24D16
Generation of Pre-formed plasma inside a cone target in Fast-ignition Scheme	 
高速点火コーンターゲット内部のプレプラズマ生成	 
Atsushi Sunahara, Tomoyuki Johzaki, Hideo Nagatomo1), Shinji Ohira1), Shinsuke Fujioka1),
Kunioki Mima1), Hiroyuki Shiraga1) and FIREX project1)
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砂原 淳, 城崎知至, 長友英夫 , 大平真司 , 藤岡慎介 , 三間圀興 , 白神宏之 , FIREXプロジェクト
Institute for Laser Technology,
1)Institute of Laser Engineering Osaka University
2-6, Yamadaoka, Suita, Osaka 565-0871, Japan
1)(財)レーザー技術総合研究所, 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２−６

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We investigated generation of pre-formed plasma inside cone targets due to a pre-pulse of the main
ultra-intense laser pulse in the fast-ignition scheme of the inertial confinement fusion. We estimated the density
gradient scale length of the pre-formed plasma inside the cone target to be 27-47 microns between the critical and1/10 of the critical density in the current FIREX experiment, based on the 2D radiation hydrodynamic simulations.
In order to reduce the effects of pre-formed plasma on the generation of fast electrons inside a cone, and increase
the energy coupling efficiency from the ultra-high intense short-pulse laser to the imploded core plasma wepropose the advanced concept of a cone target, which can relax the impact from the implosion plasma, and focus
fast electrons to the imploded core plasma by self-generated magnetic fields.

1. Introduction
In order to achieve the high-gain of thethermonuclear burn in the fast-ignition scheme ofinertial confinement fusion, we need to increase theenergy coupling efficiency between the ultra-highintense short-pulse laser and the compressed core.
Kodama et al [1] conducted the first experiment on
this fast ignition in 2000. Then, Osaka University
started the fast ignition realization experiment(FIREX) in 2009 [2]. This project has two phases. In
FIREX-I, the goal is to achieve the ignition
temperature of 5 keV by ultra-high intense laser
heating. FIREX-II aims at achieving high gain
thermonuclear burn. In order to get the coretemperature of 5 keV, the energy coupling from theheating laser to imploded core must be increased.
Baton [3] pointed out that pre-formed plasma inside
the cone significantly disturbs the fast electron flow to
the core. Cai [4] showed that existence of thepre-formed plasma inside the cone reduces creation of
fast electrons of the energy less than 5 MeV. Theseelectrons are the dominant contributor to heating of
the core plasma. Johzaki et al. [5] simulated
generation of fast electrons using ultra-high intenselaser and its transport to the core plasma, and they
estimated that 10 micron scale pre-formed plasma
lowered the coupling efficiency by ~ 20 %, comparedto the case of 1 micron. In our talk, we will show thecurrent level of pre-formed plasma formation in
FIREX experiments and we propose the advanced
concept of the cone target to improve the heatingenergy coupling from the short-pulse laser to thecompressed core plasma.

2. Pre-plasma formation
We simulated the pre-plasma formation inside thecone in FIREX experiment by 2D radiationhydrodynamic simulation [6]. The laser wavelengthand the spot diameter are set to 1.06 microns and 40
microns (Gaussian FWHM), respectively. The laser
temporal profile is a flat top profile. The diameter ofthe tip of the cone target is 30 micron. The laser
intensity is set to 1 ! 1011 W/cm2, 1 ! 1012 W/cm2, and
1 ! 1013 W/cm2, and the open angle of the cone wasset to 30 o, 45 o, and 60 o, respectively. The wall
thickness of the cone tip and the wall near the tip wereboth 10 microns. In Figs. 1 (a)-(c), we show
simulation results for various cone open angles andlaser intensities; (a) 1ns pulse duration, (b) 2ns, and(c) 10ns, respectively. In Fig. 1 (a), we see that as thelaser intensity increases, the generated pre-formedplasma fills the space inside the cone. For 30 o open
angle and the laser intensity of 1011 W/cm2, thepre-formed plasma is localized around the tip.
However, when the intensity increases to 1013 W/cm2,
the pre-formed plasma expands to the entire space.
This effect is reduced when the open-angle of the cone
increases, since a larger open- angle cone has a larger
volume to be filled with the pre-formed plasma. Also,
the pre-formed plasma flow from the sidewall collides
along the center axis of the cone, and yields a plasmajet. This jet was observed in the Hydra simulation [7],and as the laser spot diameter increases, this jet-likestructure becomes prominent.

