The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

STATUS OF S-BAND FEMTOSECOND LINEAR ACCELERATORIN OSAKA UNIVERSITY
J. Yang#, H. Tomosada, K. Takeya, T. Yamamoto, T. Kozawa, Y. Honda, Y. Yoshida, S. Tagawa
The Institute of Scientific and Industrial Research, Osaka University,
8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
Abstract
A new S-band femtosecond electron linear accelerator
was developed in Osaka University for the study of radiation-induced ultrafast physical and chemical reactions infemtosecond time regions. The femtosecond electron
accelerator was constructed with a laser driven
photocathode RF gun, a linear accelerator (linac) and amagnetic pulse compressor. The electron pulse produced
from the photocathode RF gun was accelerated by thelinac with energy modulation and compressed into a few
tens femtosecond by the pulse compressor. The
femtosecond electron pulse is expected for the study ofultrafast phenomena by means of pulse-radiolysis with afemtosecond laser light.

INTRODUCTION
Development of a pump-probe measurement techniquein the femtosecond or attosecond scale is important forthe study of a dynamic process involving the mechanicalmotion of electrons and atomic nuclei in physics,
chemistry and biology. A pulseradiolysis, which ispumped by an ultrashort electron beam and probed by an
ultrashort light, is a powerful tool for the observation ofultrafast radiation-induced phenomena, such as ionization,excitation, relaxation, electron transformation and so on.
The time resolution of the pulse radiolysis has reached
to picosecond by using a picosecond electron pulse. In
Osaka University, a subpicosecond pulse radiolysis with atime resolution of 800 fs was developed by using afemtosecond electron pulse produced in an L-band linear
accelerator, a probe femtosecond laser light, and the time
jitter compensation between the electron pulse and thelaser light[1,2]. Recently, a new femtosecond pulseradiolysis system with a time resolution of <100fs wasdeveloped in Osaka University.
However, in order to achieve such time resolution, thedevelopments of the three technologies are required: (1)afemtosecond electron pulse, (2)a probe femtosecond lightsource, and (3)precise synchronization between theelectron pulse and the probe light. To solve theseproblems, a laser-driven photocathode RF gun based
linear accelerator was constructed to produce such shortelectron pulse. It was known that the efficiency ofcharged particle pulse compression depends on the beam
quality. The photocathode RF gun produces a low
emittance short-bunch electron beam, such as <1 πmmmrad at 3 ps[3,4], resulting in an effective pulse
compression into femtosecond. Another advantage by_________________________________________#

yang@sanken.osaka-u.ac.jp

using the RF gun is easy to synchronize the probe laser
light with the electron pulse.

EXPERIMENTAL ARRANGEMENT
Photocathode RF Gun and Laser
Figure 1 shows the new femtosecond pulseradiolysis
system. A 1.6-cell S-band (2856MHz) RF gun, produced
by Sumitomo Heavy Industries (SHI)[3,4], was used inthe system. The RF gun was composed of two cells: a halfcell and a full cell. The length of the half cell wasdesigned to be 0.6 times the full cell length to reduce thebeam divergence. The coupling between the waveguideand cavity was located in the full cell. Coupling between
the cells was accomplished via the iris of the cavity. The
copper cathode used in the system was located on the sideof the half cell. A single solenoid magnet was mounted at
the exit of the RF gun to compensate the space charge
emittance. The cathode magnetic field was measured tobe less than 10G at a peak magnetic field of 3kG, resulting
in a negligible emittance growth due to the cathodemagnetic field.
The RF gun was driven by an all solid-state LDpumped Nd:YLF picosecond laser. The laser consisted ofa laser oscillator, a regenerative amplifier, and afrequency converter. The oscillator was mode-locked witha frequency of 79.3MHz, the 36th sub-harmonic of the2856MHz accelerating RF, by adjusting the cavity lengthof the oscillator with a semiconductor saturable absorbermirror (SESAM). The time jitter between the oscillator
output and the reference 79.3 MHz RF signal wasmeasured to be <0.5 ps using a phase detector technique.
After the oscillator, a Pockels cell captured a single
oscillator laser pulse to amplify the pulse energy up toabout 2 mJ in the regenerative amplifier. The repetition
rate of the regenerative amplifier is 30 Hz in themaximum. The amplified pulse was frequency quadrupled
to a 262 nm ultraviolet (UV) light with maximum pulse
energy of 300 µJ using a pair of nonlinear crystals. The
UV light was injected on the cathode surface at anincident angle of approximately 2o along the direction of
the electron beam using a prism placed downstream of thegun, as shown in Fig.1.

