24P069-P
Particle Simulations for Plasma-Plasma Intersecting Experiments
粒子シミュレーションを用いたプラズマ交差実験の挙動解析
S.Misaki1), A.Sunahara2), T.Kono1), N.Kawamoto3), T.Yabuuchi1), S.Tanaka3) and KA Tanaka1)
御前成吾1), 砂原淳2), 河野卓也1), 川本直也3), 籔内俊毅1), 田中覚3), 田中和夫1)
1)

Graduate School of Engineering, Osaka University, 2-1 Ymadaoka, 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2-12)
Institute for Laser Technology, 1-8-4 Utsubohonmachi, Nisi-ku, Osaka 550-0001, Japan
レーザー技術総合研究所, 〒550-0001, 大阪府大阪市西区靱本町1-8-43)
Department of media technology, Ritsumeikan University, 1-1-1 Norohigashi, Kusatsu, Shiga 525-8577, Japan
立命館大学総合理工学院情報工学研究科, 〒525-8577, 滋賀県草津市野路東1-1-1
Experiment is conducted on the intersections of two plasma plumes created by laser ablation. In ourexperiments, a clear material dependence has been found clearly in the visible light emission from theintersecting plumes. Two carbon plumes merge each other after the intersection. However, tungsten plumejust passes through in another tungsten plume. We have developed a particle simulation code to understandthe physics of the plume intersections. The results show that the plume collisions strongly depend on theplasma density, ionization fraction and material mass.

1. Introduction
Walls in a laser fusion reactor could beablated by exposure of high energy materials
such as x-rays and pellet debris from fuelDT-pellet implosions. Ablated plasma plumes
from reactor walls may expand toward thecenter of the reactor and may collide or
stagnate with each other.
To study the behavior of these plasma
plumes, we have conducted experiments on
the intersections of two plasma plumes
created by laser ablation[1, 2]. Figure 1 showsthe experimental setup. A laser beam fromone Nd: YAG laser (3ω, 355 ns, 6 ns, 10 Hz)
is split into two separate beams with an equalpower.
Line-focused two laser beams
irradiate solid targets. We have observed thetemporal evolution of plasma emission withan intensified CCD (ICCD) camera. Figure 2shows the results of ICCD observations. The
behavior of plasma emission after theintersection of two plasma plumes changes
between tungsten and carbon. We havedeveloped a particle simulation code tounderstand the physics of the plasma plumeintersections.

Fig.1 A schematic diagram of the experimental
setup [1].

Fig.2 Examples of the ICCD observations fromthe top of the vacuum chamber. Emission ofplasma plumes moves to the synthetic diagonal
direction after the intersection[3].

2. Particle Simulation CodeThe code developed in this study uses thedirect simulation Monte-Carlo (DSMC)
method and coulomb collision model[4, 5] tosimulate the plume intersections. The plumeplasma density is estimated to be 1013~1015
cm-3 at the intersection point frommeasurements with a Langmuir probe andalso results of hydrodynamics calculations.
Since the density is low, the intersecting
phenomena can be simulated with a particle
code. In our code, neutral-neutral, neutral-ionand ion-ion collisions are considered. All
collisions are completely elastic collisions.
We conducted simulations with various initialparticle densities (from 1013 cm-3 to 1015 cm-3)
to compare with our experimental results. The
initial temperature of ion and neutral particles
was changed from 0.1 eV to 1 eV. The
fraction of ion species was estimated based on
the corona equilibrium[6, 7].
Figure 3 shows the simulation setup. Twogroups of particles mimic the two intersecting
plasma plumes. The plume intersections ofcarbon plumes and tungsten plumes wereconsidered. The velocity of plumes is givenby the summation of the thermal velocity anda drift velocity. The drift velocities, which aredetermined by ICCD observations, are 2.9
106 cm/s in the carbon case and 1.1 106 cm/sin the tungsten case. Our simulations are inthree dimension and the particles groups have
0.5 cm thickness in z direction. We set two
observation areas(A1 and A2) to count thenumber of particles that come through theareas.

Table.1 Result table(a)Te=0.1eV-3

Particle density(cm )1314

Carbon100.0017
0.0010(b)Te=1eV

Particle density(cm-3)(=Ti)
Tungsten(=Ti)
Tungsten
Carbon13

1000.0011100.021
0.020

15

14

1000.0056

10
1.36
0.91

Ionization
fraction0(neutral)0(neutral)
Ionization

15100.19

fraction

1.40

0.8

1

The results show that the plume collisions
strongly depend on the density, ionization
fraction and material mass. Comparison of theresults with ICCD images shows that theexperimental plasma densities are considered
1015 cm-3 in carbon case by neutral-neutral orion-ion elastic collision. In tungsten case,plasma plumes may collide with the density
more than 1015 cm-3 by neutral-neutral elasticcollision.
Fig.3 Simulation setup

3. Results and Discussion
Examples of simulation results are shownin Figs.4(a) and 4(b). In Fig.4(a), mosttungsten particles do not collide with eachother at the intersection point and twoparticles groups go through withoutchanging their directionality. In Fig.4(b),two carbon particles collide with each otherat the intersection point. After theintersection, the directionality of twoparticles groups are changed and they moveto the synthetic diagonal direction. Table 1
summarizes simulation results. The values
shown in table 1 are the ratio of the numberof particles that come through the area A1and A2 in calculation time. When the value
is beyond 1, it means that the particles go to
the A2 direction are more than that of the A1
directions.
(a)

(b)

4.SummaryParticle
simulations
aboutplasmaintersecting experiments have been performed.
The result shows that the plume collisions
strongly depend on the density, ionization
fraction and material mass. From the elastic
collision model, the experimental plasma
densities are considered 1015 cm-3 in carbon
case, and plumes may collide with the density
more than 1015 cm-3 in tungsten case.
Acknowledgments
We are indebted to Prof. Yoshihiko Hirooka,
National Institute for Fusion Science, Toki
Japan, for the fruitful discussion.
Reference[1]Y.Hirooka, T.Oishi, H.Sato and K.A. Tanaka:
Fusion Science and Technology. 60 (2011) 804.
[2]Y Hirooka, KA Tanaka, H Sato, K Ishihara andA Sunahara:J. Phys. 244 (2010) 032033.
[3]K.A. Tanaka, A. Hassanein, Y. Hirooka et al:Fusion Science and Technology. 60 (2011) 329.
[4]日本機会学会編: 原子・分子モデルを用いる
数値シミュレーション (コロナ社 1996).
[5]T. Takizuka and H. Abe: J. Comp. Phys. 25(1977) 205.
[6]David Salzmann: Atomic Physics in Hot
Plasmas (Oxford University Press 1998).

Fig.4 (a)Tungsten case (b)Carbon case, (theparticle density is 1015 cm-3, the temperature is1 eV)[7]Atomic Molecular Data Service (FLYCHK atIAEA).