Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 95–98

Bunch lengthening in compact SR source HiSOR

T. Fujitaa, K. Gotoa, T. Kasugab, M. Katohb,c, Y. Kobayashib, F. Masakia,T. Obinab, M. Tobiyamab, K. Umemoria,*, K. Yadomia, K. Yoshidaa

aHiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, Higashi-Hiroshima 739-8526, JapanbHigh Energy Accelerator Research Organization, 1-1 Oho, Tsukuba 305-0801, JapancUVSOR Facility, Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan

Abstract

Current dependences of the bunch length have been measured in HiSOR electron storage ring at the beam energies of

150 and 700MeV. Significant bunch lengthening is not seen at 700MeV for the single and opposite 2-bunch operations,while the bunch is lengthened above a threshold for the full-bunch operation. Large bunch lengthening is observed at150MeV even for the single-bunch operation. Such bunch lengthening is regarded to be caused by the coupled-bunchinstabilities. # 2001 Elsevier Science B.V. All rights reserved.

PACS: 29.20.Dh; 29.27.Bd

Keywords: Synchrotron light source; Compact ring; Beam instability; Bunch lengthening; Spectrum analysis

1. Introduction

HiSOR is an electron storage ring dedicatedto the synchrotron radiation [1]. One featureof this ring is the low energy injection.The electrons are injected at 150MeV andaccelerated up to 700MeV. Since beam instabil-ities are stronger at lower energy, the maximumstored current is limited by the behavior of theelectrons at 150MeV. Another feature is thecompactness of the ring, the circumference of

which is 21.9m. The main ring parameters areshown in Table 1.

The bunch lengthening measurement is impor-tant since it represents the longitudinal motion ofthe electrons and the properties of the ring such asthe longitudinal impedance. Generally, the bunchlengthening is explained by the potential welldistortion and the microwave instability. Forcompact rings, however, the long-range wake fieldmight become important, even at the single-bunchoperation.

In this paper we report the bunch lengtheningat the injection energy of 150MeV and the storageenergy of 700MeV. Also, the measurementshave been done for different numbers of bunchesin order to study the effect of the long-range wakefield.

*Corresponding author. Tel.: +81-824-24-6297; fax: +81-

824-24-6294.

E-mail address: umemori@hisor.material.sci.hiroshima-

u.ac.jp (K. Umemori).

0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 2 4 7 - 9

2. Experiment

The bunch length has been measured by astreak camera, HAMAMATSU C1370-01,which measures a time structure of an electronbunch through synchrotron radiation fromthe bending section of the ring. The bunchlength is measured event-by-event from a singleshot of the streak camera without smearingdue to the center-of-mass motion of the bunch.In this paper, the bunch length is defined as aroot mean square deviation of a longitudinalcharge distribution. The experimental error isabout 10 ps.

The spectrum of the electron bunch is observedin order to investigate the beam instabilities. Aspectrum analyzer is used to analyze a signal fromthe button pickup electrode.

The measurements have been performed for thesingle-bunch, opposite 2-bunch and full (14)-bunch operations. The corresponding bunch spa-cings are 73, 36 and 5 ns, respectively. The numberof bunches is controlled by applying the RFknockout method with a gate circuit.

3. Results of the measurement

3.1. Bunch lengthening at 700MeV

The current dependences of the bunch length at700MeV are shown in Fig. 1 for the single, 2- andfull-bunch operations. Significant bunch lengthen-ing is not seen for the single- and 2- bunch

operations. No instabilities are observed from thespectrum. On the other hand, bunch lengthening isobserved above 7mA for the full-bunch operation.Furthermore, the peaks nfRF � ð2frev � fsÞ appearabove the threshold. The observed spectrum isshown in Fig. 2.

These features, observed at the full-bunchoperation, are regarded to be caused by thecoupled-bunch instability.

3.2. Bunch lengthening at 150MeV

In contrast to the case of 700MeV, the long-itudinal electron motion is more complicated at

Table 1

The main ring parameters of HiSOR

Energy (MeV) 150 700

Circumference (m) C 21.95

RF frequency (MHz) fRF 191.24

Revolution frequency (MHz) frev 13.66

Harmonics number h 14

Circulation time (ns) t 73

RF voltage (kV) VRF 11 203

Synchrotron frequency (kHz) Fs 65 130

Natural bunch length (ps) s0 47 110

Fig. 1. The current dependent bunch length at 700MeV.

Fig. 2. The spectrum for the full-bunch operation at 700MeV.

T. Fujita et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 95–9896

150MeV. As an example, the longitudinal chargedistributions of random-sampled events are shownin Fig. 3 for the bunch current of about 5mA. Thehigher mode oscillations, such as the quadruple orsextuple oscillations, can be seen in the singlebunch operation. Similar oscillations are alsoobserved in the 2-bunch operation. However, theoscillation amplitude is smaller than in the case ofthe single-bunch operation. The bunch distribu-tion is rather stable in the full-bunch operation.

The current dependences of the bunch length at150MeV are shown in Fig. 4. The bars in thefigure show the standard deviation of the bunchlength, in a sense, which represent the amplitude ofthe higher mode oscillations. It is observed that thebunch is largely lengthened for all operationmodes. Because of the fluctuation of the long-itudinal bunch shapes, as shown in Fig. 3, thebunch length at the single-bunch operation islarger than that at the 2-bunch operation. Thebunch shape fluctuation tends to become smallerat higher current. The peaks nfRF � 2kfrev �mfsare observed at the full-bunch operation. The fssidebands up to fourth order are observed for allthe bunch operations.

These beam behaviors at 150MeV are alsoregarded to be caused by the coupled-bunchinstabilities. We can conclude that the long-rangewake field largely affects the bunch length andthe longitudinal charge distribution at 150MeV.This effect is large even at the single-bunchoperation.

4. Discussion

4.1. Coupled-bunch instability

In order to search for sources of the coupled-bunch instabilities, the higher order modes(HOMs) in the RF cavity have been measured byusing a spectrum analyzer. A remarkable spectrumis observed at around 1.311GHz (¼ 96frev ¼7fRF � 2frev) for both 150 and 700MeV opera-tions. Higher order fs sidebands are seen up toninth or tenth order at 150MeV. At 700MeV onlya 96frev peak, without any fs sidebands, is observedfor the single and 2-bunch operations, while alarge 96frev þ fs peak is observed for the full-bunchoperation. It is notable that this 96frev þ fs peakdisappears below a threshold of 7mA.

Although more dedicated experimental andtheoretical studies are needed, the observedinstabilities might be explained by this HOM.

At the compact ring like HiSOR, the long-rangewake field can affect the beam even at the singlebunch operation, especially at low energy.

Fig. 3. The longitudinal charge distributions at 150MeV.

Fig. 4. The current dependent bunch length at 150MeV. The

bars show the standard deviation of the bunch length.

T. Fujita et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 95–98 97

4.2. Ring impedance

The potential well distortion model can describethe 700MeV single and 2-bunch data. Sincethe bunch lengthening is not appreciable, onlythe upper limit of the longitudinal impedance

is estimated from the single-bunch data to be30O.

References

[1] K. Yoshida et al., J. Synchrotron Rad. 5 (1998) 345.

T. Fujita et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 95–9898