24P045-P
Control of carbon agglomeration and hydrogen retention in low temperatureplasmas with H-C-N reactive molecular system[1]H-C-N反応性分子を有する低温プラズマ中における炭素凝集と水素吸蔵の制御[1]Yuichiro Takai, Aya Sasaki, Yoshihiko Uesugi, Yasunori Tanaka
高井裕一郎，佐々木 彩，上杉喜彦，田中康規
Division of Electrical Engineering and Computer Science, Kanazawa University, Kakuma, Kanazawa, Ishikawa
920-1192, Japan
金沢大学自然科学研究科電子情報工学専攻 〒920-1192 石川県金沢市角間町
So far, we have studied mechanisms of carbon dust formation using thermo-equiliblium inductively
coupled plasmas (ICPs). Experiments were performed with Ar/H2/N2 mixture plasmas. Addition of just afew percent of nitrogen gas to hydrogen plasmas led to significant suppression of carbon dust formation on
the graphite target. We have started a new experimental study on SOL plasma-wall interactions, such as
suppression of carbon agglomeration and hydrogen retention using steady state, non-equiliblium hydrogenplasmas in Heliotron-DR device. Here results of ICP experiments are reviewed and intial results RF plasma
production are shown.

1. Introduction
Carbon materials are used for plasma facing
components (PFCs) in present fusion experimental
devices because of its superiorly high thermal
properties. But divertor tiles made of CFC are
severely eroded by chemical and physicalsputtering of hydrogen. Consequently, carbon
redeposit layer and carbon dust particles aregenerated[1]. Carbon redeposit layer and carbon
dusts retain large amounts of hydrogen isotope in afusion device. This is a main reason why the carbon
materials will not used in DT burning ITER. The
suppression of tritium retention in the carbon
redeposit layer and dust growth is a critical issue forrevival of carbon materials as PFC in future fusion
reactors. So far we have studied the suppression
mechanism of carbon particle growth using
thermo-equiliblium ICPs[2]. It is found that thegrowth of carbon particles are strongly suppressedwith a small amount of nitrogen gas injection intohydrogen plasmas[3].
However, ICPs are thermo-equilibrium plasmas,
much different from the SOL and divertor plasmas
in fusion experiments. A new experimental study tocontrol the carbon agglomeration and hydrogen
retention in the carbon redeposit layer is started in
Heliotron-DR, where low temperature steady state
plasmas of H-C-N reactive molecular system aregenerated. This report describes a brief review ofthe dust experiments in ICP and the present status
of Heliotron-DR device for PSI experiments.
2. Experimental setup
Figure 1 shows a schematic diagram of ICP

irradiation system. The electron temperature is ~1eV. The working gas pressure is ~4 kPa. The atomic
and ionic hydrogen fluxes onto the graphite targetare ~1023 m-2s-1 and (2-8)×1019 m-2s-1, respectively.
Steady state RF plasmas in Heliotron-DR are
produced with a steady state helical magnetic fieldof 200 G. Closed magnetic surfaces generated by
the helical coils contribute to produce homogeneous
plasmas in the toroidal direction. Four antennas areplaced different toroidal sections and powered byindividual RF power source. The total RF power of
CW sources is 2.5 kW and a pulse RF source of 10
kW at 200 MHz with a pulse length of 10 ms is also
used. The working pressure is around ~ 1 Pa.
Electrostatic probes are used to measure electron
temperature and density. Mass spectrometry (QMS)
and optical emission spectroscopy are used toobserve the reactive species in the plasma withH-C-N system.

Fig. 1

Schematic diagram of ICPs irradiation system

4. Conclusions
Experiments in Ar/H2/N2 plasma irradiation tographite material show that nitrogen injection into
Ar/H2 plasmas leads to the significant suppressionof carbon dust formation with N2 injection ratio of0-1 %. A new experimental study to control thecarbon agglomeration and hydrogen retention in thecarbon redeposit layer have been started in
Heliotron-DR, where low temperature steady state
plasmas of H-C-N reactive molecular system aregenerated. These results in Heliotron-DR plasmas
irradiation are shown in 24P046-P at thisconference.

800

6H 2= 2 slpm
H 2= 3 slpm

(a)

600
4
400
22000(b)100

Average diameter (µm)

The weight loss and average numberdensity ofcarbon dust particles observed on the graphitetarget as a function of the surface temperature .

0H2= 3 slpm

3080

2 slpm2060
CN (388.3 nm)
NH (336 nm) (x 3)
C2 (516.5 nm) (x 3)
CH (431.4 nm) (x 3)4010Intensity ratioNumber density (mm-2)

Fig. 2

Weight loss (mg)

3. Results and discussion
Figure 2 shows the weight loss and average
number density of carbon dust particles observed on
the graphite target as a function of the surface
temperature in ICP experiments. Weight loss anddensity of carbon dust particles strongly depend on
the target temperature as shown in Fig. 2. Theseexperimental results indicate that the dust growthoccurs dominantly on the target surface. Figure 3
shows number density and average diameter ofcarbon dust particles observed on the target as afunction of nitrogen injection ratio, target weight
loss, and CH, C2, CN and NH emission intensity
normalized to Ar I emission. The addition of just afew percent of nitrogen gas to hydrogen plasmas
led to significant suppression of carbon dust
formation on the graphite target. From opticalemission spectroscopy, CN and NH band spectra
were observed strongly in Ar/H2/N2 plasmas. The
large increace of CN and NH emission intensity
was caused by the production of CN and NH bondformation in stead of CH and C-C bond formation.
Since CN and NH bond formation brings to form
volatile moleculars, such as HCN, NHx, etc., CNand NH bond formation might be a key suppression
mechanism of the carbon agglomeration. It issuggested that a small amount of nitrogen injection
is a possible method for tritium removal as well as
suppression of carbon dust formation in fusion
reactors using carbon PFC.
Electron temperature and density at the plasma
center are 4-10 eV and (3-5)×1016 m-3 at 2.5 kW
RF power, respectively. Electron temperature anddensity at the plasma edge are 5-10 eV and(0.4-1.5) × 1016 m-3 at 2.5 kW RF power,respectively. Hydrogen ion fluxes are (3-6)×1020
m-2s-1. These plasma features are very helpful tostudy the suppression mechanisms of carbon
agglomeration and hydrogen retention in the SOL
and remote plasmas in fusion devices.

200Fig. 302468[N2] / [H2] (%)

100

Number density and average diameter of carbon
dust particles observed on the target as a functionof nitrogen injection ratio. The lower figure
shows the variation of target weight loss and CH,
C2, CN and NH emission intensity normalized to
Ar I emission

References[1] J. Winter, G. Gebauer, J. Nucl. Mater. 226-229,228-233 (1999).
[2] Y. Tanaka et al., Appl. Phys. Lett., 89, 031501(2006).
[3] M. Kyo et al., Plasma Fusion Res. 5, 004 (2010)