JAERI-Conf 95-021

3p1-3
FEMTOSECOND ULTRAFAST QUANTUM PHENOMENA RESEARCH
Mitsuru UESAKA, Takahiro KOZAWA, Akira TAKESHITA,Mikiko AIDA, Toshiaki KOBAYASHI, Toru UEDA and Kenzo MIY A
Nuclear Engineering Research Laboratory, University of Tokyo
2-22, Shirakata-shirane, Tokai-mura, Naka-gun, lbaraki, Japan

Abstract
Femtosecond quantum phenomena research project is
proposed at Nuclear Engineering Research Laboratory,
University of Tokyo. The research facility consists of an
X-band (11.424GHz) femtosecond electron linac, afemtosecond wavelength tunable laser, two S-band(2.856GHz) picosecond electron linacs and measuring
equipments. Especially, we aim to generate a 1OOfs(FWHM) electron single bunch with more than InC at theX-band linac. Ultrafast processes in radiation physics,
chemistry, material science and microscopic
electromagnetic phenomena are going to be analyzed there.
The specification of the facility is presented.

1. Introduction
A subpicosecond (700 fs in FWHM) electron single
bunch was produced and measured at the S-band linac ofNuclear Engineering Research Laboratory, University ofTokyo, in 1994. Since then electric charge has beenenhanced up to 1 nC per bunch. Now the subpicosecond
pulse-radiolysis measurement for radiation physics andchemistry is under way. Here we propose the femtosecond
ultrafast quantum phenomena research, where both 100 fs(FWHM) electron and laser single pulses are available, as
the next project. The facility consists of an X-band( 11.424GHz) femtosecond electron linac, an femtosecond
wavelength tunable laser, two S-band electron linacs andseveral diagonostic and analyzing equipments. Thisfacility enables us to measure, analyze and visualize
excitation, ionization and relaxation of atoms andmolecules and ultrafast process of radiation damage andsurface phenomena of materials.

2. Specification of the facility
The specification of the facility is described below. The
layout of the machines are shown in Fig.1. The main
design parameters are summarized in Table I.

(i) X-band femtosecond electron linac
We have chosen X-band ( 11.424GHz) as the main
accelerating RF according to the following reason. We
think we have already optimized the operation parametersof magnetic pulse compression to produce a subpicosecond
single bunch from a I0 ps (FWHM) original bunch at theS-band (2.856GHz) linac[ 1]. Actually we have developed
and validiated the so-called nonlinear energy modulation.
Further, measured pulse widths agreed well with numericalresults based on the three dimensional particle tracking
analysis. This indicates that the beam parameters such asemittance and energy spread which we used are rather
reliable. Then we figure out that we have to have ashorter bunch and smaiJer emittance before thecompression in order to produce a 100 fs single bunch.
To fulfill the former requirement, we should introduce
higher frequency RF, namely X-band (11.424GHz). Xband klystron, accelerating tubes and other components areunder development for a future linear collider at KEK andSLAC[2,3,4]. We are to use 40 MW or less RF powerand 300 ns or longer pulse duration of the X-band
klystron. Two travelling wave acceleraing tubes of 2/3rc
mode and constant impedance are used. The length ofthe
regular acceleraing tube is 0.6 m and the field gradient is40 MY/in at the entrance and 25 MV/m at the exit. Theformer tube have a buncher section, where the RF phasevelocity varies from 0.7c to 0.9c, at the entrance. The
final energy is 35 MeV. In order to fulfill the latter
reqiurement, we have chosen the 200 kV thermionicelectron gun and to produce the peak current of I 00 A withthe pulse width of 200 ps (FWHM) by using the fast gridpulser. We found by using PARMELA that we have touse two subharmonic bunchers of 476 MHz and 2.856
GHz to get a single bunch without satellites. Pulse widthand electric charge per bunch were calculated to be I ps and1 nC, respectively. Finally, the achromatic arc-typemagentic pulse compressor achieves a I 00 fs singlebunch.
(ii) Femtosecond wavelength tunable laser
The laser is used as a probe light source in the lightabsorption pulse-radiolysis. Main oscillater and amplifier
are Ti-sapphire which produces I 00 fs light pulses of thewavelengh of 720-850 nm. The wavelength can be
expanded from 700 nm to 1.1 !J.m by using the wavelengh
converter. The repetition rate of laser pulses is tuned to be
79.3 MHz by the lock-to-clock. The single pulse
operation is also available by the pulse selector of pocket
cells synchronized by the main trigger system.
(iii) High charge S-hand electron linac
It is important to study fast quantum phenonema not
only in the femtosecond time domain but also in thepicosecond and nanosecond time domains to understandtheir whole process. From point of view of measurementwith high signal-to-noise ratio, it is beneficial to use anS-band linac for the study in the picosecond andnanosecond time domains since it can obtain more electriccharge per bunch than an X-band linac. For the purpose
we plan to use a 200 kV thermionic electron gun to get asmuch charge per bunch as 5 nC at the first S-band linac.
It consists of the 200 kV gun, the 47q MHz subharrnonic
buncher, two travelling wave accelerating tubes and the-460-

JAERI-Conf 95-021

achromatic arc-type magnetic pulse compressor so thatthe wide-ranged pulse structures such as 500 fs - I 0 ps
single bunch and 5 llS macro pulse are available.

synchronized with a specified 476 MHz RF phase and fed
to drive the electron guns, the femtosecond laser and thestreak camera.

