25G10
Fast Electron Energy Transport in Solid Target Generated by High Contrast
High Intense Laser Pulse

Tsuyoshi Tanimoto1, Mamiko Nishiuchi1, Yosuke Mishima2, Kenshiro Kikuyama2,Tomoya Morioka2, Kiyoshi Morita2, Masato Kanasaki2, Alexander S. Pirozhkov1,
Akifumi Yogo1, Koichi Ogura1, Yuji Fukuda1, Hironao Sakaki1, Akito Sagisaka1,Hideaki Habara2, Kazuo A. Tanaka2, and Kiminori Kondo1
1122
22,,,,,,
1
11
1, Alexander S. Pirozhkov1,,,,
1
12
2
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1

Quantum Beam Science Directorate, Japan Atomic, Energy Agency, 8-1 Umemi-dai, Kizugawa,
Kyoto 619-0215, Japan
2Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
1619-0215
8-1
2565-0871
2-1
The fast electron spectrum was measured simultaneously with the proton energy in the interaction between
the solid and the high intense laser. There were two components in the electron spectrum. The higher
electron temperature component was almost explained by the ponderomotive formula. The total electronspectrum including the lower temperature component was also explained by a simple model. The detected
maximum proton energy was 23 MeV with SUS thin foil target.

1. Introduction
The generation of high energetic ion in highintense laser pulse with solid target is a rapidly
growing research area. In the generation mechanism,
the target normal sheath acceleration is usuallyassumed [1-3]. Then, for the solid target, theincrease of laser intensity is important to generatehigher ion, which means increasing the electron
temperature as high as possible. Actually, themaximum energy of ion grows in proportion to theelectron temperature, and the electron temperature
increases in proportion to the square root of thelaser intensity. In this experiment, the fast electron
and the high energetic ion were measured
simultaneously.
2. Experimental and Discussion
The experiment was performed using J-KAREN(JAEA-Kansai Advanced Relativistic Engineering)laser system. J-KAREN has a measured intensity
contrast ratio of 10-10 [4]. The pulse duration was 80
fs. The laser pulses with up to 8 J were delivered ontarget. The peak laser intensity of the normalincidence was 4.0 × 1020 Wcm-2. The center
wavelength was 800 nm. The incident angle was 45
degree to the target normal.
The target materials were Stainless (SUS) andPolyimide. These thicknesses were 2.5 µm and 7.5µm, respectively.
The electron spectrometer (ESM) was placed
behind the target in the direction of laser axis, while
CR-39 stack was in the direction of target normal.
CR-39 stack was used for the detection of the ion.
The detected ions were almost protons. The
obtained maximum proton energy was evaluated tobe 23 MeV in the SUS target with taking the
stopping range of CR-39 stack into account.
Figures 1(a) and 1(b) show the measured electronspectra. These were obtained with SUS andPolyimide, respectively. The vertical axis is theelectron number and the horizontal axis is theelectron energy. The dots were obtained in theexperiment. Both spectra clearly have twocomponents. The higher energy component of themeasured energy spectra was fitted with anapproximation of Maxwellian distribution, which isshown by the broken line. The electron
temperatures of SUS and Polyimide targets were5.1 and 4.8 MeV, respectively. These values arealmost same each other in spite of the different
materials. The ponderomotive formula,()Te [ MeV ] = 0.511 1 + I18 λ2µm 1.37 − 1(1)Fig. 1. The electron spectrum for (a) the SUS and (b) thePolyimide targets. The dot and dot-line show theexperimental point and the slope temperature fitting.

where Te, I18, and λµm are the electron temperature,
the laser intensity in units of 1018 Wcm-2, and thewavelength in microns, respectively, is used forestimating the average energy of electrons
accelerated by the ponderomotive force in thetransverse field of the incident light wave [5]. It
corresponds to be the fast electron temperature of
6.5 MeV. Then it is almost the same as those which
experimentally obtained. Under 7 MeV region (lowenergy region), the electron spectrum was observed
to be different in the different target condition.
To understand this result, a simple model wasassumed. We have estimated the distribution ofenergetic electrons to the rear side of the solidtarget.
Figures 2(a) and 2(b) show the evaluated electron
spectra of SUS (a) and Polyimide (b), respectively.
The vertical axis is the electron number and thehorizontal axis is the electron energy. The solid line
and the broken line are obtained with and without
considering the effect of the re-acceleration. The
dot line was obtained by the slope temperature
fitting. In both targets, the electron temperature
became 5.6 MeV by the fitting of Maxwell
distribution.
For higher energy component, there is almost nodifference between two targets. This means that
re-acceleration of the front side plays almost noeffect to higher energy component.
When we compare the electron number under 7
MeV, the solid line is found to be almost 2 times
higher than the broken line in SUS target, while it isalmost the same level in Polyimide. These results
suggest that the re-acceleration plays an importantrole in SUS target. It could be caused by the thin
thickness of SUS in this experiment. This tendency
can be seen in the experimentally obtained
spectrum. Actually, Polyimide target was 3 times
thicker than SUS. The experimentally obtained

Fig. 2. The electron spectrum for (a) the SUS and (b) thePolyimide targets. The solid line, the broken line and the
dot line become with effect of the re-acceleration,
without effect of the re-acceleration case and the slope
temperature fitting, respectively.

electron number in SUS target was 3 times larger
than the Polyimide. It can be explained by the larger
number of electron generation in SUS withassuming the barrier suppression ionization [6]. In
the laser intensity of 4.0×1020 Wcm-2, the electron
number in SUS can be estimated to be 4 times
larger than Polyimide.
3.

SummaryThe electron distribution to the rear side of thetarget has been detected and analyzed with a simple
model. The higher temperature component isexplained by the ponderomotive formula, but thelower component was proved to be affected by there-acceleration at the front side.
In this experiment, the maximum proton energy
was measured to be 23 MeV in SUS target with theirradiating intensity of 4.0×1020 Wcm-2.
Acknowledgments
The authors gratefully acknowledge the supportof the operation group of J-KAREN laser system inthe execution of this work.
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