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

Current Status Report of Subpicosecond Electron Linacof NERL of Univ. of,TokyoMitsuru UESAKA, Toru UEDA, Koji YOSHI, Hideki HARANO,Takahiro WATANABE and Kenichi KINOSHITA
Nuclear Engineering Research Laboratory, University of Tokyo
2-22 Shirakata-Shirane, Tokai, Naka, lbaraki, 319-11 JAPAN
Abstract
The new laboratory of Beam-Material Interaction started
this April to promote generation and application of ultrashortquantum beams. 8 research subjects have been accepted andperformed in the linac facility in this fiscal year. Coherent
transition radiation interferometry is applied to picosecond
electron bunch measurement. The results agree with theresults obtained by the femtosecond streak camera The
experiment of the laser wakefield acceleration is under way
under the collaboration with KEK and JAERI-Kansai. The
18L linac of the twin linacs has been modified forfemtosecond X-ray pulse generation via Thomson backward
scattering. The laser photocathode RF gun and thechicane-type magentic pulse compressor have been installed.
The first experimemt has been carried out in August andSeptember. X-ray diffraction of an ionic nomocrystal(NaCl) was performed using subpicosecond X-ray pulse.
The low power X-band RF test for the X-band femtosecond
linac and the design study of the linac with the laser
photocathode S-band RF gun is carried out.

1 Introduction

femtoseomd elctron and laser pulses. 100 fs laser pulse by
the r (Table-Top Tera Watts) laser is used. As acounterpart we have to generate femtosecond electron pulse
whose pulse width (FWHM) is far less than 1 ps. For thispurpose we have decided to modify the 18L linac of the twinlinacs. We have moved. the 90 kV thermionic eletron gun,the subharmonic buncher and the two prebun<ners andinstalled the new laser photocathode S-band RF gun.
Additionally we designed, manufactured and installed the newchicane.type magnetic pulse compressor after the constantgradient travelling-wave-type S-band acceleraing tube. TheRF gun was designed and manufactured by KEK, Brookhaven
National Laboratory and Sumitomo Heavy Inwstries.
About 10 ps (FWHM) light pulse with tens J.l.] energy,
which irradiates the copper cathode at 10 Hz, is produced by
the YLF laser. 6 MW RF power is fed to the 1.6 cell Sband cavity to induce 100 MV/m maximum field gmdent.
4. 7 MeV, 10 ps (FWHM), 1 nC electron pulse with rather
low transverse emittance of 13t mm • mrad in normalized rms
is expected to come out. The solenoid magnet is attached tothe cavity for transverse emittance compensation against
space charge effect. This solenoid magnet is used to makethe electrons transport in parallel in the longitudinal direction.
The beam spot is about 7 mm. The Jow emittance elctron
beam is accelerated up to 17 MeV and simultaneously its
energy profile is modurated for magnetic pulse compression
in the accelerating tube where the maximum field gradient is
8.5 MV/m. The chicane was designed so as to compress tile
10 ps pulse to 200 fs (FWHM). The inner structure of theacceleration tube was inputed to SUPERFISH to calculate
electromagnetic field components of the· travelling wave.
The data of the electromagnetic components was transfered toPARMELA. Simulation of transport, acceleration, enerar
modulation and magnetic pulse compression of tile electrons
was performed by PARMEIA The schematic chwing of
the modified 18L linac is depicted in Fig.1. Calculated
result of the longitudinal phase space electron clstribution
and the pulse shape after the chicane is shown in Fig.2. 200
fs (FWHM) electron single bunch generation was confirmedby the simulation [2}.
The installation of the RF gun, the chicane 8Dd otber
components was finished in July. Since then RF power
aging of the RF cavity of the gun was carried out. The
beam test started in the beginning of August. The pulse+The new laboratory of Beam-Material Interaction wasestablished at NERL of University of Tokyo this April topromote generation and application of ultrashort quantum
beams. 8 research subjects have been accepted and performedat the linac facility in this fiscal year. The joint research
project on the laser wakefield acceleration [1] is continuing.
In the reserach the 18L linac of the twin linac; has been
modified for femtosecond X-ray pulse generation via
Thomson backward scattering. The laser photocathode RF
gun and the chicane-type magentic pulse compressor havebeen constructed and installed. Here we summarize andintrowce the updated research activities related tofemtosecond quantum beam research.

2 Femtosecond Electron Pulse Generation at theS-band linac
The major research theme in the project on the laser
wakefield acceleration in this fiscal year is femtosecond X-ray
generation via Thomson backward scattering between-65-

The 1 J'th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

100 fslaser

EnergyLaserphotocathodeRFgun

S-bandaccelerating tube

Fig.l Schematic drawing of the modified 18L linac
10881j54

27·~~I~101
relative time (ps)20220.3
18.8~!;I
fi'l

17.3
15.8
14.32

1

relative time (ps)Fig.2 Longitudial phase space electron distribution and pulse
shape after the chicane calculated by PARMELA
shape of the compressed beam is measured by thefemtosecond streak camara (FESCA-200, HAMAMATSU
PHOTONICS ) via Cherenkov radiation in Xe-gas. This
experiment and measurement will be performed till mid.
September. Upmted experimental results are introwced in
the presentation. Femtosecond X-ray pulse generation via
Thomson backward scattering and mesurement by the X-ray
subpicosecond streak camera are planned this November.
Coherent transition radiation interferometry is also aoopted
to obtain the 200 fs compressed pulse shape and its result iscompared with that by the femtosecond streak camera. For
the purpose a new Michelson interferometer is designed
This measurement and comparison will be oone thisDecember.

