CHARACTERISTICS OF THE ELECTRON LINACS OF OSAKA UNIVERSITY
AND THEIR APPLICATION TO LIGHT SOURCES
S. Okuda, K. Tsumori, J. Ohkuma, T. Sawai, N. Kimura, T. Yamamoto,T. Hori, S. Takamuku and S. Suemine•
The Institute of Scientific and Industrial Research, Osaka University
Mihogaoka, Ibaraki, Osaka 567, Japan*Unicorn System
Abstract
In Osaka University electron beams from a 38 MeV Lband linac and a 150 MeV S-band linac are available forexperimental research. Each component and the beamcharacteristics of these linacs have been investigatedfor developing new light sources using radiation fromthe high-energy electron beams. From the results of thepresent work the applicability of the linacs torealizing the intense light sources are discussed.
Introduction
One of the advanced applications of high-energy
electron beams is to light sources. A 38 MeV L-band
linac in our insti tute 1 is equipped with threesubharmonic prebunchers (SHPB's). A single-bunch beam
generated by this linac has electrons as much as 70 nC
in total charge, which has been availed for studying
ultrafast phenomena induced in liquid or solidmaterials. The peak current of the beam is about 3 kA inmaximum, which is sufficient for the high-gain freeelectron laser (FEL) experiments.
Electron beams passing through a bending magnet emit
synchrotron radiation. In the case of bunched beams theradiation becomes far more intense at wavelengths longer
than the bunch length. This phenomenon has been recently
confirmed in experiments. 2 The single-bunch beam seems
to be applicable to this experiment.
The SHPB system installed in the linac makes
micropulses of high peak-current in a macropulse. Suchfunction has been applied to some FEL systems with rflinacs. 3 • 4 The present SHPB system can be applied to theoscillator-type FEL experiments.
We have newly constructed a 150 MeV S-band linac. 5
The beam energy is high enough for visible FEL
experiments and a positron source being prepared.
The present work has been made to investigate theeach component and the beam characteristics of thelinacs for applying the beams to developing new lightof 2 ps (Fig. 2). The shape and the peak current of thebeam can be controlled.
When a long-pulse beam of a duration of 1. 5 us are
injected under operating the SHPB system micropulses aregenerated in the macropulse at an interval of 9. 2 ns
corresponding to the 12th subharmonics of 1300 MHz rf
used for acceleration. The pulse shape has beenmonitored by measuring cherenkov lights with a phototube
of a time resolution of 60 ps, as shown in Fig. 3. This
figure shows that the SHPB' s work well over themacropulse and that satellite pulses are comparativelysmall. The charge in the micropulse is evaluated fromthe pulse shape measured with a streak camera to be 0.65
nC.
threeconsists
ofaccelerator
S-bandThe

Table 1

Beam characteristics of the two linacs.

Energy (max. )Accelerator Freq.
Micropulse spacing
Chargejmicropulse
Peak current
Macropulse length
Repetition rate
Energy spread
Emittance(norm.)

L-bandLongSingle

S-bandLong38 MeV
1. 3 GHz
9.2 ns
1 nC
70 nC
50 A
3 kA
2.5 )lS)20 ps
360 pps
720 pps
1%<200:n:mm.mrad

150 MeV
2.86 GHz
0.35 ns
0.2 nC
10 A4 )lS
60 pps1%<30nmm.mrad(gun)sources.
Two electron linacs and the beam characteristics

The main components of the L-band linac and the Sband linac are schematically shown in Fig. 1. The beamcharacteristics for typical three modes are shown inTable 1.
The SHPB system of the L-band linac consists of two
12th SHPB's and a 6th SHPB. In order to generate asingle-bunch beam a pulsed beam of a duration of 5 ns is
injected from the gun to the 1st SHPB. The pulse shapeof the cherenkov lights from a radiator, which iscorresponding to the shape of the single-bunch beam, hasbeen measured with a streak camera of a time resolution

Fig.

Pulse shape of the single bunchmonitored with a streak camera.

ACC.WRVEGUIDE

(a)

(b)

2GUN H P B H BUN HL_,.,_l_A_c_c_._w_._G__JH 'i'2 Ace. w. G H +3 Ace. w. GFig.

Accelerator system of (a) the 38 MeV L-band linac and (b) the 150 MeV S-band linac.

-32-~

beam

Table 2

Wiggler parameters.

(a)LengthPeriodNumber of periods
K parameter

LongShort

198 em6 em
33
1-248 em
4 em
121-210.---------------------------~---.

