24D15
Control of Imploded Core Plasma by Using 12 beams of Gekko XII
12ビーム爆縮による爆縮コア制御
Mayuko Koga, Yoshinori Ishii, Takehiro Sogo, Hiroyuki Shiraga, Hideo Nagatomo, Tetsuo Ozaki*,
Shinsuke Fujioka and Hiroshi Azechi
古賀麻由子, 石井圭憲, 十河雄大, 白神宏之, 長友英夫, 尾崎 哲*, 藤岡慎介, 疇地 宏
Institute of Laser Engineering, Osaka University
2-6 Yamada-Oka, Suita, Osaka 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 2 - 6*
National Institute for Fusion Science322-6, Oroshi-cho, Toki-city, Gifu, 509-5292, Japan
自然科学研究機構 核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
The new laser arrangement to implode a fast ignition target, 12 beams implosion, is tested by using Gekko
XII laser in Osaka university. The 3 laser beams near a cone are shifted not to irradiate a cone. Long goldcone is used to prevent the irradiation of 3 beams with fundamental frequency. It is found that the sphericalsymmetry of the imploded core plasma is improved and the motion of the imploded core plasma towards
the cone is suppressed by using 12 laser beams. These results suggest that 12 beams implosion is apowerful concept to control the imploded core plasma.

1. Introduction
Fast ignition [1-4] is one of the proposed ways
to achieve high fusion energy gain in inertial
fusion research. It has the potential to achieve
ignition and burn with significantly less energy
than is required for the central ignition scheme.
The fast ignition realization experiment (FIREX-I)
project is going on at Osaka University. The goalof FIREX-I is to achieve heating of thecompressed core to above 5 keV. For effective
heating of imploded core, it is necessarily tocontrol the implosion hydrodynamics. Usually, 3
laser beams near the cone are not used forimplosion because they irradiate a cone. However,it was found that this laser unbalance caused an
ununiform implosion and made the imploded coreplasma move toward the cone. The imploded coreplasma may collide with the cone tip and break it.
This may become a serious problem of disturbing
the effective generation of fast electrons from thecone tip. Therefore, we proposed the new conceptof implosion, 12 beams implosion, and tried it in
the fast ignition integration experiment.
2. Concept of 12 beam implosion
Figure 1 shows the schematic diagram of acone-shell target and configurations of the 3 laser
beams near the cone. The dotted line shows an
original configuration of 3 laser beams. In 12 beams
implosion experiment, these 3 laser beams are
shifted not to irradiate a cone as shown in Fig. 1
with the solid line. Long cone is used to prevent theirradiation of 3 laser beams with fundamental

frequency inside a cone. (The conventional cone
size is shown as broken line in Fig. 1.)

coneshellFig.1. Schematic diagram of a target andconfigurations of 3 laser beams near the cone.

3. Experiment
The experiments were performed using Gekko
XII and LFEX laser system [5] at the Institute ofLaser Engineering, Osaka University. The
wavelength of the GXII laser is 0.53 micron and thepulse duration is 1.5ns. The total energy of theGXII implosion laser was about 2 kJ. The
wavelength of the LFEX laser is 1.05 micron andthe pulse duration is set to 2 ps. The targets weredeuterated plastic shells with long gold cones. The
diameter of the shells was about 500 micron. Adistance between the cone tip and the center of its
shell was about 50 micron.

X-ray framing camera (XFC) [6-8] was used toobtain the time-resolved 2 dimensional X-ray
images of the imploded core. XFC can record
images in every 80 ps with a high spacial resolution(17.3 micron). In the fast ignition experiment, hotelectrons are generated and emit sub-MeV to
MeV energy X-rays through bremsstrahlung.
These high energy X-rays cause high backgroundnoises and make measurements difficult. X-ray
reflectors made of platinum X-ray mirrors wereused to discriminate high energy X-rays andthermal X-rays. Shields made of lead are introduced
to XFC in order to block direct incidence of highenergy X-rays which make noises.
4. Results and Discussions
Figure 2 (a) and 2 (b) show X-ray images of
imploded core obtained in 9 beams implosion and
12 beams implosion, respectively. The position of
the chamber center is shown as a white cross. It isclearly seen that the imploded core plasma shows
more spherical form in 12 beams implosion thanthat in 9 beams implosion. In 9 beams implosion,the imploded core is generated near the cone tip andmoves toward the cone. On the other hand, in 12
beams implosion, the imploded core plasma isgenerated away from the cone tip and moves
toward the cone slowly. Figure 3 shows the velocityof the imploded core estimated from X-ray images.
It is shown that the velocity of the imploded coreplasma in 12 beams implosion is suppressed byabout half of that in 9 beams implosion. These
results show that 12 beams implosion is effective
way to generate spherical and stable imploded coreplasma. We are trying to estimate the motion of theimploded core plasma by using computer
simulation. The details of the imploded core plasma
generated in 12 beams implosion experiment, suchas the density and the temperature, will bemeasured in the next experiment.
(a)

(b)

Fig.2. X-ray images of the imploded core obtained in(a) 9 beams implosion and (b) 12 beams implosion.

Fig.3. The velocity of the imploded core estimated form
the X-ray images obtained in (a) 9 beams implosion and(b) 12 beams implosion.
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