22C06
Recombining Plasma Production and Spatial Distribution of Neutrals in
DT-ALPHA Device
DT-ALPHA装置における再結合プラズマ生成のための中性粒子分布制御
Atsushi Okamoto, Tsuguhiro Nishiuchi, Hiroyuki Takahashi, Yusuke Kawamura,
Takahiro Kumagai, Akira Daibo, Sumio Kitajima and Mamiko Sasao
岡本 敦，西内嗣浩，高橋宏幸，川村悠祐，熊谷孝宏，大坊 昂，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
Aiming for compatibility of a constant neutral pressure in a plasma production region and a wide range ofthe pressure in a recombining test region, orifices and a differential pumping port are installed in a radio
frequency plasma source, DT-ALPHA. When the pressure at the test region changes from 2.6×10-3 Torr to
0.16 Torr, the pressure at the production region is kept within one order of a suitable operation range. The
electron temperature at the test region decreases to a few eV as the pressure at the test region increases.

1. Introduction
Since the divertor configuration is essential tomagnetic confinement fusion reactors, it isnecessary to reduce heat and particle fluxes on
the divertor plates using the detached plasma
operation[1]. In order to study compatibility ofthe volumetric recombining processes withenergetic ions caused by the edge localized
modes (ELMs), a divertor simulating experimenthas been started using a radio frequency (RF)plasma source and an ion beam[2].
While the RF plasma source has advantages incoexistence with energetic ion beams, it wasdifficult to sustain suitable working pressure in aplasma production region for a wide range ofneutral pressure in a recombining test region. In
this article, development of pressure control using
orifices is described. Spatial distribution of theneutral pressure is experimentally evaluated andcontrolled to match with the recombining plasma
experiments.
2. Experimental Setup
The experiments were performed in a linear
plasma device, originally designed as a ``Diagnostic
Tool Assisted by Linear Plasma device for Helium
Atom beam (DT-ALPHA)''[3]. A schematic of theexperimental setup is shown in Fig. 1. The vacuum
chamber consists of a quartz pipe (0.4 m in length,36 mm in diameter) coupling an antenna to aplasma at z = 0.3 m and a main chamber (1.1 m inlength, 63 mm in diameter) made of stainless steel,
where z is the distance from an aperture at theentrance of the quartz pipe. Ionization gauges andsingle/double Langmuir probes are installed at theupstream port (z = 0.7 m) and the downstream port(z = 1.3 m) to measure neutral pressure and electron
temperature, respectively. Helium gas is used as aworking gas for plasma production and is injectedfrom a tube adjacent to the aperture, whilesecondary gas for the recombining process study is
injected from the downstream port. A converging
axial magnetic field up to 0.2 T at the downstream
port is applied. A 13.56 MHz RF power supply isused for plasma production. The RF power is about
2 kW.
(a)

(b)

Fig.1. (a) Schematic of the experimental setup
and (b) photograph of a set of orifices installedin the DT-ALPHA device.

3. Neutral Pressure Control
In order to suppress pressure variation in theplasma production region due to the secondary gas
injection, three set of 10 orifices (z = 0.8-1.2 m) anda differential pumping port (z = 1.0 m) were
designed and installed in the DT-ALPHA device.
Each set of the orifices, inner diameter of which is20 mm, reduces a conductance of a 0.15 m lengthsection of the chamber from 0.4 m3/s to 0.01 m3/s inthe molecular flow condition.
For plasma production experiments, a pressure
variation was investigated. The working gas wasinjected at a constant flow rate, while the secondary
gas injection rate was varied. Relation of thepressure measured at the upstream port and that atthe downstream port is shown in Fig. 2. Variationof the upstream pressure, which is an index of thepressure in the plasma production region, issuccessfully suppressed within one order, while thedownstream pressure changes in a more than
two-order range.

Fig. 2. Upstream pressure as a function ofdownstream pressure. Filled circle represents
experimental results. Dashed lines indicate theoperation window suitable for plasma production.

4. Plasma Production with High Pressure TestRegion
Plasma parameters were measured for a constantgas injection from the upstream end-plate. Radialprofiles of the electron temperature at the upstream
port and the downstream port are almost uniform asshown in Fig. 3.
While the pressure at the downstream portchanges from 2.6×10-3 Torr to 0.16 Torr, theupstream pressure little changes as shown in Fig. 2.
The electron temperature at the upstream port isabout 5 eV and is almost unchanged against thedownstream pressure as shown in Fig. 3(a). On theother hand, the electron temperature at thedownstream port [Fig. 3(b)] decreases to a few eV

as the pressure increases. The result indicates thatthe higher collision frequency between the electron
and neutrals makes the lower electron temperature.
In plasmas such a few eV of electron temperature,
it is expected that the reaction rates of the electron
collision excitation and ionization become small
comparable with that of the volumetric recombining
processes. Spectroscopic studies as well as
evaluation of the particle flux along the magnetic
field remains our future work.

Fig. 3. Spatial distribution of electron temperaturefor different neutral pressures. Open square
represents p = 2.6×10-3 Torr at the downstream
port; filled circle, 0.16 Torr.

5. Summary
Three set of 10 orifices and a differential
pumping system are installed in the DT-ALPHA
device. Those enable us to keep the pressure at theplasma production region in a suitable range, when
the pressure at the recombining plasma test regionchanges from 2.6×10-3 Torr to 0.16 Torr. The
electron temperature at the test region decreases to afew eV as the pressure increasing, while that at theplasma production little changes as the pressure atthat region.
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] P. C. Stangeby: The Plasma Boundary of Magnetic
Fusion Devices (IoP publishing, Bristol, 2000), Chap.
16, p. 483.
[2] A. Okamoto, T. Isono, T. Kobuchi, S. Kitajima, andM. Sasao: J. Plasma Fusion Res. SERIES 8 (2009)39.
[3] A. Okamoto, T. Isono, T. Nishiuchi, H. Takahashi, S.
Kitajima, and M. Sasao: J. Plasma Fusion Res.
SERIES 9 (2010) 428.