Phase-slip Mode

Phase-advance Mode(b)E=30 MeV

30H2 Gas Pressure at 20'C (atm)Fig. 3Pulse shape of the multi-bunch beam monitoredwith a phototube: (a) 1.5 us macropulse; and(b) micropulses at a 9.2 ns interval.

accelerating waveguides to which rf is supplied fromthree 35 MW klystrons. The overall length of the linac
has been made relatively short, according to theTheroom.
linac
theofarea
small
relatively
accelerating gradient of 19.3 MV/m could be achieved. 5
The electron gun is a main component determining thefinal beam emittance. The normalized emittance of thebeams genera ted by the gun was measured with a beamprofile monitor and a multi-whole beam mask and wasabout 30 ~mm.mrad which should be much improved for FEL
experiments.

Fig. 4Pressure dependence of the wavelength ofradiation for gas loaded FEL.

When a single-bunch beam passes through a bending
magnet the synchrotron radiation of relatively long wave
lengths compared with the bunch length becomes coherent,
and the intensity of the radiation becomes about N timesof the ordinary radiation: N denotes the number ofelectrons in the beam; and is 5xl0 11 for the presentbeams. The radiation strongly depends on the shape ofthe bunch. It is advantage of this system that the shape
can be controlled. The high-current single-bunch beams
are effective for the experiments generating intense
pulse lights of submillimeter-millimeter wavelengths bybending magnets.

Radiation from the single bunch beams
FEL experiments with the long-pulse beams
The single-bunch beam generated by the L-band linac
is available to produce coherent pulsed lights.
The peak current of the single-bunch beam iscomparable to those of the induction linacs used for FEL
experiments: 6 The energy spread and beam radius arecomparable; the present beam emittance shown in Table 1
is about one order of magnitude lower than that of thelatter; and the pulse duration (about 20 ps) is about 3
orders of magnitude shorter. By using the present beam
high-gain FEL experiment will be made to obtain intense
pulsed lights and to investigate the interaction of thebeam and the radiation in an optical cavity. In Table 2
the wiggler parameters for the present experiments arelisted. The wavelengths of the radiation is calculated
to be 10-40 pm for the beam energy of 25-35 MeV.
When an electron beam passes in gas throug,h aBy
wiggler the wavelength of the FEL can be shortened.
using the single-bunch beams and a relatively short
wiggler, whose parameters are shown in Table 2, theeffect of gas has been investigated. The relation
between the hydrogen gas pressure and the wavelength
theoretically evaluated is shown in Fig. 4. At apressure of 1.3 atm visible lights have been observed at
an intensity comparable to cherenkov lights radiated at
the same time. This method will be applied to the FEL
experiment with the single-bunch beam described above,
after relatively intense infrared lights being obtained.

-In the ordinary FEL experiments with rf linacs, thepulsed lights radiated is amplified by each electron
micropulse repeatedly in an optical cavity because theintensity of the radiation for the single passage of a
micropulse is not sufficient. Such FEL requires lowemittance and high peak-current beams. For the longpulse beam from the L-band linac the SHPB system works
well for bunching beams but the gun should be replaced
by a low-emittance gun. The S-band linac doesn't have an
SHPB system and the beam emittance measured isrelatively high. We should develope a new injector
system such as an rf gun generating low-emittance beams
with some bunching process. We have started theimprovement of the gun sys tern of the L- band linac andare planning to make a new injector system for the Sband linac.
Conclusions
The each component and the beam characteristics ofthe two linacs have been investigated for developing thesources of FEL and intense pulsed lights. The results
have shown that the single-bunch beams from the L-band
linac are effective for obtaining intense pulsed lights.
These also have shown the effectiveness of applying thetwo linacs to FEL experiments under the improvements of33-

the electron gun_or the injector system.

References
1. S. Takeda et al., IEEE Trans. Nucl. Sci. NS-32(1985) 3219.
2. T. Nakazato et al., to be published in Proc. 14th
Int. Conf. on High Energy Accelerators, Aug. 22-26,1989, Tsukuba, Japan.
3. R. W. Warren et al., Nucl. Instr. Meth. A259 (1987)

B.
4. K. E. Robinson et al., Nucl. Instr. Meth. A259
(1987) 49.
5. K. Tsumori et al., to be published in Proc. 14th
Int. Conf. on High Energy Accelerators, Aug. 22-26,1989, Tsukuba, Japan.
6. T. J. Orzechowski et al., IEEE J. Quantum Electron.
QE-21 (1985) 831.
7. A.-M. Fauchet et al., IEEE J. Quantum Electron. QE20 (1984) 1332.

-34-