3 Pulse Shape Measurement by Coherent
Transition Radiation Interferometry

subpicosecond electron bunch. The first one is to measure
C'herenkov radiation or optical transition radiation emitted by
the electron bunch by the femtosecond streak camera [3)while the second one is the coherent far-infrared transition
radiation interferometry [4). It is important to compare theresults by the two methods in order to confirm the precisionof both methods. This task was carried out under thecoHaboration with Prof.M.Ikezawa, Dr.Y.Shibata and theirresearch group of the Res. Inst. Sci. Measurement of Tohoku
University this May. We used the 28L Jinac where theachromatic arc-type magnetic pulse compressor is installed
We used the Martin-Pupplet interferometer with the wire-gridpolarizing beam spritters (cjl 10 JA.m wires with 25 JA.mspacing) and liquid.He-cooled Si bolometers. At first weobtained the interferogram of the light intensity of interfered
two transition radiation pulses as a function of the moving
mirror position; Next we calculated the power spectrum thetransition radiation by the inverse Fourier transformation.
Then we substmcted the power spectrum of the incoherent
components from the total spectrum to get the product of thelongitudinal and transverse bunch form factors.
The
transverse bunch form factor was evaluated by themeasurement of the transverse beam size. Finally theKramers-Kronig relation gives the longitudinal bunchdistribution from the longitudinal bunch form factor. In theexperiment the longitudinal bunch distribution wascontrolled by tuning energy modulation of the bunch in thesecond accelerating tube for magnetic pulse compression.
We chose 1.5, 3, 10 ps (FWHM) pulse widths and performed
the comparison between the streak camera and coherent
trasition radiation interferometry measurements.
Pulse
shapes of 1.5 ps bunch obtained by the streak camera andinterferometry are shown in Figs.3, 4, respectively. The
figures indiactes nice agreement. Nice agreement was also
confirmed in case of 3, 10 ps bunches. Thus the validity ofthe both methods has been confirmed up to 1.5 ps (FWHM)
short bunch [5]. The next subject is to check it for 200 fs(FWHM) bunch as mentioned in Sec.2.

There are two methods to evaluate pulse shape of-66-

The lith Symposium on Accelerator Science and Technology, Harima Sci~nce Garden City, 1997

year to visulali:ze ultrafast movement of atoms. Thispulse--snapshot technique is a pump-and-probe--like analysis
for ultrafast radiation induced phenomena in matter usingsynchronized different ultrashort pulses where the snapshotof the phenomena is obtained at each delayed time between
the two pulses. Here a monocrystal such as NaCl, Nal, KBr,Si, GaAs etc is irradiated by subpicosecond far-infraredcoherent synchrotron radiation (pump) to inwde movementof atoms and ·then the monocrystal is irradiated bysubpicosecond X-ray pulse (probe) to get three dimensional
snapshot of atoms via the X-ray diffraction. The irradiatioof the two pulses is repeated till the clear X-ray diffraction
imageis obtained. Since the same electron single bunch
generates the two pulse and the delay time is controlled bychanging the optical length of the coherent synchrotron
radiation, there is no time jitter between the two pulses.
Finally the ultrafast movement is visualized by computer
graphics.

Light intensity (Arbitrary unit)
3500
3000
2500
2000

1.7ps (FWHM)

1500
1000

03o20

1040soTime (ps)Fig.3 Pulse shape of 1.5 ps bunch obtained by thefemtosecond streak camera.
Light intensity (Arbitrary unit)

5 X-band Femtosecond Llnac'The resign study of a new X-band femtosecond linac
system with a laser photocathode S-band RF gun isperfonned [7]. It is confinned by PARMEIA. that 200 fs(FWHM) electron single bunch with 1 nC is prowced.
Solenoid magnet system is rather simplified. On the other
hand, the reliability and stability of low power X-band RF
components are under invetigation [8].

1.6 ps (FWHM)

o.s r-References

Longitudinal axis [mm]Fig.4 Longitudinal bunch distribution constructed by thecoherent transition radiation interferometery.

4 Ultrafast X-ray pulse
Subpicosecond X-ray pulse was generated by irradiating
30 ~-tm thick copper foil by subpicosecond electron single
bunch from the 28L linac. Ka. X-rays of 8.1 keY, 1.54 nm
were picked up and X-ray diffraction was perfonned for theionic monocrystal of NaCl. The X-ray diffraction image
was obtained on the X-ray image plate (by the courtesy ofDr.N.Niimura of JAERI). As the next step the exprimentof the pulse-snapshot X-ray diffraction [6] is planned next

[1] K.Nakajima et al., Proc. of the 17th Particle Accelerator
Conf., Vancouver, 1997 (in press).
[2] K.Kinoshita et al., Proc. of the 11th Symp. on Acceleator
Sci. and Tech., Harima, 1997.
[3] M.Uesaka et al., Phys. Rev. E, Vol.SO, 1994,pp.3068-3076.
[4] Y.Shibata et al., Phys. Rev. E; Vol.50, 1994,pp.1479-1484.
[5] Y.Shibata, M.Uesaka et al., submitted to NucJ. Instrum.
Meth.
[6] M.Uesaka et al., J. of Nucl. Materials, 1997 (in press).
[7] H. Harano et al., Proc. of the 11th Symp. on Aa:eJeator
Sci. and Tech., Harima, 1997.
[8] T. Ueda et al., Proc. of tbe 22th Lin. Accelerator Meetingin Japan, Sendai, 1997 (in Japanese).

-67-