The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Generation of ultrashort electron pulse
for laser synchronized picosecond pulse radiolysis system
Takahiro KOZA WA, Katsunori HORI, Yasuhiro MIZUTANI, Tamotsu YAMAMOTO,
Shouji SUEMINE*, Yoichi YOSHIDA, Seiichi TAGAWA
The Institute of Industrial and Scientific Research, Osaka University,
8-1 Mihogaoka, Ibaraki, Osaka 567, Japan*Unicorn System, 6-24-28 Misaki, Suminoe-ku, Osaka 559, JapanAbstract
A pulse radiolysis system by using a femtosecond
laser synchronized with electron linac was developed atOsaka University. This system has a potential to detect
ultrafast phenomena in the femtosecond region. The pulse
width of analyzing light is 100 fs. However, the presentpulse width of the electron beam (irradiation source) is 20
ps. The pulse width of electron beams and the time jitter ofthe system mainly decide the time resolution of the wholesystem. We attempted to generate a shorter pulse by
installing a magnetic pulse compressor to the L-band linacof Osaka University. The pulse width of electron beams wasmeasured by a picosecond streak camera with a timeresolution of 2 ps. 27 MeV electron pulses with the pulse
width of 30 ps have been compressed to 2.4 ps, which isequal to the time resolution of the streak camera, by themagnetic pulse compression system.
1. Introduction

A pulse radiolysis is one of the most powerful
methods to study the very fast radiation-induced reactions.
To detect so fast reaction, the so-called stroboscopic
500mm

technique is used in the picosecond pulse radiolysis. The
short-lived intermediates produced by very short radiationsuch as electron beams are detected by measuring theoptical absorption of very short analyzing light such as
Cherenkov radiation. The new picosecond pulse radiolysis
system, in which the femtosecond laser is used instead of
the Cherenkov radiation, was proposed at FST'951). The
merits ofthe laser system are as follows:
1) Obtaining the optical absorption in the wide
wavelength region from 250 nm to 2 !IDl,2) Easy to improve into femtosecond pulse radiolysis.
The most difficult point of the system is the synchronizationof both the electron pulse and the laser pulse. The
synchronization was succeeded at our laboratory of Osakauniversity first in the world in 1995, reported in FST'96 2l.
In this system, the timing is controlled by radio frequency(RF) system. The time profile of the optical absorption canbe obtained by changing the phase of the RF. The detectablewavelength is from ultraviolet to infrared by using SHG,THG, and OPO techniques etc.
The investigation on ultrafast phenomena in thepicosecond regime have been started. The time-dependent
behaviors of short-lived species in materials have been

45oBending MagnetIBeam SlitGate Valve

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Magnetic PulseCompressor

ISIR L-band Linac

Fig. 1 Magnetic pulse compressor-77-Linac RoomThe 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

obtained. The subjects are:1) Primary processes of radiation chemistry inpolyethylene and its model compounds,
2) Observation of short-lived species of a-conjugatedpolymer,
3) Study of radiation-induced reactions in materials
for electron beam and X-ray lithography.
Our pulse radiolysis system by using a femtosecond
laser synchronized with electron linac has a potential todetect ultrafast phenomena in the femtosecond region. The
present time resolution is several ten picoseconds. The
present pulse width of the electron beam is 20 ps. The pulse
width of electron beams and the time jitter of the system
mainly decide the time resolution of the whole system. The
plan to make a shorter electron pulse has been started. A
magnetic pulse compressor was installed to the L-band
linear accelerator of the Institute of Industrial and Scientific
Research, Osaka University (ISIR L-band linac). The
longitudinal distribution of electron pulse is modulated forpulse compression by adjusting the RF phase in accelerating
tube. The magnetic field in the pulse compressor effectuatesa phase space transformation, translating the energy
dispersion into a time correlated spread of trajectory lengths.
We attempted to make a ultrashort pulse by magnetic pulse
compression for improvement of time resolution of ISIR
laser synchronized picosecond pulse radio lysis system.

used avoid the pulse broadening due to optical dispersion in
the convex lenses used in the measurement. All data wereacquired by a single shot measurement to avoid effects ofjitter during accumulation. Beam sizes were also measured
by using phosphor screens (AF995R, Desmarquest Co. Ltd.)at the end of beam line.
3. Results and discussion
3 .1. Numerical simulation
The characteristics of compression are evaluatedby a numerical electron tracking method. Figure 2 shows theeffects of the beam slit at the center of pulse compressor.
Simulation parameters used in the analysis are
235nmmmrad as 90% normalized emittance, 0.2 %as theenergy spread in the same phase, 70° as the acceleratingphase of the traveling microwave in the accelerating tube
and 10 MV /m as the peak electric field in the accelerating
tube. It is found that the width of compressed pulse become
shorter by installation of slit. When the width of slit is 40mm, the width of compressed pulse becomes shorter with,--...,

