24P157-B
Intermittent Electron Flux in an ECR Plasma (I) : 2D Profile Measurement
ECRプラズマ中の間欠的電子流束(I)：二次元分布計測
Yoshiyuki Kato1, Kenichiro Terasaka1, Kazuya Furuta2, Shinji Yoshimura3, Mitsutoshi Aramaki2,Tomohiro Morisaki3 and Masayoshi Y. Tanaka1
加藤祥行1, 寺坂健一郎1，古田和也2，吉村信次3，荒巻光利4，森崎友宏3，田中雅慶1
1

Interdis. Grad. Sch. Eng. Sci., Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan
九州大学総合理工学府 〒816-8580 福岡県春日市春日公園6-1
2
Grad. Sch. Sci., Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan
名古屋大学理学研究科 〒464-8602 名古屋市千種不老町
3National Institute for Fusion Science, 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japan
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
4
Dept. Elec. Eng. Comp. Sci., Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
名古屋大学工学研究科 〒464-8603 名古屋市千種区不老町

Spontaneous generation of intermittent high-energy electron fluxes has been observed in a linear
electron-cyclotron-resonance (ECR) plasma. In order to investigate the spatial distribution of electron
flux and the spatial point of occurrence, we developed a wire-grid probe with independent 16electrodes. The wire-grid probe successfully reconstructed two-dimensional distribution of thehigh-energy electrons. It was found that the high-energy electron fluxes have a typical size of 20-50
mm in diameter and are generated at various spatial positions in a seemingly random manner.

1. Introduction
Intermittent phenomena are ubiquitous both inlaboratory and in astrophysical environment.
Intermittent particle transport called “blob” hasbeen observed in many magnetically confinedplasmas [1] and intermittent explosive energy
release takes places in the solar flare events [2].
Recently, spontaneous generation of intermittent
high-energy electron fluxes, or electron currents,
has been observed in a linear electron-cyclotronresonance (ECR) plasma. The electron fluxes havea typical duration of 10 μs and are generated on thewhole plasma cross-section. Because of intrinsic
intermittency of the phenomenon, measurementswith a few probes do not provide valuable
information on the spatial distribution of theelectron flux. To obtain the definitive information,
we have developed a wire-grid probe (WGP)
capableofreconstructingtwo-dimensional
distribution of the high-energy electrons, anddetermined the points of occurrence.

are coincidently observed with two probes andother spikes are detected by a single probe. This
result is attributable to the fact that the high-energy
electrons have finite spatial extent, and when theyFig.1. Floating potential fluctuations measured by
two LPs located at x=0 mm and x=20 mm.

2. Necessity of Two-dimensional Measurement
We utilize the floating potential (Vf) as a signalof existence of the intermittent high-energy electron
flux, since the existence of high-energy electrons
strongly changes Vf of the probe. Figure 1 shows
the simultaneous measurement of Vf fluctuations
with two Langmuir probes located at 20 mm aparteach other. As seen in the figure, some of the spikes
Fig.2. TheWGP in the HYPER-I device.

,×,(V)

Fig.3. Two-dimensional profiles (intensity maps) of the intermittent high-energy electron fluxes.

flow into the two probes, the coincident observationof negative spikes happens. The detection by asingle probe indicates that the electron fluxes aregenerated in various positions. These experimental
facts strongly suggest that two-dimensional
measurement of high-energy electrons is needed.
3. Experimental Setup
The experiment has been carried out using thehigh density plasma experiment (HYPER-I) device[3] at the National Institute for Fusion Science.
HYPER-I is a cylindrical plasma device with 0.3 m
in inner diameter and 2.0 m in axial length. A
helium plasma is generated by ECR heating with a2.45 GHz microwave. The input microwave power
and the gas pressure were fixed at 20 kW and 1.5mTorr, respectively. The typical electron density
and temperature are 1017 m-3 and 10 eV,
respectively.
The WGP consists of eight horizontal and eight
vertical electrically insulated tungsten wires with adiameter of 0.7 mm and has 64 lattice points, as
shown in Fig. 2. Each wire is separated from theadjacent wire with the distance of 10 mm. The
WGP measures 16 time-series of Vf fluctuation toreconstruct the intensity map of the electron flux
with 64 lattice points.
4. Experimental Results
Floating potential is proportional to the electron
temperature and has positive correlation to theimbalance between the ion and electron flux [4].
Therefore, the intensities of electron fluxes at 64
lattice points may be evaluated asΓ, ∝Thequantity, ×, ,, ×,(1)
hasapositive

correlation to the variation of the electron fluxes
and the symbols i ( i = 1, 2, ⋯ , 8 ) and j ( j =1, 2, ⋯ , 8 ) indicate the electrode numbers ofhorizontal and vertical wires, respectively.
The two-dimensional profiles (intensity maps) of
the intermittent high-energy electron fluxes areshown in Fig. 3, where the origin ( , ) = (0, 0)
corresponds to the center of the device. The
reconstructed profiles of the electron fluxes exhibit
finite spatial extent with a circular cross-section.
From 80 samples of such profiles, it is found thatthe electron fluxes have a typical size of 20-50 mm
in diameter. In addition, the fluxes are generated atvarious positions with various intensities in aseemingly random manner.
In conclusion, the newly developed WGP
successfully reconstructed two-dimensional profilesof the electron fluxes and determined the points ofoccurrence. The random nature of the phenomenon
requires a statistical approach to investigate theentire picture of this event.
References[1] J. A. Boedo et al.: Phys. Plasmas. 8 (2011) 4826.
[2] J. D. Fix: Astronomy, Journey to the Cosmic Frontier(McGraw-Hill, New York, 2011), 2nd ed.
[3] M. Y. Tanaka et al.: Rev. Sci. Instrum. 69 (1998)
980.
[4] M. Cercek et al.,: Contrib. Plasma Phys. 39 (1999)
541.