If we use diamond-like carbon (DLC) or aluminum of
the solid density 2~3 g/cm3, we can increase the delay
time to 20-30 ps, even though the lower-Z material
has a smaller areal density compared to that of theconventional gold tip with 10 micron. Also, weinvestigate another function of focusing fast-electrons
by TONGARI tip cone. The resistivity of DLC oraluminum pointed tip is relatively higher than that ofsurrounded CH or DT plasma. In this situation, whenfast electrons flow because of ultra-high intense laser
irradiation, a magnetic field will be generated along
the boundary between the implosion plasma and thepointed tip. Subsequently the magnetic field may
focus the electron flow to the imploded core.

Fig. 2.

Fig. 1(a)-(c): Electron number density ne (cm-3).
(a) 1 ns duration pulse, (b) 2 ns, and (c) 10 ns. IL is thelaser intensity of 1 ! 1011 W/cm2, 1 ! 1012 W/cm2, and
1 ! 1013 W/cm2, respectively. The cone open angle is,respectively, 30o, 45o, and 60o

3. “TONGARI” TIP
Another critical issue is the impact from theimploded core plasma on the tip. The protection of thecone tip is very important to the future fast-ignition
design, since when the implosion energy is increased,a high-pressure shock wave can easily destroy thecone tip. For the remedy, we propose using a pointed
cone tip (TONGARI) shown in Fig. 2. Using thepointed shape, we can increase the shock-travelingtime from the imploded core to the inner surface ofthe tip. We simulated the shock propagation in analuminum pointed tip from the implosion plasma
under the typical implosion condition of FIREX. Thelength from the inner surface to the tip is 50 microns,
and the tip is located at the center of the implosion.
The areal density is 2.7 g/cm3 ! 50 microns =0.0135 g/cm2, which is still smaller than 0.0193 g/cm2,
the value for the conventional 10 micron gold flat tip.

Schematic of the (TONGARI) pointed cone.

4. Conclusion
We estimated the density gradient scale length ofthe pre-formed plasma inside the cone to be 27- 47
microns at the densities between the critical density
ncr and 1/10 ncr by 2D radiation hydrodynamicsimulations. In order to protect the cone tip from theimplosion plasma, and increase the heating energy
coupling, we propose use of a pointed cone to protect
the cone tip from the break through of the shock wave
from the implosion plasma, before the main pulse
irradiation starts. Also the focusing fast-electrons canbe expected.
References[1] R. Kodama et al., Nature 412 (2001) 798; R.
Kodama et al., Nature 418 (2001) 933.
[2] H. Nishimura et al., AIP Conf. Proc. 1209 (2010) 83.
[3] S. D. Baton, et al., Phys. Plasmas 15 (2008) 042706.
[4] H.-B. Cai, et al., Physical Review Letters 102 (2009)
245001.
[5] T. Johzaki, H. Nagatomo, A. Sunahara, H.-B. Cai, H.
Sakagami, Y. Nakao and K. Mima, Nuclear Fusion
51 (2011) 073022.
[6] A. Sunahara, T. Johzaki, H. Nagatomo and K. Mima,
Laser and particle beams, accepted.
[7] A. J. Kemp, B. I. Cohen, and L. Divol, Phys.
Plasmas 17 (2010) 056702.