24P067-P
Ratio of Ions and Neutral Particles in Inertial Fusion Reactor Plasma Plume
慣性核融合炉におけるイオンと中性粒子の比からみたプラズマプルーム特性
Kenshiro Kikuyama, Kazuo Tanaka, Toshihiro Yabuchi, Tatsuya Oishi and Takuya Kono
木久山 健士郎, 田中和夫，藪内俊毅，大石達也，河野卓也
Graduate School of Engineering, Osaka University
Yamada-oka 2-1, Suita, Osaka 565-0871 Japan
大阪大学大学院工学研究科 〒565-0871 大阪府吹田市山田丘2-1
In inertial confinement fusion, the fusion reaction could lead the first wall ablation in reactors. It is
important in reactor design to understand the behavior of the plasma plume by wall ablation. We
simulate the ablation plasma of inertial fusion reactor by laser ablation experiments. Laser intensity is
about 2-7×10^8[W/㎠]. And we use the carbon target that is one of the candidate materials of inertial
fusion reactors. First, we measured the amount of ions and neutral particles contained in the carbon
plasma plume. Next, we removed ions by the magnetic field and measured the amount of neutral
particles only contained in the carbon plasma plume. From the result, we found out that ions andneutral particles are in the ratio of 1:4 at a distance of 6.5cm from the target surface when laser
intensity is about 7×10^8[W/㎠].

1. INTRODUCTION
The first wall of inertial confinement fusion
reactor (R ~ 5-10m) is subject to the pulse load(~1GW/ ㎠ ) and may be ablated by fusion
reaction[1]. This ablation may form an aerosol in
the reactor, and it will affect the following laser
shot. In this study, we simulated the wall ablation ofreactor by laser ablation[2, 3]. We used the carbon
target: one of the candidate materials of inertial
fusion reactors. The ratio of ions and neutral
particles in the carbon ablation; will influence thebehaviors of plume collision or stagnation. In order
to differentiate the ions out of plasmas, we used themagnetic field.
2.EXPERIMENTAL SETUP
We irradiate a cone cave shaped carbon target
with Nd:YAG laser (3ω, 1-4J/㎠, 6ns, 10Hz) toablate it. The laser is line focused (1.0cm×0.1mm)
on the target. The ablated plasmas focus at 1.4cm
from the surface. The target is placed in the vacuum
chamber ( ~ 0.01Pa).
Ablation plume is monitored as the deposition
measured by a quartz thickness monitor[4] and acharge collector[5]. A quartz thickness monitor canmeasure ions and neutral particles in ablation plume.
A charge collector can measure ions only. These
measuring instruments are placed 5.0cm in front offocus and 2.0cm below axis of the laser.
Neodymium ring magnet produces the magnetic
field to remove ions. The magnet is placed 2.0cm infront of measuring instruments.

3.EXPERIMENTAL RESULTAt first, it was confirmed by charge collector thations in the ablation plume could be removedcompletely by the magnetic field. Fig.1 shows
charge collector signal with a magnet and without amagnet when laser power density is 4 J/㎠. Time 0
is the moment the laser hit the carbon target. From
Fig.1, the ratio of neutral particles and ions could bemeasured by a quartz thickness monitor and themagnet.

Fig.1) Charge Collector signal with and withoutmagnet when laser power density is 4J/㎠

Next, Fig.2 shows the results of a quartz
thickness monitor with and without the magnet.
Because neutral particles are unaffected by themagnetic field, the decreased component is due tothe ions in Fig.2.

effect is large. It is presumed that many ionsbecome to be neutral.
We tried the same experiment for the tungsten
target. Fig.4 shows the charge collector signal witha magnet was not 0 when the laser power density
was 4J/㎠ or less. Tungsten (~ 184g/mol) is heavier
than carbon (~ 12g/mol). So tungsten is hard to beaffected by the magnetic field[6].
In future, we will improve the experimental
method and the accuracy of measuring.

Fig.2) Result of Quartz Thickness Monitor
with and without magnet

4.CONCLUSION
Fig.3 shows the ratio of ions in the carbon
ablation plasma calculated from Fg.2. It isconceivable that the more laser power density
increase, the more ratio of ions increase. But whenlaser power density is 2J/㎠ or less, the tendency is
not showed.

Fig.4) Charge Collector Signal
with and without the magnet
when laser power density is 4J/㎠
Target material is tungsten

Acknowledgments
We are indebted to Prof. Yoshi Hirooka, National
Institute for Fusion Science, Toki Japan for thefruitful discussion.

Fig.3) The ratio of ions in carbon ablation plasma
where is 6.5cm in front of the target surface

5.PROBLEMS AND NEXT SUBJECTSThis experiment was performed by 4J/㎠ or less.
When we increased the laser power density, thecharge collector signal with a magnet was not 0,that is to say, ions in the ablation plume could not
be removed by the magnetic field of this magnet.
And measuring instruments are placed 6.5cm infront of the target surface, so the plasma cooling

References
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4. G.A. Frazier et al, Journal of Physics D: Appliedphysics, 12, L113-115 (1979)
5. S. Fujioka et al, Applied Physics Letters 87,241503 ( 2005)
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Plenum Press, New York (1974)