24P068-P
Material Dependence on Plasma Shielding Induced by Laser Ablation
レーザーアブレーションを用いたPlasma Shieldingの材料依存性
Takuya Kono, Satoshi Tanaka, Toshinori Yabuuchi and Kazuo A Tanaka河野卓也1), 田中覚2)，籔内俊毅1)，田中和夫1)1) Graduate School of Engineering, Osaka University
2-1, Yamadaoka, Suita, Osaka 565-0871, Japan
大阪大学工学研究科 〒565-0871 大阪府吹田市山田丘2-12) Department of Media Technology, Ritsumeikan University
1-1-1, Noji-Higashi, Kusatsu, Shiga 525-8577, Japan
立命館大学メディア情報学科 〒525-8577 滋賀県草津市野路東1-1-1
Plasma shielding is to study the effect of ablated plasma or the intentionally created plasma plume on thefirst wall by absorbing next incoming plasma energy. Two laser ablation plasma (n~1012/cm3, Te~1eV) arecreated and are crossed each other in our experimental platform. 10~60% of the incoming plasma particles
change the direction. The ratios are different with the plasma material. By observing the material
dependence of colliding effects, the relation to plasma shielding is discussed.

1. Introduction
In nuclear fusion research, it is one of theimportant themes to study the damage on plasma
facing components (PFC). For example, expectedheat loads are 10 to 100 MW/m2 at magnetic
fusion energy (MFE) divertor and 10 9 W/cm2 or
higher at inertial fusion energy (IFE) first walls atplasma densities 10 19 /m3 or higher andtemperature 1 eV to keV or even to MeV.
However active functions are proposed to protect
the walls like vapor shielding [1] and plasma
shielding effects. Our intention is to shed light on
this effect. For this purpose, the behavior ofablated plasma should be studied. For example inIFE chamber, the stagnation of ablated plasma
plumes affects next laser irradiation. To solvethese problems, it is necessary to study how theablated plasma from IFE chamber wall behaves
with the cylindrical structure.
In our experiments, the direction of thecollided plumes changed when plasma plumes
made by laser ablation are crossed each other. As
the material of plasma plume, several target
materials are used such as Carbon and Tungsten.
2. Experimental Setup
Plasma shielding is studied using theexperimental set up “LEAF-CAP” (LaboratoryExperiments on Aerosol Formation by Colliding
Ablation Plumes) [2]. Fig.1 (a) shows the set up, athird harmonic beam of YAG laser (6ns, 10Hz) is
optically split into equal-power, and each beams areline-focused (~0.1mm by ~1cm), then these radiate
two cone-cave targets at room temperature in a
vacuum chamber (~10-3 Pa). Carbon, Aluminum,Copper, Molybdenum and Tungsten are set as thetarget. The plasma plumes ablated on the cone-cavetargets cross each other on the 14 mm point fromthe surfaces (Fig.1 (b)). The line-focused laser
energy is 10J/cm2/pulse, at this energy the plasma
density is ~1012/cm3 and the temperature is ~1eV
[3].

(b)
(a)
Fig.1. Experimental set up “LEAF-CAP”

A quartz thickness monitor is set to measure thenumber of plasma particles at (i) Front or (ii) Side(45o) in Fig.2. The depositions at each place arecompared with and without shield plasma (whichablated by the perpendicular laser to the monitor at(i) Front).

Fig.2. The place of quartz thickness monitor

3. Results and Discussion
3.1 Results of the quartz thickness monitor
Fig. 3 shows the result of the quartz thickness
monitor at each place with and without shieldplasma in case of Carbon targets. Without shieldplasma, the coat speed at Side is 0.21Å/sec and it is
1.4% of total. With shield plasma, the side is 13
times and the front is 0.41 times than the withoutcase, and the side is 31% of total. Taking deposition
from two directions into consideration, the coat
speeds in case with shield plasma at side are in half.
These results suggest that plasma collides and theratio of side increases by 30 points.

and we define the collision parameter as:
Collision parameter em 2u 3(1)In equation (1), e is the divided value with ion
charge [5]. Fig.5 shows that the collision parameteris linearly correlated with the increase of side in ourexperiment.

Fig.5. Collision parameter to increase of side

Fig.3. Results of quartz thickness monitor (Carbon)

3.2 Material dependence
The above tendency is also observed in cases ofother materials. However the ratios of side aredifferent, as shown in Fig.4 (a). In addition, Fig.4(b) shows that the ratios are depend on the atomicmass, namely more plasma is changed the line in
the smaller atomic mass.

(a)(b)
Fig.4. Material dependence of shielding

3.3 Collision parameterTo consider the difference of collision effects inmaterials, the Collision parameter is provided instanding on theoretical equation. The rate ofcollision is a function of ion charge, mass (m) andvelocity (u) in the simulation of plasma collision [4],4. Conclusion
Through the intersection of two plasma plumes,
10~60% of the particles changes the direction.
The increases of the side vary with the material.
Our results show that plasma shielding may beeffective in the smaller atomic mass plasmas.
Introducing the collision parameter in standing,
one can observe a linear correlation with theexperimental data. This parameter can be treated as
the degree of plasma collision and the plasma
shielding.
Acknowledgments
We are indebted to Prof. Yoshi Hirooka, National
Institute for Fusion Science, Toki Japan for thefruitful discussion.
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[5] Atomic Molecular Data Services, IAEA.