SI-5
慣性炉チェンバーに於けるPWIと高繰り返しペレット爆縮PWI & High Repetition Pellet Implosions in IFE chamber
田中和夫
TANKA A, Kazuo
大阪大Osaka University
In IFE reactors, the inner wall surface
could be exposed to a rather high thermal flux
stemming from nuclear fusion reactions. The
thermal flux consists of high energy (several 100
keV to MeV) H, C, He ions as well as neutrons.
Since these particles come onto the wall within amicro second order, the thermal flux intensity canbe as high as 109 W/cm2. This intensity is close orbeyond the material ablation threshold resulting in
the wall surface melting or ablation. Testing thiswhole process is essential to understand themechanisms and characteristics of ablation andsubsequent plasma plume and possibly aerosol
formation. If we replace this ion thermal flux tolaser flux, laser system can deliver almost any levelof intensity and energy densities from 100 W/cm2 to1014 W/cm2 or 10-3 J/cm2 to 1 kJ/cm2. These
numbers can be controlled relatively easily to test
the ablation and evaporation processes of basically
any target wall materials. Using ultra-intense laser
system, we can even create 10 MeV proton beamwith a psec pulse duration which can be used to test
the material damage with ion beam[1]In our study, 1J blue laser beam is split
into two beams with equal energy to irradiate two
concave shape solid targets with a mm wide line
focusing (0.1 mm width). The laser wave length is351 nm, repetition rate is 10 Hz, and pulse width is
6 nsec. The focused laser intensity can cover 107
W/cm2 to 1012 W/cm2 while the energy density
varies from 0.1 J/cm2 to 50 J/cm2. When carbon
targets are used the plumes show temperaturearound 1 to 10 eV and plasma density around 1011
to 1012/c.c. for single and double plumes measured
at 1.3 cm away from the target surface. Only one
laser beam is used for the single plume case while
the two laser beams are used for the double plumecases. The experimental configuration is shown inFig. 1. The double plumes collide and stagnate at
the crossing point at 1.3 cm away from the target.
The plume moving speed is 10 km/sec.
The process appears to be led by Carbon ion
collisions based on our particle simulation [2].

Fig. 1 Experimental Configuration
Two soli targets with cone cave shape areirradiated with two blue laser beams. The
ablated plumes cross each other at 1.3 cm.
The simulation has been conducted usinga Direct Simulation Monte Carlo method. The
model simulation has assumed 106 cells with 5x 105
particles of ions or neutral particles within a 3 cm
cubic box.

Fig. 2 DSMS simulation shows close similarity
to the observation.
The stagnated carbon plumes result in carbon large
molecules such as nano tube formation [3]. Actuallycreated are onion, carbon nano-tubes, etc. as shownin Fig. 3. These created carbon products actuallyaviate much longer than the plasma plume life time
which has been measured with another laser probe
beam using a scattering techniques.

be related to the molecular formation processes
which are not applicable to the metal materials.

Fig.3 Carbon molecular products recoveredfrom the plasma plume collision andstagnation.
The life time of these products are peaked at 3
micro seconds with roughly 2 microsecond life timeafter the laser pulse irradiation while the plasma
plume peak emission comes at around 2
microseconds.
The collision and stagnation has beentested for other materials such as W, Al, and Mo.
If we take into account the fraction of ionization,
average Coulomb collision frequency is,

The efficiency of collision can be expressed as“Shield rate”. The shield rate is defined as the ratioof transmitted particles of one plasma plumethrough the second plume crossing normal to thefirst one to the particles directly counted without
the secondary plume (namely single plume). The
shield rate is plotted for Carbon, Aluminum,Mlybdenum, and Tungsten materials in Fig. 4.

Fig. 4. Shield rate vs. collision frequency
Target materials are Aluminum, Carbon,Mlybdenum, and Tungsten. The number/cm2
indicates the laser energy density.
Figure 4 indicates that the collision
efficiency is linearly proportional to the collision
frequency. The carbon is an exception showing
60 % of shield rate. This high number of C should

In recent theoretical study, another possible issue isindicated as afterglow plasma evolution in IFE
chamber [4]. The averaged residual gas densities
from the target reactions only could reach as high as
1015 /cm3. Cryogenic targets could survive from theblack body radiation heating only if the gas density
is below 3 x 1013 /cm3. This new issue could bestudied in an independent experimental platform as
proposed here.
In a summary, experimental platform based on laser
systems can be a versatile tool[5] to study theablation or evaporation from the inner wall surfaceof IFE reactors. Our recent studies show ablated
plasma plumes can collide and stagnate at thecrossing point. In the carbon plasma case, thecollision is effective more than the other metaltargets. As a result of collision and stagnation, thecarbon case shows efficient molecular formationsuch as carbon-nano tubes etc.
Reference
1. Kenshiro Kikuyama, KA Tanaka, private
communication (2012).
2.

“Atomic number Z dependence of dynamics oflaser-ablated matter” A. Sunahara and K.A.
Tanaka, Fus. Eng. Des. 85 (2010) 935; Seigo
Misaki, M Eng., Thesis, Osaka University, Mar.
2012.

3.

“Aerosol formation and hydrogenco-deposition by colliding ablation plasma
plumes of carbon”
Y Hirooka, KA Tanaka et al., Fusi.Sci.Tech.
60, 804 (2011).

4.

“Simulation of afterglow plasma evolution in
an inertial fusion energy chamber”,
K Frolov, A Yu Pigarov,, SI Krachieninnikov,
RW Petzoldt, DT Goodin,
J. Nucl. Mat., 337-339, 206-210 (2005).

5.

“Carbon plume stagnation: platform for vapor
shield study” .A. Tanaka, A. Hassanein, Y.
Hirooka, T. Kono, S. Misaki, T. Oishi, A.
Sunahara, and S. Tanaka,Fus. Sci. Tech. 60 (2011) 329.