volts.

quantities have a close relation to the thermal deformationof the rf cavity. All of them were measured as a functionof time.

RESULTS0.0-0.2-0.4-500

0500

1000

Vertical beam displacement(µm)

Figure 2: Calculated response η against the beamdisplacement.

Observation of the electron beam instability
In order to see the influence of the excitation of theHOM on the electron beam, the longitudinal andtransverse beam profile measurements were made byusing the streak camera (Hamamatsu C1587) and thestandard profile monitor using a focusing lens,
respectively. By observing the beam signal from the RF
cavity pick-up using a spectrum analyzer (HEWLETT
PACKARD 8561E), the magnitude and the frequency ofthe excited bunch oscillation mode were measured. Inaddition, the rf cavity pick-up voltage and the temperatureof the rf cavity wall were recorded because these-80

Pver-140

PHOM(dBm)

40-160-60

Vcav (mV)

41-120Vflux
dipole mode

quadrupole mode
1.325280

no mode1.325275-70

PHOM-80

FHOM

1.325270
1.325165
1.325160
1.325155

0.84

dT/dt

0.04
0.02

0.83

0.00
-0.02

0.82
12:00

Vcav
13:00

14:00

15:00

16:00

17:00

Time (h.m.)
Figure3: Example of the observation of the photon beam stability.

-0.04

FHOM (MHz)Pver (µm)-10042o-1000

An experiment was carried out in the usual operation ofthe ring. Figure 3 shows the example of the results ofobservation. At the top in Fig. 3 shows the measured
photon beam position Pver and the flux Vflux, in which thecompensation has been made for the reduction of the Vflux
depending on the stored beam current. They represented
that the drastic changes of beam position give the flux
reduction and fluctuation. The middle in Fig.3 shows thechange in the magnitude PHOM and frequency FHOM of theobserved bunch bunch oscillation mode due to the LCBI.
We see the mode changes between dipole (fHOM,d = 6frf+13frev +fs = 1.325163 GHz) and quadrupole oscillation(fHOM,q = 6frf +13frev +2fs = 1.325278 GHz) arising from afrequency shift of the longitudinal HOM. The bottom inFig. 3 shows the rf cavity pick-up voltage Vcav and thegradient of the increase in the temperature of the rf cavity
wall dT/dt. It is seen that the change in Vcav depends onthe dT/dt, suggesting that thermal load of the cavity wall
induces the deformation of its shape. Since theconsiderable changes in all measured quantities have been
occurred at the almost equal time, these measured
quantities are strongly related to each other. When nobunch oscillation mode appears, the photon beam position
was somewhat disturbed, showing the other possible
sources of instability are dominant.

Vflux(mV)

0.2

dT/dt( C/min.)η=(Iu-Id)/(Iu+Id)

0.4

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

In the streak camera measurements, the quadrupole
bunch oscillation was observed as shown in Fig. 4. It acts
to increase the effective bunch length resulting in an
increase in the beam life time and the result is shown inFig. 5.
time(ps)220

Int ensit y (a.u.)

180
140
10060

observation of the LCBI, it seems that the frequency ofthe longitudinal HOM shifts with a thermal deformationof the rf cavity and the LCBI is excited. New rf cavity are
now being installed to overcome the thermal load of it
and reduce the effect of higher order modes.
For the change in the vertical beam profile measured, it
is necessary to take into account the effect of the ion
trapping or the ring vacuum, resulting in a more reductionof the photon beam quality. Simple analysis showed thatthe error arising from the beam profile change is includedin the measured beam displacement and it is roughly
estimated to be over 30%.

20- 20300

500

7009001100 1300 1500 1700

Time (ps)Figure 4: Streak camera image (left side) and profile(right side).
1000

550

Acknowledgement
We would like to thank Prof. M. Taniguchi for his
encouragement of this work. We also would thank Mr. K.
Yadomi, Dr. H. Tsutsui and Dr. K. Umemori for theiraid and in data collection and many useful discussion.

Bunch length
Bunch length (FWHM, ps)

900
800

450

700
600

400

500
350
400

quadrupole mode
300
11:00

Beam life time (min.)Beam life time

500

300
12:00

13:00

14:00

15:00

16:00

17:00

Time (h.m.)

Figure 5: Average bunch length and beam life time.
In the vertical direction, we observed the beam profilechange at different bunch oscillation mode as seen in Fig. 6.

no modequadrupole mode

Intensity(arb.units)

200
150
100
500-600-400-200

0200

400

600

Vertical position(µm)
Figure 6: Vertical beam profile when quadrupole modeand (∆) no mode was observed.

DISCUSSION
Our results show that the stability of the photon beam isconsiderably disturbed by the excitation of thelongitudinal bunch oscillation due to the LCBI. In theREFERENCES[1] T. Mitsuhashi, K. Haga and T. Katsura, Proceedingsof IEEE Particle Accelerator Conference, 1987, pp.
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