22P122-P
Measurement of Plasmoid Characteristics using Interferometer
干渉計によるプラズモイドの特性評価
Tetsuya Uchiyama1, Tsuyoshi Akiyama2, Takuya Nihashi1, Shin Kajita3, Noriyasu Ohno1,Yusuke Kikuchi4, Naoyuki Fukumoto4, Masayosi Nagata4
内山徹也1, 秋山毅志2, 二橋卓哉1、梶田信3, 大野哲靖1, 菊池祐介4, 福本直之4, 永田正義4
1

Graduate School of Engineering, Nagoya University, Nagoya 464-8603,
National Institute for Fusion Science, 322-6 Oroshi, Toki, Gifu 509-5292, Japan
3
EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan4
Graduate School of Engineering, University of Hyogo, Hyogo 671-2280, Japan

21

名古屋大学工学研究科エネルギー理工学専攻 〒464-8603 名古屋市千種区不老町
2
自然科学研究機構 核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
3名古屋大学エコトピア科学研究所 〒464-8603 名古屋市千種区不老町4
兵庫県立大学大学院工学研究科電気系工学専攻 〒671-2280 兵庫県姫路市書写2167
The electron density of a plasmoid produced in a developed magnetized coaxial plasma gun device, whicha linear plasma device is equipped with, is measured with an interferometer using a Zeeman laser at awavelength of 633 nm. The line averaged electron density is estimated to be 1-2×1021 m-3 from themeasurement.

1. Introduction
Control of transient heat and particle loads due toELMs and its impact to divertor materials will be an
important issue in future fusion devices. To
simulate the harsh condition, extensive experiments
have been conducted by using a laser [1], anelectron beam [2], and a plasma gun [3]. It has been
pointed out that simultaneous irradiation of aplasma and transient loads can lead to a synergistic
effect that enhances the surface damages [4,5].
We are developing a plasma gun device
combined to a steady-state linear plasma device. In
this study, characteristics of a plasmoid producedby the plasma gun device were evaluated with aninterferometer.
2. Experimental setup
Figure 1 is a picture of the magnetized coaxial
plasma gun device developed in Nagoya University.
A compact torus plasma with poloidal and toroidal
currents is produced and is ejected in the direction

Figure 1: A picture of the developed plasma gun device
in Nagoya University.

of the arrow in Fig.1. The ejection of the plasma
can be controlled with an external bias field coil. Atungsten coated material is used for an internal
electrode in the gun to reduce the release ofmaterial from the electrode during the discharge.
The gun is equipped with a magnetic probe andviewing ports for the interferometer (not shown inFig.1) at the end part of the chamber.
Figure 2 is a schematic of the optical system of
the heterodyne interferometer (actually the probe
beam goes back and forth in a plasma). A He-Ne
Zeeman laser, which oscillates orthogonallinearly-polarized components with a frequency
difference Δω of 208 kHz, is used as a light source.
The Zeeman laser enables to build a heterodyne
interferometer with a simple optical system. Thelaser beam is split into two: probe and local beams.
Polarizers with orthogonal transmission directions

Figure 2: A schematic of the optical system of theHe-Ne Zeeman laser interferometer.

4-1

2-2-3

Magnetic Flux

Digital Demodulation

00.7

0.8

0.9
Time (ms)11.1

Figure 3: The temporal evolutions of the electron
density measured with interferometer and the signal of amagnetic probe. The density is evaluated two methods:
the analog phase counter and the digital demodulation
technique.

at each chord select one of polarization components.
Following polarizers whose transmission directions
are 45 deg. against orthogonal linear polarizations,interference signals with a beat frequency of Δω aredetected with photodetectors. An analog phasecounter [6] or the digital demodulation technique[7] are used for extraction of the phase shift.
3. Experimental Results
Figure 3 shows the temporal evolutions of theelectron density and the magnetic flux measured
with the interferometer and a magnetic probe,respectively. The reason of the smoothed signal
with a delay time of about 50 μs of the density
evaluated with the phase counter is expected to be
an insufficient time response of the phase counter.
It will be replaced with the one with a time
response of 5 μs. Supposing that the plasma
diameter is almost the same as the inner diameter ofthe chamber (~5 cm), the both density shows thatthe maximum electron density reaches 1.5×1021 m-3.
Some of variations of the density signal correspondto these of the magnetic flux. Hence these density
variations are possible to reflect the structure of theplasmoid. Drifts of the zero line before and after thedischarge are caused by mechanical vibrations. The
frequency of a small oscillation during wholemeasurement time is the same as that of the beatfrequency. When the beat signals have DC offsets,such an oscillation is superimposed on the actual
density signal in the digital demodulation
technique.
To confirm the validity of the measurement,
the density measurement was conducted withoutejecting the plasmoid to the measurement region.
Figure 4 shows the measurement results of different
bias voltages applied to the external bias field coil.
The optimum voltage for the plasmoid ejection is0.3 kV and the further voltage prevents the42200-24

Magnetic flux (a.u.)

0Bias Voltage: 0.3 kV 6-2
Bias Voltage: 0.4 kV 642200-24

Magnetic flux (a.u.)

64-2
Bias Voltage: 1.5 kV 642200-2

Magnetic flux (a.u.)Phase Counter1

Averaged Density (×1021 m-3)Averaged Density (×1021 m-3) Averaged Density (×1021 m-3)

8L=0.05 (m)×2

Magnetic Flux (a.u.)

-3m )

212

Averaged Density (×103-2
0.6

0.8Time (ms)1Figure 4: The temporal evolution of the electron
density measure with interferometer and the signal of amagnetic probe.

plasmoid from being ejected because the plasmoid
is confined by the strong magnetic field around thebirth place. The evaluated density decreases similar
to the reduction of the magnetic flux in the case of abias voltage of 0.4 kV. Applying a voltage of 1.5
kV, the plasmoid completely disappears from theline of sight of the interferometer. This indicatesthat the measured density is definitely originatedfrom the plasma formation and its dependence on
the bias voltage is reasonable.
4. Summary
The magnetized coaxial plasma gun device tostudy the synergistic effect of simultaneous
irradiation of a plasma and a transient heat load on
divertor materials is being developed. The Zeeman
laser interferometer is installed and the electron
density is found to be 1-2×1021 m-3.
References[1] S. Kajita, S. Takamura and N. Ohno, Nucl. Fusion,49 (2009) 032002.
[2] D. Nishijima, Y. Kikuchi, M. Nakatsuka, M.
Baldwin, R. Doerner, M. Nagata and Y. Ueda
Fusion Sci. Technol. 60 (2011) 1447.
[3] I.E. Garkusha, et al., Journal of Nuclear Materials
337–339 (2005) 707–711.
[4] S. Kajita et al. J. Appl. Phys. 100, (2006) 103304.
[5] S. Kajita et al., Appl. Phys. Lett. 91 (2007)
261501.
[6] Y. Ito et al., Fusion Engineering and Design 56–57,965–968 (2001).
[7] Y. Jiang, D. L. Brower, L. Zeng, and J. Howard,
Rev. Sci. Instrum. 68, 902 (1997).