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

STATUS REPORT OF L· & S·BAND LINAC AT ISIR, OSAKA UNN.
Y. Yoshida, S. Okuda, N. Kimura, Y. Honda, T. Kozawa,T. Yamamoto, G. Isoyama, and S. Tagawa,
The Institute of Scientific and Industrial Research, Osaka University
8·1 Mihogaoka, Ibar~ki, Osaka 567, Japan
Abstract
The L·band and S·band linacs at ISIR, Osaka
Univ. were used for quantum beam science, suchas picosecond and subpicosecond pulse radiolysis,
slow positron beam, FEL, coherent radiation. The
recent activities are reviewed.
!.Introduction
The radiation laboratory, the institute of scientificand industrial research, Osaka University, has twoelectron accelerators, a 38 MeV L-band linac and a 150
MeV S-hand linac for the quantum beam science. Figure
1 show the location of two linac and experimental areasfor pulse radiolysis, slow positron, FEL and coherent
radiation. The 32 themes as shown below were adopted in
2001.

Pulse radiolysis (material)
12
Pulse radiolysis (biology)
3
Irradiation effect3
Application of subpicosecond pulse
2Slow positron
3
FEL & coherent Radiation Lab
5Machine study
4Many themes are closely related with nano science andnano technology by using quantum beams. The detail ofthe theme will be mentioned in section 3
2. Application of Quantum Beam
2.1 Picosecond pulse radiolysis
The performance of the L·band linac has beenused in the study of picosecond pulse radiolysis toinvestigate the primary process of radiation
chemistry.
Thedevelopmentof
thelaser-synchronized picosecond pulse radiolysis
was started in 1995.
Figure 2 shows a block diagram of laser
synchronized picosecond pulse radiolysis. The
electron pulse (pulse widthe: 20 ps, energy: 28MeV,) was used as the irradiation source, and the200 fs white continuum light pulse which wasproduced by Ti·sapphire laser amplifier system,
was used as the analyzing light. The transientabsorption was obtained by using so-calledstroboscopic method. The time-resolution wasdecided by the pulse width of the irradiation
source and the analyzing light and time-jitter
between the irradiation source and the analyzingsource.
The Ti-sapphires laser amplifier system wascomposed of a Ti·sapphire oscillator excited withan Ar-ion laser and a Ti·sapphire regenerated
amplifier excited with an Nd:YAG laser. The
amplified laser pulse was focused into a water cellto produce the continuum white light. The
analyzing light through a sample cell wasdetected by a Si·photo detector located after amonochromator.

LlNAC

Water cell
M@>'Lens

Opiical delay

Figure 1 Location of L·band and S·band linacs andexperimental section at !SIR. Osaka Univ..

Figure 2 Block diagram of laser synchronized picosecond
pulse radiolysis.

- 74-

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

Figure 3 Double pulse method in subpicoaecond pulse

radiolysis pulse radiolysis.

The L-band linac and the Ti·sapphireoscillator were synchronized with the common RF
to reduce the time jitter between the irradiation
source and the analyzing light pulse.
All components of the system were controlled
by a personal computer. The tome-resolution ofthe system was about 30 ps.
2.2 Subpicosecond pulse radiolysis
The subpicosecond pulse radiolysis system has beendeveloped by advancing the laser-synchronizedpicosecond pulse radiolysis system. The
subpicosecond electron pulse was generated by
the magnetic compression system at the L-bandlinac. The time jitter between the electron pulse
and the laser pulse reduces the time-resolution
strongly in the subpicosecond region. To avoid theproblem, the timing detection system as shown infigure 3 was introduced. The concept of the timing
detection system is that the time jitter is detectedby a femtosecond streak camera. The achieved
time resolution was 800 fs.
The performance of the subpicosecond pulse
radiolysis system had not been enough todetection the transient absorption .due to the low
SIN. This was caused by cause the vibration of theoptical components, such as the mirror, lens, andso on in the laser line. The double-pulse method
solved the vibration. The stability of the laser
intensity was kept in 0.8 % which was reduced
from 5 %. The details of the system will bereported elsewhere.
2.3 Slow positron beam
In order to estimate the free volume size andsub-surface structure of the material, the lifetime

