The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

OPERATION OF THE ISIR L-BAND ELECTRON LINAC
IN VARIOUS MODES
S. Okuda, T. Yamamoto, S. Suemine, G. Isoyama,ISIR, Osaka University, 8-1 Mihogaoka, lbaraki, Osaka 567-0047, Japan

Abstract
High-intensity electron beams of the 38 MeV L-bandlinac are being generated in various pulse modes andirradiation doses for applications to experimental
researches. Single-bunch and multibunch beams aregenerated under the different operational conditions of thegun and the subharmonic prebunchers at macropulse
lengths below 2 J..lS. For the irradiation experiments withthe single-bunch beams the irradiation doses are in aremarkably wide range of 0.05-91 nC/bunch or 30 fC-1 0
The applicability of the beams tonC/mm 2/bunch.
experimental researches are discussed.

1 INTRODUCTION
High-intensity single-bunch and multibunch electron
beams are generated with a 38 MeV L-band (1300 MHz)linac [ 1,2] at the Institute of Scientific and Industrial
Research (ISIR) in Osaka University. The beams· arebeing applied to basic experimental researches foranalyzing ultra-fast phenomena induced in matters
irradiated with the beam. High-intensity far-infrared lightsources using free-electron lasers [3] and the coherent
synchrotron or transition radiation [4] from the beams
have been developed. In these experiments the electron
beams having various pulse modes at macropulse lengths
below 2 J..lS have been used.
Recently, high-current thermionic triode electron gun
has been developed [5]. By the improvement of the pulse
characteristics of the beams injected from the gun theelectron charge in the single-bunch beams has increased
to 91 nC/bunch. Irradiation experiments in a remarkably
wide dose range are being performed after thedevelopment of the method for monitoring a low electron

2 LINAC COMPONENTS
The components of the ISIR linac are schematicallyshown in Fig. 1. In the electron gun a cathode-grid
assembly YU-156 (EIMAC) is installed. The maximum
peak current of the beam from the gun measured with atest facility is 30.1 A at an anode voltage of 100 kV [5].
The pulsed electron beam injected from the gun iscompressed longitudinally with three subharmonicprebunchers (SHPBs) (two at an rf frequency of 108 MHz
and one at 216 MHz). The SHPB has a coaxial single-gapcavity. Pulsed rf generated with a 20 kW rf amplifier at apulse length of 20 J..lS is supplied to each SHPB. Thissystem has been optimised to generate the high-intensity
single-bunch beams. In this case the pulse length of thebeam from the gun is 5 ns FWHM.
Two bunchers are driven by a 5 MW L-band klystron.
An accelerating waveguide 3 m long is driven by a 20
MW L-band klystron. The pulse lengths of the rf on theflattop are 3.2 and 3.9 J..lS, respectively. The maximum
Table I: Beam pulse modes
Beammodes
Singlebunch
TwobunchMultibunch 1
Multibunch 2

dose using radiation dosimeters.

Macropulse
lengthBeam charge(nC/bunch)
91 (max.)20

9.2xNns
770ps

5 ns- 2J..ls

0.2

9.2 ns2J..ls (max.)2

1.3 GHz
20MW

1.3GHz5MW---~--... ~~--... ;--'----.:Bunchers

108 MHz 108 MHz 216 MHz
20 kW
20 kW
20 kW
rf source rf source rf source

Micropulse
spacing

Accelerating
waveguidecompressor

BCM: Beam current monitor
BPM: Beam profile monitor
OM: Quadrupole magnet

Figure 1: Schematic diagram of the components of the L-band linac at ISIR.

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magnetThe 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

repetition rate of the pulsed rf is 360/s. The operational
conditions of the rf components are described in ref. 2.
Electron beam current monitors and beam profile
monitors are placed in the beam transport system, some ofthem being shown in Fig. I. The energy spectrum of thebeam is measured with a bending magnet and a Faradaycup.
Under the various operational modes for the linac
components single-bunch and multibunch beams asindicated in Table 1 are generated. Two-bunch beams aregenerated by injecting two pulsed beams succeedingly
from the gun [6]. The micropulse spacing of themultibunch beams depends on the operational conditionsof the SHPB system, and can be changed by selecting thenatural number N.
The beams are transported toirradiation rooms and are being applied to various kindsof experiments.