Linear Accelerator and RF source
The electron beam produced by the RF gun wasaccelerated with a 2 m long S-band travelling-wave linear
accelerator (linac). The linac was located at a distance of
1.2m from the cathode surface. The energy of the electron
pulse was also modulated by adjusted the RF phase for

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

pulse compression, as described below. The operating
temperatures of the RF gun and the linac were 32oC and30oC, respectively. The temperature fluctuation of both
the RF gun and the linac were within 0.1oC.
The peak RF inputs of the RF gun and the linac were 10MW and 30 MW, respectively, which was produced by a40 MW Klystron. The stability of the RF power was 0.1%peak-to-peak. The effective pulse width of the RF was4µs. The peak on-axis electric fields in the RF gun andthe linac were approximately 100 and 20 MV/m,respectively. The repetition rate of the operation was 10Hz in the experiment. A high-power phase shifter installed
in a 30 MW RF line, as shown in Fig. 1, was used toadjust the RF phase of the linac for energy modulation.

Magnetic Pulse Compressor
The magnetic pulse compression, which wasconstructed with two 45o-bending magnets and four
quadrupole magnets, is a technique to longitudinallyfocus a charged beam by rotating the phase space
distribution in a magnetic field. The picosecond electron
pulse generated in the RF gun and accelerated in the linac
with energy was compressed into femtosecond by
adjusted the magnetic fields of the quadrupole magnets.
However, it is noted that, in order to obtain a femtosecond
electron pulse, the generation of a low-emittance electron
beam should be important in the system.

Femtosecond Pulse Radiolysis SystemA new pulseradiolysis based an S-band photocathodefemtosecond linac is shown in Fig.1. The compressed
femtosecond electron pulse was used as an irradiation
source, while a mode-locked Ti:Sapphire femtosecond
laser was used as a probe light source. The Ti:Sapphirelaser oscillator output was phase-locked with the 79.3
MHz RF (the 36th sub-harmonic of the 2856MHz
accelerating RF). The femtosecond oscillator light wasstretched to 200ps by a pulse stretcher, and amplified thepulse energy up to about 1 mJ in a regenerative amplifier
with a Pockels cell. The repetition rate of the regenerativeFig. 2 Time synchronization between the lasers and RFamplifier is 1 KHz. An optical parametric amplifier (OPA)
was located at exit of the regenerative amplifier. The
probe light wavelength from 300nm to 1200nm can beselected by changing BBO crystals for applications.
However, the time jitter between the electron pulse andthe probe femtosecond laser was occurred in the system
due to the stabilities of the accelerating RF source and thelaser oscillator. In order to obtain a femtosecond time
resolution in the pump-probe measurement, a jittercompensation technique was used. The time interval of a
Cherenkov light emitted from the electron pulse and theprobe laser light is measured precisely by a femtosecond
streak camera. The data is used to compensate the effectof the time jitter with shot-by-shot in the pump-probe
measurement.
Finally, in the femtosecond pulseradiolysis system, thetime resolution is limited by the degradation of velocity
difference between the electron and the laser light insamples. The use of a thin sample leads to a higher timeresolution, but the S/N ratio would be decreased in themeasurement. An oblique incidence of the probe light wasconsidered in the system[5].

