22P051-P
Formation and confinement properties of repetitively generated magnetizedplasmoids by an IGBT inverter control
IGBTインバータによる連続生成磁化プラズマの生成・保持特性	 
Hirotomo Itagaki1, Midori Yamazaki2, Yuki Fujita2, Mikio Takatsu2, Haruhisa Koguchi3,Michiaki Inomoto1 and Tomohiko Asai2
板垣宏知1, 山崎翠2，藤田侑希2，高津幹夫2，小口治久3，井通暁1，浅井朋彦21) The university of Tokyo Graduate school of frontier science
5-1-5, Kashiwanoha, Kashiwa city, Chiba 277-8561, Japan2) College of Science & Technology, Nihon university
1-8-14, kandasurugadai, Chiyoda-ku, Tokyo 101-8308, Japan3) Innovative Plasma Technologies Group, Energy Technology Research Institute, National Institute of Advanced
Industrial Science and Technology (AIST), Tsukuba Central 2, Umezono 1-1-1, Tsukuba 305-8568, Japan1) 東京大学大学院新領域創成科学研究科 〒277-8561	  千葉県柏市柏の葉5-1-52) 日本大学大学院理工学研究科	  〒101-8308	  東京都千代田区神田駿河台1-8-143) 産業技術総合研究所エネルギー技術研究部門先進プラズマ技術グループ〒305-8568	  つくば市梅園1-1-1、つくば中央２
A power inverter with IGBTs was developed and has been applied in a mini-magnetized plasma gun
experiment. A magnetized plasmoid was generated by M2PG with 10 kHz of discharge frequency. Then
repetitively generated plasmoid are ejected into metal chamber as a flux conserver. In the repetitive
discharge case, an electron temperature up to 140 eV and an electron density up to 2×1019 m-3 weremeasured. In contrast with repetitive discharge case, an electron temperature and density is up to 50 eV and1×1019 m-3, respectively. These results indicate that the formation and confinement of plasmoid byrepetitive discharge are improved compared to single discharge case.

1. Introduction
In this work, we have developed a power
inverter to control electrode discharge current,and it has been applied in a small sizedmagnetized coaxial plasma gun (MCPG) device.
A generated plasmoid by a MCPG can easily beaccelerated to the range of the Alfven speed and has
significant amounts of thermal and kinetic energy,
particles and magnetic helicity [1]. Therefore a
MCPG has been utilized in nuclear fusion research.
Repetitive operation of a MCPG is required to
inject significant amounts of helicity, particles andenergy into the fusion core of plasma. Also, therepetitive operation produces a continuous plasma
source, for a UV light source for example [2].
However, conventional high-power fast switches(e.g., ignitron and spark gap) are not adequate forrepetitive operation. Therefore, we have developed
a power inverter with IGBTs to control thedischarge current on a MCPG and have evaluatedformation and confinement characteristics ofgenerated plasmoid by MCPG with repetitivelygeneration.
2. Experimental device
An experimental device has been consisted apower inverter with IGBTs, a MCPG, and a metal

Fig.1. A Schematic view of M2PG-device.

chamber as a flux conserver. Figure 1 shows atypical diagram of the experimental device.
2.1. Mini-magnetized plasma gun (M2PG)
In this work, magnetized plasmoid has been
generated on a small size MCPG named
Mini-magnetized plasma gun (M2PG). The M2PG
has a simple configuration consisting of inner andouter electrodes and a magnetized coil as shown in
Fig.1. The outer electrode has an 8 mm of I.D., andthe inner electrode has a 3 mm of O.D. Theseelectrodes are insulated by a spacer flange made ofacetal copolymer (Duracon).

Fig.2. Typical discharge current waveforms of (a)repetitive discharge case and (b) single dischargecase.

2.2. Power inverter with IGBTs
A power inverter using IGBTs consists of apower supply, IGBTs (Nihon Inter Electronics
Corporation, PHMB600BS12), snubber circuit, and acurrent limited resister. In this work, the power
inverter was connected to a capacitor of 640 µF
which is charged by DC power supply.
3. Experimental Result
3.1. Typical discharge waveforms
Typical waveforms of discharge current areshown in Fig. 2. A discharge current was measuredby a Rogowski-coil. A series of repetitive discharge
was operated with 10kHz of discharge frequency
and 50% of duty-cycle. Duty-cycle indicates a ratioof pulse width to a cycle of the dischargefrequency.
3.2. Properties of repetitively generated plasmoid
In this study, we have measured a line spectrum
emission of HeI (471.3 nm), electron temperatureand density as shown in Fig. 3. A thin (red) line
indicates the case of repetitive discharge (Case 1)
and a bold (blue) line indicates the case of single
discharge (Case 2). The emission of HeI wasmeasured by optical measurement system
consisting of a collimator and a photo multi pliertube with interference-filter (470±5 nm). Figure 3(a) shows a time evolution of emissions of HeI. In
the Case-1, intensity of HeI emission is decreased
compared to the Case-2.
Figure 3 (b) and (c) show an electron temperature
and an electron density, respectively. The electron
temperature and density were measured by triple

Fig.3. Time evolutions of (a) emission of HeI,(b) electron temperature and (c) electron density.

Langmuir probe. The triple probe measured at 18mm from a center of flux conserver. In the Case-1,
an increasing of electron temperature and density
have been observed as shown in Fig. 3 (b) and (c),and at the pulse-peak of an electron temperature anddensity in the Case-1 are increased as compared toCase-2.
These results are considered an effect ofrepetitive discharge. In the case of repetitive
discharge, collision between two ejectedmagnetized plasmoid is occurred. Additionally,increasing of an electron temperature has beenmeasured at the pulse peaks of repetitive discharge.
Therefore it seems that there is possibility ofplasma heating via magnetic reconnection event.
4. Summary
In the repetitive discharge case, we haveobserved an increasing of an electron temperatureand density and a decreasing of line spectrum
emission of HeI compared to single discharge case.
This is considered that plasma confinement
property is improved with repetitive discharge.
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