23P065-P
Magnetic helicity injection into a field-reversed configuration
by a magnetized coaxial plasma gun.
磁化同軸ガンを用いたFRCプラズマへの磁気ヘリシティ注入	 
Hiroyuki Matsunaga1, Atsushi Takase1, Hirotomo Itagaki2, Tsutomu Takahashi1, L.C.Steinhauer3,Tomohiko Asai1
松永宏幸1, 高瀬敦司1，板垣宏知2，高橋努1, L.C.Steinhauer3, 浅井朋彦11) College of Science ＆ Technology, Nihon University
1-8-14, Kandasurugadai, Chiyoda-ku, Tokyo 101-8308, Japan.
2) Department of Complexity Science and Engineering, Graduate school of Frontier Sciences The University of Tokyo
5-1-5,Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan.
3) Department of Aeronautics ＆ Astronautics, University of Washington
211 Guggenheim Hall, Seattle, WA 98195-2400. USA.
1)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1-8-14
2)東京大学大学院 新領域創成科学研究科複雑理工学専攻 〒277- 8561 千葉県柏市柏の葉5-1-53)ワシントン大学 航空宇宙工学科 98195	  シアトル.	  A coaxial helicity injection (CHI) onto a field-reversed configuration (FRC) has been applied. As one of the effect ofCHI, delayed onset time of toroidal deformation with a mode number n = 2. Based on the theoretical prediction that themodest amount of toroidal magnetic field could stabilize the rotational instability [Milroy, R.D and L.C. Steinhauer:
Phys. Plasma 15, 056101, (2008)] , the injected helicity and its stabilization effect have been experimentallyinvestigated on a theta-pinch type of FRC device, NUCTE-III. Magnetic helicity is injected into a FRC through aspheromak generated by a magnetized coaxial plasma gun coaxially mounted at the end of cylindrical chamber. The
experimental results have indicated advantage of CHI for suppression of the rotational instability while it keeps
confinement property.

1. Introduction
A field-reversed configuration (FRC) plasma has
a self-generated toroidal rotation. It causes
rotational instability with toroidal mode number n =2 which it could destroy the configuration, In order
to control this destructive instability, several
methods had been proposed and demonstrated. The
application of static multipole fields demonstrated
suppression of the instability [1,2]. However, it has
been shown that non-axisymmetric multipole fields
have adverse effect on confinement [3]. The otherreport that the onset time of rotational instability isdelayed by presence of a low toroidal field in FRC
had been came out [4].
In this work, use of a magnetized coaxial plasmagun (MCPG) has been proposed as an effective
method to control FRC. Indeed, these initial resultsof coaxial spheromak injection demonstrated
suppression of the most prominent FRC instability.
This observation has been made on the Nihon
University Compact Torus Experiment (NUCTE)
which is a flexible theta-pinch-based FRC facility[5].
2. Experimental device
This experiment has been performed on thetheta-pinch based FRC device that named NUCTE– III [6].

NUCTE-III has a set of theta pinch coil consist of25 coil elements. The discharge tube made oftransparent fused silica is 0.256 m in diameter and 2m in length. The central part of the theta pinch coil
is 0.9 m in length and 0.34 m in inner diameter, anda passive mirror coil of axial length 0.275 m isarranged at each end. These coils connected to1920µF of capacitor bank (charged up to 2 kV) that
produces magnetic bias field of 0.032 T. Two set offast capacitor banks of a 32 kV, 67.5µF and 25 kV,18.75µF are employed for main compression andtheta pre ionization respectively.
yNUCTE-IIIzMCPG-1.0-0.5

00.5

1.0 [m]Fig.1 Schematic view of NUCTE-III with a magnetizedcoaxial plasma gun.

As basic diagnostic system, magnetic probes,
flux loops, He-Ne laser interferometer, an ion
Doppler spectroscopy (IDS), and 60 channels

collimators are arranged on the NUCTE-III device.
The collimators measure radiation profile fromplasma using the interference band-pass filter withwavelength range of 550±5 nm which does not have
any line spectrum from impurity and deuterium ions[7]. In this experiment, two set of 7 channels
collimator array with the interval of 24 degrees arearranged at z = -16.5 and -49.5 cm. Other 28
collimators are arranged along the x and y axis at z= -22 cm. Two chords of He-Ne laser passing
through discharge tube at z = -16.5, -55 cm andmeasure averaged electron density. The IDS system
is placed at z = -22cm, measuring the ion
temperature and toroidal ion flow with a line
spectrum of CV 227.2 nm.
The MCPG consists of the inner and outerelectrodes, insulator between both electrodes and anacceleration region. The dimension of MCPG is46.5 cm in length and diameters of inner and outerelectrodes are 1.6 cm and 2.9 cm, respectively. It
operated with a 6kV, 250µF capacitor. A fastsolenoid valve is employed for gas puffing. A bias
coil is directly wound on middle of outer electrodeto magnetize the generated plasmoid. The coil isconnected to a 200V, 800µF capacitor bank. The
MCPG is mounted on the end of theta-pinchchamber as shown in Fig.1.
3. Experimental ResultA magnetized plasmoid generated by the MCPG
is ejected into the vacuum chamber and get to theedge of FRC in approximately 30 microseconds.
Fig.2 shows typical time sequence of FRC
formation and MCPG discharge.

process.
At a FRC with helicity injection, lifetime ofseparatrix radius has slightly been prolonged.
Observed time evolution of Bremsstrahlung
indicates that helicity injection also makes delayed
onset time of rotational instability. However, thiseffect is strongly depends on the injection timing.
Especially, plasmoid injection after the formation
process brought a destruction of confinement. In theseries of experiments, injection timing of -20 ~-30µs before the main compression shows better
effect of CHI. It is consistent with the travelingtime of 20 ~ 30µs of plasmoid from gun muzzle tothe end of FRC.

Fig.3 Time evolutions of (a) separatix radius rs and (b)bremsstrahlung at midplane.

Acknowledgements
This work was partially supported by Grant-in-Aidfor Scientific Research (KAKENHI) 21740403,
Nihon University Symbolic project and Nihon
University Strategic Projects for AcademicResearch.

Fig.2 Typical time sequence of FRC formation andMCPG discharge

The trigger timing for plasmoid injection has set at-25 ~ -30µs before the initiation of maincompression.
Figure 3 shows a time evolution of separatrix
radius and bremsstrahlung at midplane. Because ofless reproducibility, these shot are selected by thesimilarity of the condition in the pre-ionization

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