24C04
Steady State Production of Recombining Plasma in DT-ALPHA
直線型RFプラズマ源DT-ALPHAにおける再結合プラズマの定常生成実験
Hiroyuki Takahashi, Atsushi Okamoto, Yusuke Kawamura, Takahiro Kumagai, Akira Daibo,
Tsuguhiro Nishiuchi, Sumio Kitajima and Mamiko Sasao
高橋宏幸, 岡本 敦，川村悠祐，熊谷孝宏，大坊 昂，西内嗣浩，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 仙台市青葉区荒巻字青葉6-6-01-2
Steady state gas-puffing experiment is carried out in a radio frequency plasma source, DT-ALPHA. To
prevent secondary gas from affecting the plasma production, three orifices are installed. The neutralpressure at a test region is changed from 0.3 Pa to 21 Pa. In the pressure below p ≤ 3 Pa, the electron
density increases due to the electron collision ionization. In the high pressure regime, both the electron
temperature and density decreases as the neutral pressure increasing. Those results are obtained in a steadystate plasma, which suggests that the experimental device is suitable for long time-exposing spectroscopyof recombining plasma study and for transient beam-injection experiments.

1. Introduction
Detachment operation of the divertor plasmausing volumetric recombining processes isrequired for steady state fusion reactors. Whilefundamentals of the volumetric recombining
plasma has been understood using many divertor
plasma simulators [1-3], what should be studied
is sustainability of steady state detached plasmas
against transient heat and particle fluxes. Since a
beam component injection penetrating through aplasma source is a suitable method for such a
study, we have started to produce a steady staterecombining plasma using a radio frequency (RF)plasma source instead of the direct current (DC)arc discharge sources.
In our previous experiments [4], a sudden dropof the ion saturation current was observed when asecondary gas was puffed into a test region.
However, the phenomenon was transient due to aleakage of the secondary gas to a plasma
production region. In order to sustain a steadystate high neutral-pressure environment in the test
region, orifices have been installed in theexperimental device. In this paper, experimental
results of a steady state gas puffing operation arepresented in terms of a pressure dependence and aradial profile.
2. Experimental setup
Experiments are performed in a linear RFdivertor plasma simulator, DT-ALPHA [5]. A
schematic of the DT-ALPHA device is shown inFig. 1. Total length of the device is about twometers. Vacuum vessel consists of a stainless steel(SUS) chamber and a quartz pipe. There are 10

coils to produce a magnetic field. The magnetic
field strength is up to 0.2 T. Helium working gas issupplied into the DT-ALPHA device throughupstream end-plate, flow rate of which is controlledwith a mass flow controller. In addition, helium
secondary gas is supplied in a test region for highpressure experiments. To prevent secondary gas
from affecting the plasma production, three orifices
made of SUS are installed in the DT-ALPHA
device, of which dimensions are 20 mm in diameterand 110 mm in length. Three turbo molecular
pumps are equipped. Two of them are placed at theboth end of the device and one is placed between
the orifices. To produce a plasma, a 13.56 MHz RF
oscillator, whose maximum power is 3 KW, is used.
Measurements of the plasma parameters wereperformed using the double probe method at the test
region as shown in Fig. 1.

Fig. 1. A schematic of the DT-ALPHA device.

3. Experimental results
Experiments using helium plasma with helium
secondary gas puffing were performed. The
pressure dependence of the electron temperatureand density measured at the test region are shown inFig. 2. The horizontal axis represents the neutralpressure measured at the test region. The electron
temperature monotonically decreases from 12 eV to2 eV when the pressure increases from 0.3 Pa to 21
Pa. On the other hand, the electron density
increases from 2.9 × 1016 m-3 to 3.1 × 1017 m-3 in alow pressure region then it decrease to 5.5 × 1016
m-3 as the pressure increases. In the low pressure
region below p ≤ 3 Pa, the increasing electron
density is considered as a result from the electron
collision ionization in the test region. Since thevariation of the electron temperature is small, theionization frequency is proportional to the neutral
density. In the high pressure region over p > 3 Pa,
possible processes of the decreasing electronpressure are radiation and diffusion losses. Detailed
investigation
includingaspectroscopic
measurement is on going.

Fig.2. Neutral pressure dependence of the electron
temperature(a) and density(b) obtained at the test
region, r = 0 mm.

Figure 3 shows radial profiles of the electron
temperature and density measured at the same axial
position as those shown in Fig. 2. Closed and open
squares are obtained in the case of p = 21 Pa and p= 0.3 Pa at the test region, respectively. The profileof the electron temperature is unchanged against theneutral pressure variation. A ratio of the twoelectron temperature profile is about 5 throughout
the plasma column. On the other hand, it is revealed
that the profile of the electron density changes
between these two pressures. In the high pressure
operation, the peak position of the profile somewhat
shifts to about 3 mm downward (r = -3 mm). A
plateau is also observed at r = 0 - 5 mm.

Fig.3. Radial profiles of the electron temperature(a)and density(b) with (closed squares) and without(open squares) gas puffing obtained at the test region.

4. Summary
Steady state gas-puffing experiment is carried out
in the DT-ALPHA device. Changing the neutralpressure at a test region from 0.3 Pa to 21 Pa,
electron temperature and density is measured. In thepressure below p ≤ 3 Pa, the electron density
increases due to the electron collision ionization. In
the high pressure region, both the electron
temperature and density decrease as the neutralpressure increasing. Those results are obtained in a
steady state plasma, which suggests that theexperimental device is suitable for longtime-exposing spectroscopy of the recombining
plasma study and for transient beam-injection
experiments.
Acknowledgments
This work was partly supported by the Ministryof Education, Culture, Sports, Science andTechnology of Japan (MEXT) Grant-in-Aid forYoung Scientists (B), 22740357, 2011, and that for
Challenging Exploratory Research, 23656585,
2011.
References[1] N. Ohno, D. Nishijima, S. Takamura, Y. Uesugi,
M. Motoyama, N. Hattori, H. Arakawa, N. Ezumi,S. Krasheninnikov, A. Pigarov and U. Wenzel:
Nuclear Fusion 41 (2001) 1055.
[2] K. J. Gibson, P. K. Browning, B. Mihaljcic,
D. A. Forder and J. Hugill: J. Nucl. Mater. 313-316(2003) 1253.
[3] A. Tonegawa, M. Ono, Y. Morihira, H. Ogawa,T. Shibuya, K. Kawamura and K. Takayama: J. Nucl.
Mater. 313-316 (2003) 1046.
[4] T. Nishiuchi, A. Okamoto, T. Isono, H. Takahashi,
S. Kitajima and M. Sasao: Meeting Abst. Phys. Soc.
Jpn. Volume 65, issue 2, P6.2, p. 199 (2010).
[5] A. Okamoto, K. Iwazaki, T. Isono, T. Kobuchi,
S. Kitajima and M. Sasao: Plasma and Fusion Res. 3(2008) 59.