S3-3
Physics of the plasma-wall boundaries in inertial fusion
慣性閉じ込め核融合に於けるプラズマ-壁境界層現象の物理科学	 
Toshinori Yabuuchi1), Kazushige Takaki1), Takuya Kono1), Seigo Misaki1),
Ryo Shibata1), Kiyohide Nishimura1), Hiroyuki Oda1),
Yoshi Hirooka2), Atsushi Sunahara3) and Kazuo A. Tanaka1)
籔内俊毅1)，高木一茂1)，河野卓也1)，御前成吾1)，柴田怜1)，西村清秀1)，小田浩之1)，
廣岡慶彦2)，砂原淳3)，田中和夫1)1) Graduate School of Engineering, Osaka University
2-1 Yamada-oka, Suita, Osaka, 565-0871, Japan
大阪大学大学院工学研究科 〒565-0871	  吹田市山田丘2-1
2) National Institute for Fusion Science322-6 Oroshi-cho, Toki, 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322-63) Institute for Laser Technology
1-8-4 Utsubo-hommachi, Nishi-ku, Osaka, Osaka, 550-0004, Japan
レーザー技術総合研究所 〒550-0004	  大阪市西区靭本町1-8-4
Interactions of intense energetic particles with matter in inertial fusion reactors are discussed. A
hydrodynamics simulation code coupled to the calculations of the energy deposition from the particles has
been developed to study the interactions. It has been found that the ionization processes of the ablated
matter are important to determine the energy deposition of the incoming particles. Similar effects ofionization on particle interactions have been found in laboratory scale experiments of laser-ablated
plasma-plasma interactions.

1. Introduction
In inertial fusion reactors, the reactor wall isexposed to high-energy particles such as fusion
products, unburned fuel, and ions contained in thefuel pellet as well as high energy x-rays. The
particle intensity can be above the ablationthreshold of the first wall. The wall ablation isparticularly crucial for solid wall reactors. It canbe also important for liquid wall reactors sincethe ablated matter may collide each other at thecentral region of the reactor chamber and produce
aerosols[1]. Ways to avoid the wall ablation canbe increase the chamber size or reduce the fusion
power, however, these solutions could reduce
fusion efficiency. Another approach to minimize
the effects of energetic particle irradiation to thewalls uses a buffer gas[2]. The buffer gas works
as an energy absorber of the energetic particles.
However, the gas density is limited to avoid theoptical breakdown that prevents the irradiation ofthe implosion laser to the fuel target.
We propose a new concept that could reducethe material ablation caused by intense, energetic
particle beam irradiations. In the concept, the firstpart of the beam ablates the matter, and then, theablated matter itself behaves as a shield, which issimilar to the “vapor shielding” around thedivertor in the magnetic confinement fusion[3].
Here, the common key of the concepts is heatingof ablated matter due to the incoming energy flux.
Energy deposition efficiencies of the energetic
ions in matter are determined as a stopping power.
We have calculated the stopping power of matteras a function of its temperature (i.e. ionizationlevel). For the study of the shielding phenomenain the ablated matter from energetic particle
irradiations, a hydrodynamics simulation code
has been developed that is coupled to the energy
deposition calculation due to energetic particle
injection to the matter using the temperaturedependent stopping power.
We have also performed an experimental studyof particle shielding using two laser-ablated
plasma plumes colliding each other. In theexperiments, it has been observed that the plasma
particles are ionized to higher levels due to theinteraction.
2. Simulation of Ablation Dynamics in HighEnergy Particle Irradiation to MatterOne dimensional hydrodynamics code with
two-temperature model has been developed for thisFig. 1. Ablated tungsten profile after the irradiation ofintense alpha particle beam from the left in this figure.
Initially there is a tungsten slab with a solid density atx>0 with an initial temperature of 300 K.

study. In the code, the stopping power of theinjected ions in matter is calculated as theinteractions of ions with nucleus, bound electrons,and free electrons. The total stopping power in thematter depends on its density and temperature since
the ionization level of matter depends on theseparameters. The stopping power originated from thebounded electrons decreases as the increase of thetemperature. In the contrary, the free-electronoriginated stopping power increases in hotter matter.
As a result, the total stopping power is higher for
hotter matter. This trend indicates that when theablated matter is heated more, it has higher stopping
power i.e. absorbs more energies from the incoming
particle flux resulting in becoming even hotter; apositive feedback loop to avoid further ablation ofthe matter. The keys of this “shielding effect” arethe ionization process and the ionization level of theablated matter. The developed code treats thephenomena self-consistently. As an example, theirradiation of intense alpha particle beam to solid
tungsten is studied. The beam is assumed to have apeak intensity of ~108 W/cm2 with energies up to afew MeV[4]. Figure 1 shows the hydrodynamics
simulation results just after the peak of the beam
injection. The ablated tungsten plasma is heated toabove 1-2 eV that can have larger shielding effectthan the one at the initial temperature.
3. Experiments of Colliding Plasma Plumes
The similar phenomena can be seen in low
density (ni ~ 1013 cm-3), low temperature (Te ~ 1 eV)
plasma particle interaction. In laboratory scale
experiments, the interaction of laser ablated plasma
plumes has been studied. The detail of theexperimental setup can be found elsewhere[5]. It
has been observed that the intersecting two plasma
plumes behave differently depending on its
parameters; density, temperature, ion species, andvelocity. For example, the intersecting two tungsten

Fig. 2. Snapshots of intersecting plasma plumes producedby laser ablation of concave targets; (a) tungsten, (b)aluminum. The white allow indicates the plasma flowing
direction after the interaction.

plasma plumes just penetrate through in each otheras shown in Fig. 2(a), however, the aluminumplasma collides each other (i.e. shielding) andmerged after the intersection region as shown in Fig.
2(b). It has been confirmed that the rate of thescattering of the interacting plasma flow is quasi
proportional to the ion-ion collision frequency thatis dominantly depends on the charge state of ionsand also its velocity. It has been also found in thetime-resolved spectroscopy of the interacting
plasmas that the plasma particles are ionized further
during the intersection in the case showing highshielding effect.
4. Summary
The phenomena of energetic particle irradiation
to matter and the following ablation process havebeen discussed from the viewpoint of theplasma-wall interactions in inertial fusion reactors.
A positive feedback loop can occur spontaneously
because of the ionization process of ablated matterthat enhances the shielding effect of matter fromthe further ablation caused by the incoming
particle flux. It is expected that the reactor designusing the ionizing and shielding phenomena
effectively can reduce the reactor wall ablation.
Acknowledgments
Part of this work is supported by the JSPS underthe Asian CORE Program for high energy density
science using intense laser photons and also thePartnerships for International Research and
Education Program for nuclear energy systems andmaterials under extreme conditions.
References[1] Y. Hirooka et al., Fus. Eng. Des. 97 (2012) 1760.
[2] A. R. Raffray et al., Fus. Sci. Tech. 46 (2004) 417.
[3] A. Hassanein et al., Fus. Eng. Des. 60 (2002) 527.
[4] K. Yamamoto et al., JPFR 82 (2006) 441.
[5] Y. Hirooka et al., J. Phys. Conf. Ser. 244 (2010)
032033.