The 11th Sy17!posium on Accelerator Science and Technology, Harima Science Garden City, 1997

A Short Pulse X-Ray Generation via Laser Thomson Scattering
on Relativistic Electron Beams
Hideyuki Kotaki*, Kazuhisa Nakajima* •**, Masaki Kando*, Hyeyoung Ahn*, Hideki Dewa*,
Syuji Kondoh*, Fumio Sakai*, Tak:ahiro Watanabe***, Toru Ueda***, Kenichi Kinoshita***,Koji Yoshii***, Hiroshi Nakanishi**, Atsushi Ogata**, Mitsuru Uesak:a****JAERI, Tokai, lbarak:i, Japan 319-11,**KEK, Tsukuba, lbarak:i, Japan 305,***NERL, The University of Tokyo, Tokai, Ibarak:i, Japan 319-11

Abstract
Short pulsed X-rays were experimentally generated by
90° Thomson scatterings of 2 TW, 90 fs laser pulses by 17
MeV electron beams. Synchronization between the laser
pulses and electron beams were achieved within a few ps. A
100 fs X-ray pulse will be generated via backward Thomson
scatterings from a 100 fs electron bunch made by a bunch
compression chicane. A high peak power, high brightness,high repetitive laser synchrotron radiation source consistingof counter-colliding laser and electron storage rings is proposed.

1 Introduction
A compact, narrow bandwidth, ultrashort pulses of hard
X-rays have basic and industrial applications in a number offields, such as solid-state physics, material, chemical, biological and medical sciences. An ultrashort pulse X-ray source
will allow measurements of time resolve atomic motion providing important information about the material properties,
chemical and biological reactions on ultrafast time scales.
The present high-brightness hard X-ray sources have been developed as third generation synchrotron light sources basedon large-scale high energy electron storage rings and magnetic undulators. Recently availability of compact terawatt
lasers referred to as table-top-terawatt (T3) lasers based on
chirped pulse amplification arouses a great interest in the useof lasers as undulators of which a period is ....., 1if shorter thanthe conventional undulator. This feature of laser undulators
allows the use of....., 100 times less energetic electrons to generate X-rays of a particular wavelength. The laser undulator
concept using T 3 lasers makes it possible to construct an attractive compact synchrotron radiation source which has been
proposed as a laser synchrotron radiation source (LSRS)[1].
In order to generate an ultrashort X-ray pulse, we attempted 90° Thomson scattering experiments, where a femtosecond laser pulse interacts with a relativistic electron beamat 90° [2]. In this configuration the X-ray pulse length is determined by the transit time of the laser pulse across the electron
beam waist as long as the laser pulse length is much shorter
than the electron bunch length. It is, however, essential toachieve an exact timing between the laser pulse and the electron pulse. Experiments were carried out by using a 1'3 laserdelivering laser pulses of 90 fs duration with the peak powerof 2 TW and a 17 MeV electron beam of 10 ps bunch length
produced from the RF linac synchronized with laser pulses at
the repetition rate of 10 Hz.
If the electron pulse duration is as short as a femtosecond, head-on collisions of laser pulses with the electron beam
should generate femtosecond X-ray pulses through backseat-tering. The head-on configuration can produce twice higher
energy photons than the 90° configuration as well as thehigher photon intensity. In this scheme a difficulty in timing between laser and electron pulses can be relaxed as longas a spatial overlapping of two beams can be accomplished.
In order to test the backward Thomson scattering to generate
femtosecond X-ray pulses, we prepare the picosecond photocathode injector and the bunch compression chicane in theelectron linac.
As an example of a practical high-brightness quasimonochromatic, high repetitive ultrashort hard X-ray source,
we propose a compact synchrotron radiation source consisting of counter-colliding laser and electron storage rings
where the compressor and the stretcher for both beams are installed. A conceptual design is discussed for providing highpeak power as well as high average power comparable to thepresent conventional synchrotron light source.