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2. Experimenta l setup
Figure 1 shows the ISIR L-band electron linac. TheISIR linac consists of an thermionic electron gun (YU-156,EIMAC), two 108 MHz (1/12 of the main accelerating
microwave frequency of 1300 MHz) subharmonic buncher(SHB), a 216 MHz (116 ofthe main accelerating frequency)
SHB, a 1300 MHz traveling wave type prebuncher, a 1300
MHz traveling wave type buncher, a traveling wave type
accelerating tube and focusing system. The accelerating
potential of the electron gun is provided by 90 kV DC.
A pulse compressor was installed at ISIR L-bandin Fig. 1. The achromatic pulse compressor
shown
linac as
45° sector magnets, four quadrupole
twoconsists ofslit in the horizontal direction.
beamaandmagnets
Identically, the longitudinal distribution of fully accelerated
relativistic beam should be modulated for magnetic pulse
compression by adjusting the RF phase in an accelerating
tube. However, ISIR L-band linac has only one accelerating
tube. Therefore, in this· experiment, acceleration andmodulation were carried out by one accelerating tube,simultaneously.
Pulse width of a relativistic beam was evaluated bymeasuring Cherenkov radiation emitted by the relativistic
electrons in air at the end of the beam line. The Cherenkov
radiation was measured by using the picosecond streak
camera which has the time resolution of 2 ps (Hamamatsu
Photonics Co. Ltd.). An optical band pass filter, which iscentered at 461.5 nm and has a half width of 10.7 nm, was-78-Iau

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Width oflnitial Pulse (ps)Fig. 2 Effects of beam slit at the center of the pulse
compressor on efficiency of pulse compression
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Fig.3 Relation between beam quality and width ofcompressed pulse

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 199731

decreasing of the pulse width of initial beam before
compression. However, in the case of 10 mm, the width ofcompressed pulse is independent of that of initial pulse.
Figure 3 shows the relation between beam quality(transverse emittance and energy spread before modulation)and the width of compressed pulse. It is found that anefficiency of pulse compression depends on beam quality.
High quality beam is necessary for the generation of ultra
short pulse. Figure 4 shows the relation between theaccelerating phase and the width of compressed pulse. It isnecessary that the accelerating phase is less than 70° for theenergy modulation. One of the typical results of simulationis shown in Fig. 5. The beam parameters are 235 nmm mrad
as 90 % normalized emittance, and 0.2% as the energy
spread before the energy modulation. The width of slit is 10mm.
Energy ~pread (%)
Normalized Emittance (,--.,

7r>' 30.5~'-'@ 30~~ 29.52950Time (ps)(a) Phase Space Distribution of Compressed Pulse~mm mrad)5000

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Accelerating Phase
Fig. 4 Relation between the accelerating phase andthe width of compressed pulse

3.2. Magnetic pulse compression experiment
The RF phase in the accelerating tube was tuned soas to accelerate electrons in the early phase of the pulse
more than those in the later phase of the pulse. The peakelectric field in the accelerating tube was adjusted to 10
MY /m. The magnetic fields of pulse compressor were
adjusted so as to make the shortest pulse while monitoring
its width by using the streak camera. The measured pulse
shape of the shortest compressed pulse riding on the phaseof 70° is shown in Fig. 6. The pulse width was 2.4 ps andthe horizontal and vertical beam sizes (full width) of thecompressed pulse were 5.0 mm and 6.0 mm, respectively.
The charge was 2.0 nC/pulse.

to the time resolution of the streak camera. Now, thepicosecond pulse radiolysis system using the compressedpulses is under construction. This system will achieve thehighest time resolution in the world. Furthermore, we
attempt to make a shorter pulse (femtosecond electronpulse) by improvement of beam quality and to construct afemtosecond pulse radio lysis system.
References[1] Y. Yoshida et al., Proc. Femtosecond Techno!. '95,(1995) 63.
[2] S. Tagawa et al., Proc. Femtosecond Technol. '96,(1996) 31.
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4. Summary:;-; 20

27 MeV electron pulses with the pulse width of 30
ps were compressed by the magnetic pulse compression
system at ISIR of Osaka university. 2.4 ps single pulse wasgenerated with the charge of2.0 nC. The length of the pulse
has been measured by a picosecond streak camera with atime resolution of 2 ps. The measured pulse width is equal$ 10________ .; __________ ,__ _''(/)-79-cc50

100

150200Time (ps)Fig. 6 Measured pulse shape of compressed single pulse