Timing Synchronization
Figure 2 shows a block diagram of the timing synchronization system for the pulseradiolysis. A RF signal

Fig. 1 A femtosecond electron linac and a femtosecond pulseradiolysis

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

generator supplies a 2856 MHz radio frequency. The RF
signal was divided into two: one was used to drive theKlystron, while another is divided in frequency by 36 to79.3 MHz to drive both the picosecond Nd:YLF laser andthe femtosecond Ti:Sapphire laser. A trigger signal
generator generates two 1 KHz signals: one issynchronized with the 79.3 MHz RF signal and used for
the Ti:Sapphire laser and measurement system (such as a
streak camera), while another is divided to a 10 Hz trigger
signal and used for the trigger signals of both the Nd:YLF
laser and Klystron. Two low-power phase shifters
installed in the 79.3 MHz RF lines are used to adjust boththe laser injection phase in the RF gun and the timing of
the probe femtosecond laser light. The timing jitter
between the electron pulse and the laser light is expected
to be a few hundreds femtosecond.

simulation indicates that an electron beam at the exit of
the linac with a transverse normalized emittance of 0.7
πmm-mrad, a longitudinal emittance of 15 KeV-deg andan energy spread of 3-4 % can be achieved at the bunchcharge of 0.1 nC.
By using the output of the PARMELA simulation, we
calculated the pulse length of the electrons passing
through the magnetic pulse compressor by using TRACE3D code. By optimising the magnetic fields of four
quadrupole, we obtained the compressed pulse length ofthe electrons as a function of the bunch charge, as shownin Fig. 4. The data indicated that the minimum rms pulse
length of 20 fs is achieved at the bunch charge of <0.2 nC.
The pulse length of the electrons increases for the highcharge, 130 fs for 1.0 nC.

Accelerator Control System
The reappearances and the stabilities of both theelectron beam and the laser light are important in thepump-probe measurement. An accelerator control system
by using a programmable language controller (PLC)
technique[6] was developed. All magnetic fields of themagnets and the laser ON/OFF are controlled by apersonal computer (PC). All parameters after optimization
are automatically saved into the memory of the PC for thenext experiment.

Fig. 4 The calculated pulse length versus bunch charge

CONCLUSIONS

Fig. 3 The calculated emittance and beam size

SIMULATIONS
The generation and acceleration of the electron pulse inthe RF gun and the linac were simulated by using
PARMELA code with a 3D particle-to-particle spacecharge calculation. Figure 3 shows the results of thephase-space distributions of the electron beam versus thedistance from the cathode. In the simulation, the charge ofthe electron pulse was fixed to be 0.1 nC. The laser
injection phase in the RF gun was 30o from the zerocrossing of the RF. The solenoid field was 1.2kG, which
was given the best emittance compensation at the bunchcharge of 0.1 nC. The RF phase of the linac was 70o to
give an optimum energy modulation for the pulse
compression. The beam energy was 20 MeV. TheA new S-band femtosecond electron linear accelerator
based a laser-driven photocathode RF gun was developed.
A femtosecond pulseradiolysis system was constructedwith a femtosecond laser light and expected for the studyof radiation-induced physical and chemical reactions in
femtosecond time regions.
A simulation with beam acceleration and magnetic
pulse compression was presented. The simulation of thepulse compression was done with a linear space-charge
effect. However, the effects of high-order space-charge
effects and coherent synchrotron radiation should beconsidered in the next step for producing an electron
pulse in a few femtosecond or attosecond.

REFERENCES[1] Y. Yoshida, et al., Radit. Phys. Chem., 60 (2001), 313-318..
[2] K. Kozawa, et al., Nucl. Instrum. Meth. A 440 (2000),251-254.
[3] J. Yang, et al., J. Appl. Phys., 92 (2002), 1608-1612.
[4] J. Yang, et al., Nucl. Instrum. Meth. A 491 (2002), 1522.
[5] H. Tomasada, et al., Proc. of this conference.
[6] K. Takeya, et al., Proc. of this conference.