3 CHARACTERISTICS OF THE SINGLEBUNCH BEAM
The characteristics of the single-bunch beam have beenmeasured at an energy of 27 MeV, which is a common
energy in usual experiments. The electron charge of thesingle-bunch beam increases with the peak current of apulsed electron beam injected from the gun. At a peak
injection current of 28 A the electron charge has themaximum value, 91 nC/bunch. The width of the energy
spectrum is 0.8-1 % FWHM at charges below 45 nC, andat the higher charges it increases with the charge.
The single-bunch beam of the ISIR linac generally hasa triangular bunch shape and the bunch length is 20-30 ps
FWHM. In the previous work the bunch shape of thebeam has been investigated by analyzing the longitudinalbunch form factor given from the spectrum of thecoherent transition radiation emitted from the beam [7].
The beam emittance has been measured with themethod using a quadrupole magnet and a beam profile
monitor. Figure 2 shows the electron charge dependenceof the normalized rms beam emittance measured atcharges of 9-63 nC/bunch. The averaged value ofemittance for vertical and horizontal directions is plotted
and the error in the measurement is indicated. This figure
shows that the emittance increases as the charge and is ina range of 65-160 mnm mrad. These values are below
20% of those of the linac at Argonne National Laboratory[8] in which an electron charge of 100 nC/bunch wasobtained.

4 IRRADIATION AT A WIDE RANGE OFDOSES
The irradiation dose of a beam in a certain volume of atarget sample is determined mainly by the intensity andthe quality of the beam. Assuming a bunch length of 25
ps the beam brightness are given from the electron chargeof the single-bunch beam and the emittance, as shown inFig. 3. The brightness has a peak at a charge of 40-50
nC/bunch. Assuming the irradiation area of 2x2 mm2, theirradiation dose is about 10 nC/mm2/bunch in maximum.
On the other hand, for the irradiation at relatively low
doses it is important to monitor beam intensity andirradiation doses accurately. The experimental setup formonitoring relatively low irradiation doses isschematically shown in Fig. 4. By using the beam current
monitor the bunch charge at 0.05 nC/bunch can be• "C"'....

2·ew-g~(/)

150~ ~:.=-

as50z0!

!

••

1:10

""CICIJN0 ..... -~-~~-.L.._____]_ .....~ ......

I I

2040

..... J . . . . . . .

~-.

0~

Charge of Electrons (nC/bunchl

50

Al plate

E00L~

100

Figure 3:
Electron charge dependence of thenormalized brightness of the single-bunch beam,given by the results shown in Fig. 2.

....

E E 100... E"O E•CIJ0c:•"'"'Q)as•.....c..c200 .--------------- -----------.

:::::

360

80

100

Charge of Electrons (nC/bunch)

TLD------~~~~ D
Imaging plate

Figure 2: Electron charge dependence of thenormalized rms emittance of the single-bunch beam,given by being averaged for vertical and horizontal
directions.

Figure 4: Schematic diagram of the experimental
setup for monitoring relatively low irradiation
doses.

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The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

monitored. The diameter of the electron beams isexpanded by the vacuum window and an aluminiu?I plate
2 mm thick used as a beam expander. The beam diameterthus obtained is 50 mm<!> at 450 mm from the plate. In
this case the lowest irradiation dose corresponds to 30
fC/mm 2. The density distribution of the beam in thelateral direction has been measured by a wire-scanning
method and with an imaging plate. The irradiation dose
has also been measured with a thermoluminescence
detector (TLD) used for personal dosimetry. The dosecorresponds to 1 mSv. In the irradiation the effect of thegrid emission from the gun is avoided. In this experimentirradiation doses have been expanded as 0.05-91
nC/bunch or 30 fC-1 OnC/mm2/bunch.
The minimum irradiation dose is determined by thesensitivity of these dosimeters and can be decreased more
by about two orders of magnitude.

5 BEAM APPLICATIONS
The brightness of the single-bunch beams of the ISIR
linac is about two orders of magnitude lower than that of
the linac using photocathode rf gun while the intensity of
the former is one order of magnitude higher that the latter.
For the self-amplified spontaneous emission free-electron
laser experiments the brightness and the intensity of thebeam are important factors. On the other hand, in thepulse radio lysis experiments using the beams in· air
atmosphere the beam intensity is the most importantfactor when the irradiation area is more than a few mm2.
For the experiment of the coherent radiation in a
submillimeter to millimeter range the intense beams of
the ISIR linac have good features.
The irradiation experiments at relatively low doses arebeing performed for characterizing highly sensitive
dosimeters such as imaging plates and TLDs, and for
imaging experiments with the imaging plates.
The various beam modes of the ISIR linac and theirradiation experiments at a wide range of irradiation
doses will expand the research fields using the beams.

6 CONCLUSIONSThe single-bunch and the multibunch beams weregenerated with the ISIR linac in various pulse modes
given by the various operational conditions of the gun andthe SHPB system. The electron charge of the singlebunch beam is 91 nC/bunch in maximum. By using thebeam the irradiation doses of 0.05-91 nC/bunch or 30 fC10nC/mm2/bunch were achieved. By these features in thebeams of the ISIR linac the application research fields areexpected to be widely expanded.

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