2 X-ray generation via Thomson scattering
When a laser beam interacts with an electron beam at anangle 1/J, Thomson scatterings of relativistic electrons in thelaser undulator field generate frequency up-shifted radiation
with the peak frequency given by
2!2 (1- cos 1/J)

wx=

1 +a~/ 2

wo,(1)

where 1 is the Lorentz factor of the electrons, w0 the incident laser frequency and ao the undulator strength or thenormalized vector potential of the laser field given by a0 .....,
0.85 X 10- 9 J 112 [W/cm2]Ao[Jtm] for the peak intensity I inunits ofW/cm2 , the laser wavelength Ao 21rcjw0 in units ofJtm. For the 1/J = 90° configuration, the maximum radiation
photon energy is=_3

Ex[keV] :::bwx =9.5 x 10~[MeV]

Ao(Jtm](l + ~/ 2 ),(2).

where Eb is the electron beam energy. The radiation wavelength is Ax[A] = 12.4/Ex[keV]. In the head-on configuration with 1/J = 180°, the maximum photon energy turns out tobe twice as high as the ljJ = 90° configuration. The angular
distribution of radiation with the spectrum A > Ax is withina cone of half angle,(}= (lh)V(A.;... Ax )/Ax.

2.1 90° Thomson scattering
Assuming the Gaussian temporal and spatial distributionsof both the electron and laser beam with the transverse and-449-

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

longitudinal beam sizes of (J' x and (J'z and the laser transverseand longitudinal laser beam sizes of (J'w and (J'£, the rms pulse
length of the X-ray radiation is obtained fromTo Streak Camera
17MeV
Electron Beam

Shutter

rr===============~~=rfC=9~==0 Camcra(3)

The number of photons per pulse within the spectral width
flw I w is given by[2]Electron
Beam(4)

where Ne is the total number of electrons per bunch, J thelaser pulse energy in Joules, and .A 0 and the beam sizes aremeasured in Jlm.

2.2 Backward Thomson scattering
In the head-on configuration x-rays are generated toward
the direction of the electron beam propagation. It turns outthat the x-ray pulse length is determined primarily by the electron pulse duration, i.e. (J'x = (J' z. The number of photons per
pulse within the spectral width flw I w is obtained from[ 1]11n = 4 X 103 NeJ

flw,

ZR[Jlm] w(5)where Z R = 1rr5/ .A 0 is the Rayleigh length, r0 the spot radiusof the Gaussian laser profile and r 0 ,..., rb is assumed with theelectron beam radius rb.

3 Experiments at 17 MeV Linac
Experiments of X-ray generation through Thomson scattering are made by the use of the 17 MeV electron linac and
the T 3 laser system[3]. The Ti:sapphire T 3 laser system basedon the chirped-pulse amplification at .Ao = 790 nm produces
output pulses compressed by a grating compressor to 90 fs
with an energy of > 200 mJ corresponding to a peak powerof> 2 TW at the repetition rate of 10Hz. The electron beam
is delivered from the 2856 MHz RF linac to produce a 17 MeV
single bunch beam with a 10 ps FWHM pulse duration at therepetition rate of 10 Hz. An electron pulse is synchronized
to laser pulses with the phase locked control of the modelocked oscillator. The phase locked loop maintains synchronization of the oscillator repetition period (79.33 MHz) withevery 36th RF period ofthe linac (2856 MHz). We measured
a timing jitter between the laser pulse and Cherenkov radiation from the electron beam with the streak camera with a time
resolution of 200 fs. Synchronization between two pulses wasachieved within the rms jitter of 3.7 ps.
The setup for the 90° Thomson scattering experiment isshown in Fig. 1.
Laser pulses were focused with f/10 off-axis parabolic(OAP) mirror with a focal length of 480 mm. The electron
beam from the linac is focused by a permanent quadrupole(PMQ) triplet in the chamber. Since the electron beam spot
size was 480 Jlm, the X-ray pulse duration of 1.6 ps was expected. The linac was separated with a 50Jlm thick titanium

X-ray
Detector

Fig. 1 The experimental setup for 90° Thomson scattering.

