24F04
Effects of Laser Spatial Profile on Fast Electron Generation
レーザーの空間プロファイルが高速電子生成に及ぼす影響	 
Masayasu Hata, Hitoshi Sakagami1, Tomoyuki Johzaki2 and Hideo Nagatomo3
畑	  昌育, 坂上仁志，城崎知至，長友英夫
Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan
名古屋大学大学院理学研究科 〒464-8602	  名古屋市千種区不老町
1
Fundamental Physics Simulation Research Division, National Institute for Fusion Science,Oroshi-cho, Toki 509-5292, Japan
核融合科学研究所基礎物理シミュレーション研究系 〒509-5292	  土岐市下石町2Institute of Laser Technology, Yamadaoka, Suita 565-0871, Japan
レーザー技術総合研究所 〒565-0871	  吹田市山田丘3Institute of Laser Engineering, Osaka University, Yamadaoka, Suita 565-0871, Japan
大阪大学レーザーエネルギー学研究センター 〒565-0871	  吹田市山田丘
2D PIC simulations for fast ignition with a planar target and 200 fs ultrahigh intense laser have been
performed to investigate effects of laser spatial profile on fast electron generation. Two cases of different
laser spatial profiles, namely Gaussian and super Gaussian are compared, assuming that total energies,
temporal profiles, and FWHMs of transverse intensity profile for both lasers are the same. High-energy
electrons (> 5 MeV) are much generated near the laser axis and angle divergence is large at the area far
from the laser axis in the super Gaussian case compared to the Gaussian case.

1. Introduction
In 1994, fast ignition scheme was proposed [1].
Many studies have intensively been performed
since this proposal [2,3]. In Japan, the FIREX-I
project [4], of which purpose is the achievement ofheating a core up to 5 keV, has been started.
Cone-guided targets are used in this project and thedesign optimization of the target has been being
performed [5-7]. In contrast, the laser profile is not
optimized so much and its effects on fast electron
characteristics are not clarified yet. Therefore, it isnecessary for full optimization to investigate effectsof laser temporal and spatial profiles. In this paper,
we paid attention to laser spatial profiles andestimated those effects on fast electron generation.
2. Simulation Conditions
Laser-plasma interactions on normal incidence ofan ultrahigh intense laser to a gold planar target aresimulated for 200 fs with 2D PIC code, FISCOF2.
We compared two cases of different spatial pulseshapes, namely Gaussian with peak intensity of 5 ×
1019 W cm-2 and super Gaussian (α = 4) of 5.36 ×
1019 W cm-2 as shown in Fig. 1. Each laser has same
total energy, flat temporal profile (τflat = ∞), andFWHM (full width half maximum) of transverse
intensity profile (20 µm). The target is made of thegold with the atomic number of 197, the charge
state of 40, and the ion-to-electron mass ratio of1836. The initial temperatures of electrons and ionsare 10 and 1 keV, respectively. The target consists

Fig.1. Spatial pulse shape of laser in y directionof a preplasma, whose electron density profile isexponential from 0.1 to 20 times critical density(ncr) with the scale length of 4 µm, and a main
plasma that has flat electron density profile with thelength of 15 µm. The density profile in y directionis uniform and its width is 70 µm. To neglect thecirculation of fast electrons, we introduce anartificial cooling region from three edges of theplasma, except the surface of the laser irradiation,
to a 6 µm inner side. In this region fast electrons aregradually cooled down to the initial temperature.
We observe electrons in the main plasma, 5 µm
behind the boundary between pre- and main
plasmas in x direction at y = 0–5 and 20–25 µm.

3. Electron Beam Intensity
Figure 2 shows time evolutions of electron beam
intensity in the cases of Gaussian and superGaussian at y = 0–5 and 20–25 µm. At y = 0–5 µm,
the electron beam intensity in the super Gaussian
case exceeds that of the Gaussian case because thelaser intensity of the super Gaussian case at thisregion is higher than that of the Gaussian case as
shown in Fig. 1. On the other hand, the electron
beam intensity at y = 20–25 µm in the superGaussian case is lower than that of the Gaussian
case as the laser in the super Gaussian case is lessintense than that of the Gaussian case at the area far
from the laser axis.

electrons is similar. In contrast, at the area far fromthe laser axis, low-energy (< 3 MeV) electrons inthe super Gaussian case are less generated than thatof Gaussian because the laser of super Gaussian isless intense than that of Gaussian.
5. Divergence angle of electrons
It is important for efficient core heating toinvestigate divergence angle of fast electrons.
Figure 4 shows HWHMs of angle as a function ofelectron energy on electron momentum distribution
at y = 0–5 and 20–25 µm. Near the laser axis,
HWHMs in both cases are similar. On the other hand,
HWHMs in the super Gaussian case are larger than thoseof the Gaussian case at the area far from the laser axis. In

the super Gaussian case, most of the observed
electrons are generated not at the outer region but atthe area close to the laser axis, as the outside laser
intensity is so low compared to the Gaussian case.
Therefore, at the outer region, electron divergence
becomes large in the super Gaussian case.

Fig.2. Time evolutions of electron beam intensity

4. Time-integrated electron energy spectra
In fast ignition, the electron energy distribution isextremely significant because the stopping distanceof the electron in the core varies with its energy.
Figure 3 shows time-integrated energy spectra ofelectrons in the cases of Gaussian and superGaussian at y = 0–5 and 20–25 µm. Near the laser
axis, high-energy (> 5 MeV) fast electrons in thecase of super Gaussian are more generated than thatof the Gaussian case as the laser intensity of thesuper Gaussian case is higher than that of theGaussian case, but the number of low-energy

Fig.3. Time-integrated electron energy spectra

Fig.4. HWHMs of angle as a function of electronenergy on electron momentum distribution
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