Proceedings of International Conference on Physics in ... - KEK

Abstract
X-RAY GENERATION VIA LASER COMPTON SCATTERING BY
LASER-ACCELERATED ELECTRON BEAM ∗
E. Miura † , R. Kuroda, H. Toyokawa, AIST, Tsukuba, Ibaraki 3058568, Japan
S. Ishii, K. Tanaka, Tokyo University <strong>of</strong> Science, Noda, Chiba 2788510, Japan
X-ray generation by laser Compton scattering using a
quasi-monoenergetic electron beam with a narrow energy
spread obtained by laser-driven plasma-based acceleration
is reported. X-rays are produced by the collision <strong>of</strong> a
femtosecond laser pulse (140 mJ, 100 fs) with a quasimonoenergetic
electron beam with a peak energy <strong>of</strong> 50
MeV and a charge in the monoenergetic peak <strong>of</strong> 30 pC produced
by focusing an intense laser pulse (700 mJ, 40 fs) on
a He gas jet. A well-collimated X-ray beam with a divergence
angle <strong>of</strong> 5 mrad is obtained. The maximum photon
energy and the yield <strong>of</strong> the X-rays are estimated to be 60
keV and 10 5 photons/pulse.
INTRODUCTION
In laser-driven plasma-based acceleration, electrons are
accelerated by the electric field <strong>of</strong> a plasma wave driven
by an intense laser pulse[1]. To realize next-generation
electron accelerators, the research has been intensively
conducted over a few decades. Since 2004, the generation
<strong>of</strong> a well-collimated electron beam with a narrow energy
spread, that is quasi-monoenergetic electron (QME)
beam, has been demonstrated by several groups[2, 3, 4, 5].
The generation <strong>of</strong> a QME beam with a peak energy <strong>of</strong> 1
GeV [6], and a QME beam containing 0.5 nC electrons in
the monoenergetic peak [5], which is comparable to that
achieved with radio-frequency (rf) accelerators, has been
also demonstrated. The road toward the realization <strong>of</strong> a
practical laser electron accelerator is now open.
In laser-driven plasma-based acceleration, a high acceleration
field more than 100 GV/m, which corresponds to
thousands <strong>of</strong> those achieved by rf accelerators, is obtained.
A compact electron accelerator can be realized using such
a high acceleration field. Furthermore, the wavelength <strong>of</strong>
the acceleration field, that is the wavelength <strong>of</strong> the plasma
wave, is short, <strong>of</strong> the order <strong>of</strong> tens <strong>of</strong> micrometers. Then,
the electron pulse duration is extremely short, <strong>of</strong> the order
<strong>of</strong> a few tens <strong>of</strong> femtoseconds. The set <strong>of</strong> such unique characteristics
<strong>of</strong> laser-driven plasma-based acceleration enables
us to realize a compact, all-optical, ultrashort X-ray
source based on laser Compton scattering, that is scattering
<strong>of</strong> photons by energetic electrons.
∗ A part <strong>of</strong> this study was financially supported by the Budget for Nuclear
Research <strong>of</strong> the Ministry <strong>of</strong> Education, Culture, Sports, Science, and
Technology, Japan, based on the screening and counseling by the Atomic
Energy Commission.
† e-miura@aist.go.jp
So far, generation <strong>of</strong> an ultrashort X-ray pulse by laser
Compton scattering has been demonstrated by using a femtosecond
laser pulse and a picosecond electron pulse from
rf accelerators[7, 8]. The X-ray pulse duration is determined
by the interaction time between the laser and electron
pulses. To obtain a femtosecond X-ray pulse, 90 ◦
scattering geometry should be adopted for a picosecond
electron pulse. In contrast, 180 ◦ scattering (head-on collision)
geometry is available for a femtosecond electron
pulse. There are some advantages <strong>of</strong> using 180 ◦ scattering
geometry. Even though the electron energy and the charge
<strong>of</strong> an electron beam are the same, the photon energy and the
yield <strong>of</strong> X-rays are higher than those for 90 ◦ scattering geometry.
Recently, X-ray generation has been demonstrated
using a laser-accelerated electron beam[9]. However, the
X-ray energy was around 1 keV and the yield was not so
high, because an electron beam with Maxwell-like energy
distribution was used. To obtain a bright X-ray source with
higher photon energy, a QME beam with a higher energy
and a larger charge is necessary.
In this paper, we report X-ray generation by laser Compton
scattering using a QME beam obtained by laser-driven
plasma-based acceleration.
EXPERIMENTAL CONDITIONS
Figure 1 shows the experimental setup. In the following
part, laser pulses for electron acceleration and laser Compton
scattering are called ”main laser pulse” and ”colliding
laser pulse”, respectively. A p-polarized main laser pulse
(700 mJ, 40 fs, 800 nm) was focused on the edge <strong>of</strong> a He
gas jet using an <strong>of</strong>f-axis parabolic mirror with a focal length
<strong>of</strong> 720 mm. The laser spot diameter in vacuum was 13 µm
at full width at half maximum (FWHM). The peak intensity
was 4.7 × 10 18 W/cm 2 . The gas jet was ejected from
a supersonic nozzle with a conical shape. The diameter <strong>of</strong>
the nozzle exit was 1.6 mm. The focal position was set at
1 mm above the nozzle exit. A p-polarized colliding pulse
(140 mJ, 100 fs, 800 nm) was focused around the exit <strong>of</strong> the
main laser pulse from the gas jet using an <strong>of</strong>f-axis parabolic
mirror with a focal length <strong>of</strong> 300 mm. The laser spot diameter
in vacuum was 9 µm at FWHM. The incident angle <strong>of</strong>
the colliding laser pulse was 20 ◦ to the propagation axis <strong>of</strong>
the main laser pulse.
X-rays produced by laser Compton scattering were emitted
on the coaxial direction <strong>of</strong> an electron beam. The
electron beam was bended by a magnetic field and spatially
separated from the X-rays. Both X-rays and electrons
were incident on a phosphor screen (DRZ-HGH, Mit-