23P158-P
Laboratory simulation experiments on the aerosol formation and hydrogenco-deposition in a high repetition rate inertial fusion reactor
高繰り返し慣性炉内エアロゾル形成と水素共体積の模擬実験
Yoshi Hirooka1), Naoki Omoto2) and Kazuo Tanaka2)
廣岡慶彦1), 大本直輝2) , 田中和夫2)1)National Institute for Fusion Science322-6 Oroshi, Toki, Gifu509-5292, Japan
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
2)

Osaka University
2-1Yamadaoka, Suita, Osaka 565-0871, Japan
大阪大学 〒565-0871 大阪府吹田市山田丘2-1
In a high-repetition rate inertial fusion reactor, along with pellet implosions, the interior of a target chamber is tobe exposed to high-energy, short pulses of X-ray, unburned DT and He ash particles and pellet debris. As a result,wall materials will be subject to ablation, ejecting particles in the plasma state to collide with each other in thecenter of volume. The interaction dynamics of ablation plasmas of lithium and lead, candidate first wallmaterials, has been investigated in the deposited energy density range from 3 to 10J/cm2/pulse at a repetition rateof 10Hz, each 6ns long. The plasma density and electron temperature of colliding ablation plumes have beenfound to vary from the order of 108 to 10131/cm3 and from 0.7 to 4eV, respectively. The formation of aerosol in
the form of droplet has been observed with diameters between 10nm and 10m. Hydrogen co-deposition has
been found to occur particularly for colliding plumes of lithium, resulting in the H/Li atomic ratio varying from
0.15 to 0.27 in the hydrogen partial pressure range from 10 to 50Pa.

1. Introduction
Following the recent 192-beam experiments at theNational Ignition Facility (NIF) [1], much attention
has been directed towards the next step to design a
DEMO ICF reactor with a reasonably high repetition
rate of implosion (e.g. 10Hz [2]). However, it isrequired that the target chamber be cleared
immediately after every implosion so as not to affectthe subsequent shot, which may not readily be done
for the following reason.
Along with pellet implosions, the interior of target
chamber is repeatedly exposed to intense pulses ofDT-fusion neutrons, X-rays, unburned fuel particles,
He-ash and pellet debris, perhaps in the form of CxHy,
the total power of which amounts to the order of
10J/cm2/pulse [3]. As a result, wall materials will be
subject to ablation, ejecting particles mostly in theplasma state. Ablated particles will either be
re-deposited elsewhere on the wall or collide with eachother in the center-of-symmetry region, if any, of thetarget chamber. Colliding ablation plasma particles
may lead to the formation of clusters which can growinto aerosol, possibly floating thereafter. Also, it ispredictable that tritium be incorporated into
re-deposited materials, which would then affect theICF reactor operation scenario, depending upon theon-site radiation safety regulation. Despite theircritical importance, the chamber clearing and hydrogenco-deposition issues have not widely been addressed in
the inertial fusion research community.
In our previous work, first-of-a-kind laboratory
experiments on colliding ablation plasma plumes wereconducted for selected materials in order to investigate
the fundamental aspects of cluster/aerosol formation[4].
More recently, it has been reported thathydrogen can be co-deposited with colliding ablation
plasma plumes of carbon up to the atomic ratio of(H/C), reaching the order of 0.1 [5]. The present
work is intended to study these issues associated withlithium and lead, the eutectic alloy (Li17Pb83) of which
is currently envisaged to be used as the liquid metal toprotect the target chamber wall [6].

2. Experimental
Used in the present work is a setup, referred to as
the Laboratory Experiments on Aerosol Formation by
Colliding Ablation Plumes (LEAF-CAP), the detailsof which have already been presented elsewhere [4].
For completeness, the important features are describedhere again. A schematic diagram of the LEAF-CAP
setup is shown in Fig. 1. A 3-converted YAG laser
with a pulse length of 6ns and a repetition rate of10Hz is split into two equal-power beams, eachline-focused to an area of about 0.1cm by 1cm, toirradiate two arc-shaped targets positioned in vacuum< 10-3Pa such that ablation plasma plumes collide witheach other in the center-of-arc (CoA) region. The laser
power density is varied between 2 and 10J/cm2/pulse,relevant to the inertial fusion reactor situation [3].
A variety of techniques are employed todiagnose colliding plasma plumes in real-time,
including CCD and ICCD cameras, quadrupole
mass analyzer, visible spectrometer, Langmuirprobe, and quartz crystal film thickness
monitor. Also, ablated material deposits areanalyzed after-the-fact with optical microscopy
for surface characterizations, and with thermal
desorption spectrometry (TDS) for hydrogen
retention measurements in a separate system.
Selected up to present in the LEAF-CAP seriesof experiments as the target materials havebeen copper, aluminum, tungsten, carbon(isotropic graphite), lead, and lithium. In thepresent paper, unless it is for a comparative
purpose, the results from lithium and leadexperiments will be described in detail.

(a)

(b)

(c)

Fig. 2 Aerosol particles generated by colliding
ablation plumes for (a) Li-Pb, (b) Pb-Pb and (c) Li-Li
target combinations on Pyrex glass substrates.

Fig. 1 A schematic diagram of the LEAF-CAP
setup [4].

3. Results and discussion
Shown in Fig. 2 are aerosol particles collected forabout 1 hour, i.e. ~3.6 x 104 pulses, on Pyrex glass
substrates replacing the mass analyzer in theLEAF-CAP setup (see Fig. 1). As can be seen,
aerosol particles are generally in the form of droplet,the diameter of which ranges from the order of 100nm
to 10m. Colliding Li-Pb plumes appear to haveproduced droplets with the smallest diameter.
Similar aerosol formation experiments have been
conducted at hydrogen partial pressures from 10 to50Pa, relevant to reactor conditions.
Selectedthermal desorption spectra, taken from Li-Li+H
co-deposits, are shown in Fig. 3. The atomic ratio ofH/Li has been found to reach ~0.3, close to that of
H/C measured under the identical conditions in theLEAF-CAP setup in our recent work [5].

Fig. 3 TDS data taken from Li-Li+H codeposits.
References[1] E. I. Moses, J. Phys. Conf. Ser. 244(2010)012006.
[2] M. Dunne et al. Fusion Sci. & Technol. 60(2011)19-27.
[3]T. J. Renk et al., Fusion Eng. & Design 65 (2003) 399-406.
[4]Y. Hirooka et al., J. Phys. Conf. Ser. 244(2010) 032033.
[5]Y. Hirooka et al. Fusion Sci. & Technol. 60(2011)804-808.
[6]T. Kunugi et al., Fusion Eng. & Design 83(2008)1888.