04pC01
PWレーザー駆動高速点火プラズマにおけるMeV高速電子によるエネルギー輸送
Energy transport by MeV hot electrons in fast ignition plasma driven withLFEX PW laser
Z. Zhang1, 西村博明1, 藤岡慎介1, 有川安信1, 中井光男1, 尾崎哲2, 長友英夫1, H. Chen3, J.
Park3, G.J. Williams3, 城﨑知至4, 砂原淳5, 白神宏之1, 小島完興1, 宮永憲明1, 河仲準二1, 中田芳樹1, 實野孝久1, 疇地宏1
Z. Zhang1, H. Nishimrua1, S. Fujioka1, Y. Arikawa1, M. Nakai1, et. al.
阪大レーザー研1, 核融合研2, LLNL3, 広大4, レーザー総研5	 
Osaka Univ. ILE1, NIFS2, LLNL3, Hiroshima Univ.4, ILT5
Fast ignition is recognized as a promising pathway to efficient thermonuclear fusion in laser- driven inertial
confinement fusion. A cone-guided CD-shell has been used as a base-line target for the fast ignition
realization [1, 2]. It has long been expected to provide more quantitative information about the hot electrongeneration and transport in the cone than those derived only with x-ray imaging and neutron detection. In
this research, we propose an absolute Kα line spectroscopy dedicated for quantitative measurement of hotelectron generation and transportation in high-Z targets. This diagnostic provides local information about thehot electrons propagating through specific materials composing the cone-guided target.
In this study, Au and Ta were chosen as tracers since they are representative highest-Z materials which areavailable for the guiding cone, thus better matching with MeV-hot electrons than lower-Z tracers such as Cu.
A Laue spectrometer was developed to cover the high energy Kα lines from Mo (Kα1: 17.48 keV, Kα2:17.37 keV) to Au (Kα1: 68.80 keV, Kα2: 66.99 keV)[3]. The spectrometer consists of a cylindrically curvedquartz (10-11) plate and a detector. The quartz plate is bent such that the diffracted x-rays are focused at anintermediate slit. X-ray components propagating in a straightforward manner are shielded from the detector
directly with a lead pinhole plate located in front of the crystal and a pair of lead shields located at theintermediate x-ray focus. By varying the distances from the crystal to the source and detector, thisspectrometer can cover the energy range of either 10-60 keV or 22-100 keV.

Fig. 1. Electron trajectory in the Ta cube: (a) all electrons, (b) electron with energy less than 0.1 MeV, (c)electron with energy from 0.1 to 1 MeV, (d) electron with energy more than 1 MeV.

The hot electron trajectory inside the solid target is tracked with Monte-Carlo simulation. A 3-dimensional
code PHITS was applied. The code applies a continuous slowing down approximation (CSDA), whichmeans that the electrons change their direction of motion due to elastic scattering, and lose their energy
between two scattering points continuously. An example is shown in Fig. 1. The black square in the center
indicates a 1 mm Ta cube, which is surrounded by low density air with a pressure of 10−5 Torr. The hotelectron beam was irradiated on the Ta cube from the left side. An initial divergence of 45o and a Gaussian
spatial profile were assumed. The energy spectrum was set based on the electron spectrometer (ESM)measurement in experiments. From Fig. 1, it is clearly seen that most of the electrons with energy less than
0.1 MeV were reflected at the front surface. The penetration depth increased along with electron energy; andonly part of the electrons with energy higher than 1 MeV can propagate through the Ta cube and escape fromthe back surface.
The laser transfer efficiency, ηTE, is defined as the fraction of energy deposited into hot electrons. The
number of Kα photons NKα generated from hot electrons with number Nh and temperature Th can be estimatedby the following model:

N Kα = N hdnAω Kα ∞xdEσ Kα (E) × ∫ dxfh (E0 , x)exp(−)∫04π Th 0λmfp cos(θ )(1)where σKα, ωKα, and nA are, respectively, the cross section for K-shell ionization, the Kα fluorescence yield,
and the atomic number density. The term exp(−x/ λmfp cos(θ)) describes the reabsorption of Kα photons
during the propagation through the target material, where θ is the angle between the spectrometer and target
normal. fh(E0,x) describes the energy spectrum for the hot electron propagating inside the Ta cube with adepth x, where the information was achieved with PHITS simulations. Considering the absolute Kα photon
number NKα measured by the Laue spectrometer, the transfer efficiency ηTE can be estimated by comparing
the experimental measurement and simulation results.The transfer efficiency from LFEX laser to target has
been estimated in the cases of planar and cone-guided geometry. In the case of planar target, an Au plate wasplaced at the target chamber center and the LFEX laser with maximum energy of 1.8 kJ was focused on thefront surface with an incident angle of 10o. The Kα from Au was recorded by the Laue spectrometer and anESM was located from the back surface for the hot electron temperature. The ηTE as a function of laser
intensity is shown in Fig. 2. As a general trend, the ηTE is increased by increasing the laser intensity.
The cone-guided configuration is shown in Fig. 3(a). A cone is attached with a hemi-CH shell, which wasirradiated by three beams of Gekko-XII laser [1]. A dense plasma surrounding the tip of the cone wasproduced to mimic the condition of the fast ignition Au cone+CD shell target. The Ta cube was attached as
the Kα tracer after the hemi-CH shell. Four types of cone were used, as shown in Fig. 3(b)-(e): (b) is thestandard Au cone with 7 μm thickness; (c) is the open-cone without tip; (d) is the W-cone with double Aulayers; and (e) is the diamond like carbon (DLC) cone. The estimated ηTE was shown in Fig. 3. Comparedwith the planar geometry, the LFEX laser transfer efficiency is significantly enhanced with a guiding cone.
(a)Transfer efficiency (%)

Au−plate60(b)(c)(d)(e)StandardOpen
W–cone40DLC

2004681012Intensity (x1018 W/cm2)

Fig. 2. The ηTE as a function of laser
intensity for the planar and cone-guidedtargets.

Fig. 3. (a). The configuration of a cone+CH hemi
shell+Ta cube target; (b). Standard Au cone; (c). OpenAu cone; (d). W-cone; (d). DLC cone.

[1] H. Shiraga et. al., 2012 High Energy Density Phys. 8 227[2] R. Kodama et. al., 2001 Nature 412 798
[3] Z. Zhang et. al., 2012 Rev. Sci. Instrum. 83 053502