(iv) Cherenkov radiator S-hand electron linac
The second S-band linac is used as a Cherenkov radiation
source in the light absorption pulse-radiolysis. Its
specification is the same as the first S-band linac except
the 90 kV gun which is the same one as the current
machines have. Further, an advanced RF electron gun
with a laser photocathode is to be tested at the secondlinac.

(vi) Pulse width measurement eqiupments
A new femtosecond streak camera with the time
resolution of 50 fs is now under development. After thedevelopment is completed, we plan to introduce it for bothon-line pulse diagnosis and femtosecond pulse radiolysis.
We are also going to construct the Michelsoninterferometer for off-line pulse diagonosis via opticaltransition radiation[5].

(v) RF control and trigger system
476 MHz has been chosen as the main RF generated by
the master oscillator. S-band RF (2.856MHz) and X-band
RF (11.424GHz) are generated from 476 MHz RF by thefrequency multipliers~ On the other hand, 79.3 MHz RF
which is 1/6 of 476 MHz is generated by the frequncy
devider to synchronize the femtosecond laser with thelinacs. A trigger pulse is generated so as to be(v) Analyzing equipments

The following analyzing eqiupments plan to be installed
for wide-ranged utilization; sample temperature control
system, electron spin resonance analyzer (ESR),
electrospectroscopic chemical analyzer (ESCA), Fourier
Transform Infrared Spectrometer (FTIR), time-resolved
plasma monochrometer and mass-spectrometer, highpower wide-ranged wavelengh tunable laser with narrow
bands etc.

Fig. ) Layout of the facility for the femtosecond ultrafast quantum phenomena research-461-JAERI-Conf 95-021

Table 1

3. Summary

Main design parameters of the facility

X-band femtosecond electron linac
100 fs with compression
Pulse width
1 ps without compression(FWHM)
1 nC per bunch
Electric charge
Maximum energy 35MeV
200 kV thermionicElectric gun
10 A at peak
200 ps (FWHM)
single bunch
Bunch mode
50 ppsRepetition rate
< l psJitter476 MHz (coaxial)
Subharmonic
2.856 GHz (nosecone)
Buncher
two tubes
Acceleratingtravelling wavetubes
2/3rr. mode
0.6 m for regular40-25 MV/m
0.7c -> 0.9c
Buncher<40MW
Klystron> 300 ns
Achromatic arc-type magnetic pulse compressor
Femtosecond wavelength tunable laserl 00 fs
Pulse width
700- 1100 nm
Wavelength
79.3 MHz
Repetition rate

Current design of the facility for the Jemtosecond
ultrafast quantum phenomena research is described. We aregoing to evaluate three dimensional wake field in thebuncher and regular cells at the X-band linac. Detailed
design has also started. We have submitted the proposalto the government via University of Tokyo. The
construction of the facility is expected to be done in 1996
-1997.

Acknowledgement
The authors would like to thank the following
researchers for their technical advises; Drs. H.Mizuno,THigo, H.Kobayashi, S.Ohsawa, S.Fukuda, A.Ogata,
H.Nakajima and Y.-J.Choi of KEK and Dr. G.A.Loew ofSLAC.

References[l] M.Uesaka et al., Phys. Rev. E, 50, 4(1994),p.3068.
[2] T.Higo, Proc. of the 1994 Int. Linac Conf.(NationalLaboratory of High Energy Physics)(l995), p.621.
[3] H.Mizuno et aL, ibid, p.466.
[4] A.D.Yeremian, R.H.MiJier and J.W.Wang, ibid,p.89.
[5] P.Kung, H.Wiedemann et al., Phys. Rev. Lett., 73,7(1994),p.967.

High charge S-hand electron linac
500 fs with compression
Pulse width(FWHM)
l 0 ps without compression< 5 l..lS for macro-pulse
5 nC per bunch
Electric charge
Maximum energy 35MeV
200 kV thermionic
Eletron gun
IOMW
Klystron
5 l..lS
50 ppsAchromatic arc-type magnetic pulse compressor
single and multi
Bunch mode
Cherenkov radiator S-hand electron linac
90 kV thermionicElectron gun
Laser photocathode RF gun-462-