24P094-O
Collection of Carbon Dust Particles Formed Due to Plasma-Wall Interactions
Between High Density H2 Plasmas and Carbon Wall onto Substrates
by Applying Local DC Bias Voltage
高密度水素プラズマとカーボン壁間のプラズマ・壁相互作用により形成されるカーボンダスト粒子の局所バイアス電圧印加による捕集
Katsushi Nishiyama1, Yasuhiko Morita1, Daisuke Yamashita1, Kunihiro Kamataki1,
Giichiro Uchida1, Hyunwoong Seo1, Naho Itagaki1,2, Kazunori Koga1, Masaharu Shiratani1,
Naoko Ashikawa3, Suguru Masuzaki3, Kiyohiko Nishimura3, Akio Sagara3,
the LHD Experimental Group3, S. Bornholdt4, and H. Kersten4
西山雄士1，森田康彦1，山下大輔1，鎌滝晋礼1，内田儀一郎1，徐	  鉉雄1，板垣奈穂1,2，古閑一憲1，白谷正治1
芦川直子3，増崎貴3，西村清彦3，相良明男3，LHD 実験グループ3
S. Bornholdt4, and H. Kersten4
1

2

Kyushu University, Fukuoka, 819-0395, Japan
PRESTO, Japan Science and Technology Agency, Tokyo, 102-0075, Japan
3
National Institute for Fusion Science, Gifu, 509-5292, Japan
4
Institute of Experimental and Applied Physics, CAU Kiel
1
九州大学	  〒819-0395	  福岡市西区大字元岡744
2
JSTさきがけ	  〒102-0075	  東京都千代田区三番町53
核融合科学研究所	  〒509-5292	  岐阜県土岐市下石町322-6
4
Institute of Experimental and Applied Physics, CAU Kiel

We collected dust particles generated by interaction between a plasma and a carbon target in helicondischarge reactor, by which diverter plasmas in fusion devices are simulated. The dust particles are classified
into three kinds: small spherical particles, agglomerates, and large flakes, suggesting three formation
mechanisms: chemical vapor deposition (CVD) growth, agglomeration, and peeling from redeposited layeron the reactor wall. The area density and flux towards substrates of dust particles exponentially increase withthe substrate bias voltage. The energy influx of plasmas towards the target and ion density were measured toclarify the dust formation mechanisms. They are nearly constant irrespective of the substrate bias voltage
suggesting that the production rate of dust particles does not depend on the substrate bias voltage and theircollection efficiency does.

1. Introduction
Dust particles are formed in fusion devices due
to plasma-surface interactions. Dust particles pose
potential problems for the safety of future fusion
devices, as they can accumulate tritium and may
explode violently, and they have health andenvironmental risks of spreading of dust particles
that contain tritium. Dust particles are also known
that they lead to deterioration of plasma
confinement in future fusion devices [1]. We havefound large amount of carbon nanoparticles formed
due to interactions between hydrogen (H2) plasmas
and carbon walls in the large helical device (LHD)and in our helicon discharge reactor [2, 3]. Here we
describe experimental results of collection of dust
particles on substrates by applying dc bias voltages
to the substrates and measurement results of energy
influx towards a target using a calorimetric probe in

the helicon discharge reactor.
2. Experimental
Experiments were carried out with a helicondischarge reactor as shown in Figure 1. Dust
particles formed due to interaction between H2
plasmas and a graphite target. Gas of H2 wassupplied to the reactor and the pressure was 5Fig. 1. Experimental setup.

3. Results and Discussion
Dust particles collected are classified into threekinds: small spherical particles, agglomerates, andlarge
flakes,suggesting
threeformation
mechanisms: CVD growth, agglomeration, andpeeling off from wall. The size ranges of dust
particles are below 1 µm for small dust particles,
100 nm to a few µm for agglomerates and over 1µm for large flakes. These features are similar to
those of dust particles collected in LHD [2, 3].
Figure 2 shows substrate bias voltage
dependence of area density and flux towards
substrates of dust particles in the helicon discharge
reactor. The area density and flux of sphericalparticles exponentially increase with the substratebias voltage, while those of agglomerates and flakes
are almost constant. These results imply that theflux of dust particles can be controlled by thesubstrate bias voltage.
Figure 3 shows influence of substrate bias
voltage on the energy influx of H2 plasma measured
with the calorimetric probe. The value of energy
influx is approximately from 10 to 12 W/cm2. Theenergy influx is nearly constant irrespective of the9108107107106105-2 -1

10sphericalagglomerate
flakeall

20 40flux (m s )area density (m-2)

10V - Vs (V)bias-100-80-60-40 -20 0
10-80 -60 -40 -20 0 20 40 60 80
V (V)biasFig. 2. Substrate bias voltage dependence of area
density and flux of dust particles.

1210energy influx (W/cm2)

mTorr. H2 plasmas were generated by applying 1.1
kW, 13.56 MHz pulsed RF voltage to a heliconantenna. The graphite target was located at thecenter of the discharges. These particles werecollected on Si substrates which were set on aholder located at 111 mm below the target. A dc
bias voltage was applied to some of the Si
substrates during the collection. The total
discharging period was 600 s for the collection. The
plasma parameters were measured with a Langmuirprobe, and energy influx were measured with acalorimetric probe [4]. Then, size and shape of dust
particles were obtained with a SEM.

864
20bias offbias on

12
3
4measurement No.

5Fig. 3. Influence of substrate bias voltage on energy
influx towards the target.

substrate bias voltage. This result is consistent withthe results of ion density measured using aLangmuir probe. The substrate bias voltage,
therefore, has little effects on plasma parametersand hence the production rate of dust particles.
Hence, the collection efficiency of spherical dust
particles exponentially increases with the substratebias voltage.
4. Conclusions
We collected dust particles in the helicondischarge chamber. There are three kinds of dust
particles: small spherical particles, large flakes, andagglomerates. They are formed by three formation
mechanisms: CVD growth, peeling off from wall,
and agglomeration. The flux of dust particles
towards substrates exponentially increases with thesubstrate bias voltage. These results suggest that theflux of dust particles can be controlled by substratebias voltage. The substrate bias voltage has little
effects on the energy influx to the substrate.
Acknowledgments
This research was supported in part by a General
Coordinated Research Grant from the National
Institute for Fusion Science and by JSPS.
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Ashikawa, K. Nishimura, A. Sagara, and the LHD
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