window from the interaction chamber to maintain ultrahighvacuum in the linac. The incident electron beam was swept
off with the a magnet at a bending angle of 270°. The X-ray
radiation was detected by a scintillator with the 1 x 6 cm2 sensitive area to be coupled to the photomultiplier tube. Plenty ofthe bremsstrahlung background was generated from the titanium window and the upstream beam line. In order to subtract
the background signal, two sets of X-ray signals were takenwith laser pulses and without them as the background. The
signal was averaged over 500 tolOOO shots to reduce a signal
fluctuation. A net signal height proportional to the X-ray flux
was obtained from subtracting the background signal from thesignals with interaction. Fig. 2 shows the net X-ray signals
observed as the timing between laser and electron pulses wasscanned.
In order to test ultrashort pulse X-ray generation by the
backward Thomson scattering in the head-on configuration,
the bunch compression chicane. is installed in the beam line
following the linac to produce the pulse duration of,....., 100
fs. The present thermionic electron gun is replaced by thephotocathode RF gun. which can provide a small normalized
emittance of,....., l1rmm-mrad and the electron pulse duration of
1 ps. Estimates of experimental parameters are summarizedin Table 1.

4 Laser Synchrotron Radiation
Consider that LSRS is composed of a compact electron
storage ring and a storage ring of laser pulses with the low
gain amplifier to compensate the power loss due to X-ray
radiation. Ultrashort intense X-ray pulses can be generated
by the head-on interaction between ultrashort electron andlaser pulses made through the pulse compressors for bothbeams at the interaction region. After passing through theinteraction, both pulses are stretched in the other section of
the ring to avoid beam instabilities or damages in the optical components. This system will allow both a high peak
power and a high average power operations ofLSRS in a com-

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

Table 1
Parameters of X-ray generation experiments

Delay (ps)Fig. 2 X-ray signals observed as a function of the time delay between laser and electron pulses.

Laser beam
Wavelength 790 nm
Peak power 2 TW
Pulse energy 0.2,J
Pulse duration 90 fs
Focal spot radius 150JLm
Electron beam
Energy 17 MeV(/= 33)
Pulse duration 100 fs
Charge/pulse 1 nC
Beam radius 150JLm
X-ray pulse
Photon energy 7 keV (Ax = 1.8A)
Pulse duration 100 fs
Number of photons 5 x 10\flw / w = 0.1)
Collection angle (20) 2 x 9 mradpact size. Table 2 summarizes design parameters of LSRS
performance[ 1].

5 Conclusion
A short pulse X-ray generation was observed by the 90°
Thomson scattering of 2 TW, 90 fs laser pulses from the 17
MeV electron beam. A femtosecond X-ray pulse is efficiently
generated via the backward Thomson scattering with the electron pulse compression. A compact, high peak power, highbrightness laser synchrotron radiation source is proposed toprovide attractive features comparable to a large storage ringbased synchrotron light source.

References[1] E. Esarey, P. Sprangle and A. Ting, NIM A 331, 545(1993).
[2] K.-J. Kim, S. Chattopadhyay, C.V. Shank, NIM A 341,
351 (1994).
[3] K. Nakajima, Phys. Plasmas, 3, 2169 (1996).

Table 2A design of laser synchrotron radiation
Laser pulse parameters
Wavelength 0.8JLm
Pulse energy 1 J
Peak power 10 TW
Pulse duration 100 fs
Spot radius 50JLm
Rayleigh length 1 em
Repetition rate 10 MHz
Electron pulse parameters
Energy 150MeV
Peak current 100 kA
Average current 100 rnA
Pulse duration 100 fs
Beam radius 50JLm
Revolution frequency 10 MHz
X-ray pulse parameters
Photon energy 530 keV max.
Pulse duration 100 fs
Peak photon flux 2.5 x 1023 photons/s
Peak brightness t 2 x 1020
Peak radiation power 3.6 GW
Average photon flux 2.5 x 1017 photons/s
Average brightnesst 2 x 1014

t photons/s mrn2 mrad2 0.1 % BW-451-