spectroscopy and S parameter measurement of
the positron are very useful method. Figure 4
shows the experimental setup of the short-pulsed
positron beam and the high brightness one basedon the s·band linac.
The present pulse width of the positron beam
is about 550 ps, which is mainly caused by thewide energy spread of the positron beam. Thewidth will be reduced below 200 ps for the studyof polymers. Moreover, the background is too highto measure the lifetime. This problem will beimproved by modifying the detection system.
The high brightness · positron beam isobtained with the aid of W re·moderator. The
transport efficiency of the high brightness
positron beam was about 2 %, which is not enoughfor the measurement.
To improve the lowefficiency, a new small moderator was redesigned
by using numerical calculation.
Solid Ar is one of the most favorite materials
to get the high reemission efficiency, in which freeelectron and free volume do not exist in principle,
and the diffusion length is quite long (500nm).
The efficiency increased up to a factor of 1.3 in the
preliminary experiment by adopting thismoderator. The details of the solid Ar
re·moderator will be studied.
Further, a new positron beam experiments
has been planed by using the L-band linac. By
synchronizing the femtosecond laser system withthe positron pulse, the behavior of thepositronium in the solid material can be studied.
The experiment is quite new method to advancethe positron science.
2.4FELR&D of the far infrared free electron laser(FEL) based on the L-band linac is in progress.
We are analyzing the power gain of the FELHigh brightness beam• Low energy positron diffraction

Figure 4 Experimental setup of the short-pulsed
positron beam and the high brightness one based on thes·band linac.

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

measured so far, using the one-dimensional theory
and the three-dimensional simulation code,
TDA3D. The energy spread of the electron beam
integrated over the macro pulse accelerated with
the L-band linac is 4.4 %. We evaluated the gainfrom exponential growth of the measured powerin the later half of the macro pulse, where theFEL signal is largest. In order to know thetemporal variation of the energy spread over themacro pulse, we measured the energy spectra bychanging the macro pulse duration, and obtained
the energy spread in the later half to beapproximately 2 %. The experimentally measured
gain is in good agreement with FEL gain
calculated with TDA3D, using the energy spread
2 %, peak current 50 A and the normalized
emittance 150 1t mm mrad. The analysis of thegain with the TDA3D shows that the high-gain
effect and the optical guiding effect play
important roles.
The
proof-of-principle
experimentofself-amplified spontaneous emission (SASE) in theinfrared region is also conducted using the L-band
linac. The intense light was radiated when theelectron beam passed through the wiggler. The
light was assigned to be SASE, because
dependency of the optical intensity on the K·value
agreed well with the prediction of theone-dimensional theory. Moreover, the second andthe third harmonics were observed in the SASE
spectrum, which was the first observation of
non-linear harmonic generation of SASE.
In order to study the optical properties of theFEL and SASE, stability of the electron beam isvery crucial, though it is not realized.
Fluctuations of the power and the phase of the RF
for accelerator cause those of the electron beam.
We therefore analyzed fluctuations of RF system
togetherwithenvironmental
parameters
including temperatures at varied locations andthe AC line voltage using Autoregressive model. It
has been found that the fluctuation of the RF

system is closely related to fluctuations of the ACline voltage and the temperature of the klystronroom.
2.5 Coherent Radiation
The generation mechanism of the coherent
radiation has been studied by using the electron
beams of the L-band linac. The bunch shape of thebeam observed with a streak camera has been
compared with that obtained from the spectrum of
the coherent radiation emitted by the beam.
According to the results, the operationalparameters of the L-band linac have beenoptimized.
The peak intensity of the coherent radiation
at ISIR is more than five orders of magnitude
higher than that of the tera Hz wave light sources
using femtosecond pulse lasers. The application
researches of the new light source to absorption
spectroscopy and pulse radiolysis in a
submillimeter to millimeter wavelength region
have been started.
3. Nano science and technology by using quantum
beams
The picosecond and subpicosecond pulse
radiolysis, slow positron beam, FEL, and coherent
radiation are powerful methods to investigate thenano science and technology. Basic researches
have been started as follows.
a) Femtosecond pulse radiolysis system foranalysis of reaction mechanism in nano space.
b) Pulse radiolysis system for nano material.
c) Analysis system for nano structure by using
positron beamd) Basic research of FEL and SASE for nanomaterial application.
e) Development of THz source for spectroscopy ofnano materials.

- 76-