22A04
High Density Plasma Production by a Particle Injection Electrode in TU-Heliac
東北大学ヘリアック装置における粒子注入型電極による高密度プラズマ生成実験	 
Mamoru Kanno, Kiyohiko Nishimura1, Keiichi Ishii, Yu Sato, Jo Tachibana, Shigeru Inagaki2,Masayuki Yokoyama1, Yasuhiro Suzuki1, Hiromi Takahashi1, Atsushi Okamoto, Sumio KItajima
and Mamiko Sasao
菅野	  守, 西村清彦1，石井啓一，佐藤	  優, 立花	  丈, 稲垣	  滋2, 横山雅之1, 鈴木康浩1, 	  	 高橋裕己1, 岡本	  敦, 北島純男, 笹尾眞實子
Department of Quantum Science and Energy Engineering, Tohoku University
6-6-01-2, Aza-Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan
1
National Institute for Fusion Science, Toki 509-5292, Japan
2
Research Institute for Applied Mechanics, Kyushu University, Fukuoka 816-8580, Japan東北大学大学院工学研究科量子エネルギー工学専攻 〒980-8579 宮城県仙台市青葉区荒巻字青葉6-6-01-2
1
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
2
九州大学応用力学研究所 〒816-8580 福岡県春日市春日公園6-1
In the Tohoku University Heliac (TU-Heliac), the high density plasma (~1019 m-3) was successfully
produced with a particle injection electrode biasing in Helium working gas. The threshold in the neutral
working gas pressure on the high density plasma production was also observed. The measurements of aradial profile were performed by the spectroscope with interference filters in virtue of successive highdensity shots.

1. Introduction
Stellarators have no hard limit of the highdensity, which is one of great advantages in fusion
reactors [1]. In the Tohoku University Heliac(TU-Heliac), which is one of stellarators, we havebeen trying to produce high performance plasmas.
With a hot cathode electrode biasing, we showed
the role of the radial electric field for the improved
confinement mode [2]. In addition, with the particle
injection electrode biasing, we produced the highdensity plasma (>1019 m-3) with working gas Argon[3]. In the present work, we changed the working
gas to Helium, which has the smaller sputtering
effect than Argon, and successfully produced thehigh electron density similar to that in case of
Argon and succeeded in successive high density
plasma productions.
2. Experimental Setup
The TU-Heliac is one of heliac devices (majorradius R = 0.48 m, average plasma radius a = 0.06
m and toroidal magnetic period n = 4). The
TU-Heliac consists of three sets of coils, and themagnetic field on axis B0 is 0.3 T. The vacuum
vessel was filled with a working gas before
discharges. The target plasma was produced by
Joule heating with an 18.8 kHz alternating current
in additional poloidal coils. The main diagnostic
system was a triple probe for measuring electron
density. In addition, we prepared the spectroscope
composed of three interference filters (centerwavelengths: 468.6 nm, 667.8 nm and 728.1 nm).
The experimental set-up of the particle
injection electrode is shown in Fig. 2. The electrode
was made of palladium coated with gold (Pd-Au).
Before discharges, the Pd-Au electrode wasimmersed in hydrogen gas (pin ~ 20 kPa) at room
temperature for about 18 hours in the upperchamber (conditioning chamber) separated from thevacuum vessel by the gate-valve. After theconditioning, the decrease of hydrogen gas pressure∆pin in the chamber was about 10 kPa.

Fig.1. The position of the particle injection
electrode and the computed cross-section of thevacuum magnetic flux surfaces at toroidal angle
φ = 0˚.

[10-2 Pa]

10gas_in86
not high density420Pd-Au(H charge)

LaB60210202.5-3468.6 nm
667.8 nm728.1 nm

102030-2667.8, 728.1

40Shot numberFig.2. The electron density in the series of thehigh density production experiments. The firstten shots of the series were discharges with thehot cathode (LaB6). From 11th shot, circle andtriangle symbols show the high density shots andfilled symbols show the low density shots.

Figure 3 shows the neutral gas pressure pgas_in
before discharges, which was the summation of theHelium working gas pressure and the Hydrogen gas
pressure desorbed from the Pd-Au electrode. In Fig.
2, it is clear that the neutral gas pressure increased
rapidly at the beginning of the discharge with thePd-Au electrode, and then decreased gradually. The
high density plasma was not produced in thecondition below pgas_in ~ 7×10-2 Pa. This resultshows the threshold in the neutral gas pressure on
the production of high density plasmas.
As described above, we succeeded in the highdensity plasma production enough to measure aradial profile of plasma. And we tried to measure
the density profile by the spectroscope withinterference filters. Figure 4 shows radial profiles ofthe emission intensities, which had a peak around ρ~ 0. The calculation of intensity ratio of theemissions from the neutral Helium (I(667.8nm)/I(728.1 nm)) with the Collisional Radiativemodel [4, 5] allows us to estimate an electron
density of plasma.

41

20.5

I

02

1.5

-1

Pd-Au(H charge)

LaB6

6

[10 V]

22468.6

not high density

4[10 V]

8high density

Ie40

106030

Fig.3. The neutral gas pressure through theseries.

8n [1018 m ]#81830~81859#81860~81895

high density

12

Shot number#81830~81859#81860~81895

1014p

3. Experimental results
The Pd-Au electrode inserted into the plasmaat ρ ~ 0.3 was biased negatively. In the experiment,
two series (shot number 81830 ~ 81859, 81860 ~81895) were performed. Figure 2 shows theelectron density ne measured by the triple probe at ρ~ 0.58. We successively produced the high density(~8×1018 m-3) plasma discharges over 20 times
similar to that in case of Argon. The density in caseof the Pd-Au electrode was about twice of that incase of the hot cathode electrode (LaB6).

0-1

Electrode

-0.5

00.5

10!
Fig.4. The radial profiles of the emission
intensities (center wavelengths: 468.6 nm,667.8 nm and 728.1 nm) measured by thespectroscope with interference filters.

4. Summary
We successfully produced the high density
plasma (~8×1018 m-3) discharges over 20 times withthe working gas Helium and observed the thresholdin the neutral gas pressure on the high density
plasma production. Utilizing successive highdensity shots, we measured radial profiles of the Heemission intensities by the spectroscope withinterference filters.
Acknowledgments
This work was supported by the LHD Joint
Planning Research programme at the National
Institute for Fusion Science.
References[1] A. Iiyoshi et al.: J. Plasma Fusion Res. 85 (2009)487[2] S. Kitajima et al.: Nucl. Fusion 46 (2006) 200[3] H. Utoh et al.: J. Plasma Fusion Res. SERIES 8(2008) 1052
[4] T. Fujimoto: J. Quant. Spectrosc. Radiat. Transfer 21(1979) 439[5] M. Goto: J. Quant. Spectrosc. Radiat. Transfer 76(2003) 331