24P046-P
Control of carbon agglomeration and hydrogen retention in low temperatureplasmas with H-C-N reactive molecular system[2]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[2]Aya Sasaki, Yuichiro Takai, Yoshihiko Uesugi and Yasunori Tanaka
佐々木彩, 高井裕一郎，上杉喜彦，田中康規
School of Electrical and Computer Engineering, Kanazawa University,Kakuma, Kanazawa, Ishikawa, 920-1192, Japan
金沢大学理工学域電子情報学類 〒920-1192 石川県金沢市角間町
Carbon dust particles and redeposited layer formed by plasma-surface interactions in fusion experimental
devices retain large amount of hydrogen isotopes. Nitrogen injection into hydrogen plasmas has been used
as one of the methods for suppression of carbon dust growth and reduction of tritium inventory. In thisreport, we have conducted experiments to investigate influences of nitrogen injection into H2/CH4 lowtemperature plasma. From optical emission spectroscopy, volatile CN band spectra were observed inH2/CH4/N2 mixture plasmas. From mass spectrometry of H2/CH4/N2 mixture plasmas, volatile CN andHCN spectra were observed, which are key particles for suppression of the carbon dust formation.

1. Introduction
Dust particles are generated by plasma surface
interactions, and contain large amount of tritiumthat is used as fuel gas in fusion reactors. Sincecarbon dusts retain large amount of hydrogen, thedust particles in fusion reactors bring several safety
problems, such as tritium inventory and diffusion at
an accidental air leakage. In addition, the dust
particles containing tritium are transported into
scrape off layer. The control of tritium inventory
and suppression of dust particle release is important
issues in future fusion reactors.
Nitrogen injection has been considered and tested
as one of the methods for suppression of carbon
dust growth and reduction of tritium inventory. Inthe experiments which are performed with Ar/H2/N2
plasma irradiation to graphite targets, using
high-power inductively coupled plasmas (ICPs), wegot the results that nitrogen injection into Ar/H2
plasma leads to the significant suppression ofcarbon dust formation in the number of carbon dust
particles[1,2].
In this paper, we have investigated effect ofnitrogen injection into H2/CH4 plasma in theexperiments using Heliotron-DR, which generates
low density and temperature, and pure H2 plasma insteady state condition. These conditions are near toremote plasma rather than ICPs’s one.
2. Experimental setup
Low temperature RF plasmas with H-C-N
reactive species are generated in hydrogen plasmas
with small amount of CH4 and N2 injection. H2 gas

flow rate is 20 sccm. CH4 and N2 gas flow rate isabout several sccm. The working gas pressure is~1Pa. The electron temperature is 5-10eV. The
electron density is (0.4-1.6)×1016m-3 at the plasmaedge. Four target stands for plasma irradiation areplaced at different troidal sections. A silicon targetof 10×10 mm2 is placed on a BN plate. ScanningElectron Microscope (SEM) is used to observe thegenerated dust particles on the targets. Energy
Dispersive X-ray spectrometer (EDS) is also used
to investigate the components of the dust anddeposited layer. A film thickness meter is used toinvestigate the film thickness on the targets. Surface
temperature of silicon is not actively controlled, but
can be measured by a thermocouple which is placed
behind. Mass spectrometry (QMS) and opticalemission spectroscopy are used to observe thereactive species in the plasma with H-C-N system.
3. Results and discussion
Figures 1(a), (b) and (c) show calculation resultsof particle composition under thermal equilibrium
plasmas. Figure 1(a) shows particle composition in
C-H system. Figures 1(b), (c) show particle
composition in C-H-N system. The composition ofcarbon containing particles widely changes between
T=1000-3000K whether N2 is injected or not. The
density of high order carbon molecules like C4, C5
are markedly decreases and the generation ofvolatile HCN molecule is remarkably enhanced asshown in Fig.1(c). These results show that atomic
nitrogen in H-C-N plasma system hinders C-C
combination necessary for nucleation in the carbon

agglomeration process. In addition, generation ofvolatile molecules like HCN, NH is expected tocontribute the removal of hydrogen isotope fromthe carbon deposit layer.

Radiation intensity [a.u.](a)C-H system

1E23

H2

1E21

CH4

1E19eH
C2H

C2H4+HC

C3

1E17
1E15

C5

1E13C4

1E111000

10000

Temperature [K](b)C-H-N system

1E23H21E21eHCH4HCC2H

1E17C2H4C4C5-3

1E23
1E21

N2NH3N+NHCNCNC2N
NH

1E17CN2(σ=2)

1E15
1E13

H2+CH4 plasma

Hα0.01

0.06
0.00
300

350

0.04

400

H2+CH4+N2 plasma
H2+CH4 plasma
Hβ0.02
0.00
300

400

500

600

700

10(c)C-H-N system (N-including species)

1E19

CN
0.02

NH

C3

1E13

0.03

0.08

1E15

1E11
1E25

Number density [m ]H 2+CH4+N2 plasma

0.10

Wavelength [nm]Fig.2. Optical emission spectra from H2/CH4 andH2/CH4/N2 plasma.

+1E190.04CH4NH28NH3H2+CH4+N2 plasma

C2H3HCN-11-3

Number density [m ]1E250.12Ion current [10 A]-3

Number density [m ]1E25

found that the dusts are mainly composed of carbon.
These results in preliminary experiments aredifferent from those in ICPs experiments shown in
24P045-P.

6CH3NH

4C2H4N2

H2+CH4 plasma

20C2N2C2H2CN

101520253035

Mass

1E111000

10000

Temperature [K]Fig.1 Calculation of particle composition in(a)C-H system (b)(c)C-H-N system

Following is the results of the experiments using
Heliotron-DR. The surface temperature of silicon in
the irradiation is Ts=320-370K. Figure 2 shows
optical emission spectra of H2/CH4 and H2/CH4/N2plasmas. In this case, gas flow rates of H2, CH4 andN2 are 20, 5, 2 sccm, respectively. Irradiation timeis 300 minutes. The molecular band spectra ofvolatile CN and NH radicals are clearly observed in
H2/CH4/N2 mixture plasmas.
Figures 3 shows mass spectra of H2/CH4 andH2/CH4/N2 mixture plasmas. In H2/CH4/N2 plasmas,mass spectra of volatile nitrogen particles like CN,
HCN and NH increase strongly compared to those
without nitrogen.
The carbon film thickness of silicon surface is60-400 nm after 300 minutes H2/CH4 plasma
irradiation. On the other hands, it is 240-800 nm
after H2/CH4/N2 plasma irradiation. In this case, thethickness of carbon film notably increases bynitrogen injection. From analysis by EDS, it isFig.3. Mass spectra from H2/CH4 and H2/CH4/N2plasma.

4. Conclusions
Mass spectra and optical emission spectra showthat generation of volatile molecules such as CN,
HCN and NH by nitrogen injection into H2/CH4
plasmas. These volatile molecules bring thesuppression of carbon dust formation in theexperiments using ICPs. In the first experimentusing Heliotron-DR, this suppression effect was
not seen. The difference between ICPs andHeliotron-DR experiments is the surface
temperature of targets besides working pressure
and so on. Carbon dust formation strongly depends
on the surface temperature[2]. In this firstexperiment,thesurface
temperatureisconsiderably low compared to ICPs’s one. In orderto confirm the suppression effect by nitrogen
injection, using Heliotron-DR, higher surface
temperature of targets may be needed.
References[1] M. Kyo et al: Plasma Fusion Res. 3, 2010.
[2] M. Kyo et al: 19th Int. Symp. on Plasma Chem.,
Bochum, Germany, 2009.