23P117-P
Internal structure of a field-reversed configuration plasma in translation process
移送過程における磁場反転配位プラズマの磁気構造
Naoki Tada, Kazuhiro Akimoto, Takanori Yamauchi, Tadafumi Matsumoto
Tomohiko Asai and Tsutomu Takahashi
多田直樹, 秋本和宏，山内貴紀，松本匡史，浅井朋彦，高橋 努
Nihon University College of Science and Technology
1-8-14, Kand-Ssurugadai,Chiyoda-ku, Tokyo 101-8308, Japan
日本大学理工学部 〒101-8308 東京都千代田区神田駿河台1−8−14Time evolutions of poloidal magnetic field structure and flux of a field-reversed configuration in atranslation process have been investigated in NUCTE-III/T facility. One of the purposes for this study is torevel the relation between the time evolution and a translation speed. The translation velocity has
controlled using a pair of assist coils installed in the translation region. The FRC plasma has been simulated
as a set of several ring currents including induced current rings as a metal boundary and the poloidal magnetic
structure and flux have been estimated. Preliminary experimental results will be presented.

1. Introduction
A field-reversed configuration (FRC) plasma
belongs to a compact troid (CT), which is atoroidal magnetic system confined by only apoloidal field and no or little toroidal magnetic field.
[1]. The FRC has very extremely high beta value.
The poroidal confinement field (Bze) consists of themagnetic fields of an external coil (Bz0) and a self
magnetic field of toroidal plasma current (Ipθ).
Since this plasma is an axial symmetric, liner,
simple connected plasma and no link with theexternal coil system and another component
structure, FRC plasma is able to translate along theconfinement field.
Recently, supper alfven translation trapping [2]and collision and marging [3] experiments havebeen performed, and improved ploidal flux
confinement properties, increases of the poloidal
flux and the rethermalization efficiency, a toroidal
field generation, and a flux conversion have been
observed. The development of these phenomena has
related with a translation speed (supper alfvenspeed) and processes of a relaxation andself-organization of FRC plasma [4].
The Nihon University Compact Troid

Experiment III/ Translation (NUCTE-III/T) device
has been constructed and translation-trapping
experiments have started [5]. One of the purposes
for the research is to revel the time evolution of thepoloidal magnetic field structure and the poloidal
flux in the translation-trapping process. Since thisprocess is a transient time evolution, it is difficult toapply an equilibrium model and solution of theGrad-Shafranov equation. In this study, the FRC
plasma has been simulated as a set of several ring
currents [6]. The conducting boundary (the metalchamber) and FRC are simulated as a set of ring
currents. Here, the parameter of the ring currents(positions and strengths) is determined for theexperimentally observed magnetic field density andflux. Then, the magnetic flux surface of the FRC
plasma is estimated, including the effects of themetallic chamber.
In this paper, to investigate a time evolution ofseparatrix structure in the reflection process of thedownstream mirror coil, an array of 12 magnetic
probes has been inserted in the mirror coil region. A
pair of pulsed coils has been also installed in thetranslated region (confinement region) to control of(a)(b)
Fig.1 (a) Asist coil inserted in comfinement region and (b)magnetic probe array installed indownstream mirror field region.

(a)(b)Fig. 2 Time evolutions of excluded flux
radius: (a) without the assist coil, (b) withthe assist coil

the translation speed and preliminary experiments
have been performed.
2. Experimental apparatus and diagnostic
The experiments have been curried out by theNUCTE-III/T device, which the details of thedevice have been described in the reference of [5].
The Figure 1 shows the schematic of a pair of assist
coils installed in the confinement region of
NUCTE-III/T (a) and the inserted magnetic probearray in the downstream mirror region (b). The
axial motion of FRC plasma and magnetic structure
is monitored by an excluded flux method offourteen pairs of magnetic probe and magnetic flux
loop and the newly inserted array of 12 magnetic
probes and two arrays of 3 magnetic probes
installed in up and down stream mirror region [6].
These magnetic data uses in the reconstruction ofmagnetic structure.

3.

Preliminary experimental results
The time evolution of the FRC in theconfinement region is shown in Fig. 2. The contour
indicates the time evolution of r. At the first pass,the FRC rapidly decompressed to a volume fourtimes larger than that in the formation region. The
plasma accelerated to a velocity of 150 km/s andwas then reflected at the downstream mirror with amirror ratio of about three. At this point, theFig.3 Estimated external coil field (dashedline), confinement field (chain line), induced
current in vacuum chamber ( × ) andmagnetic field of induced current (solid line).

translated velocity decreased to 90 km/s. After threereflections, the translation speed decreases to about50km/s and the plasma entered a static phase. Whenthe assist coil is applied, FRC plasma has been
trapped between the assist coil and upstream mirror
coil at third pass. By the assist coil, magnetic humpof mirror ration of 1.25 and magnetic countor of theration of 0.8 have been produced in the confinement
region. At the first and second passes, thetranslation speed is decrease and increase of 10% at
backward and forward of the coil, respectively.
Magnetic field density strengths of the reflection
region (-2.0<z<-0.8) produced by external coil andinduced current in the metal chamber are shown inFig. 3, which are estimated by 28 ring currents of
the induced current. Its current profile in the metalboundary is shown in the same figure. The
estimated time is just before the FRC injection. Buttime evolution of the plasma current, after theinjection of FRC, doesn’t have yet been obtained.
In near future, the current profile of the FRC
plasma and magnetic field structure in the reflection
process will be revealed.
References[1] L. C. Steinhauer: Phys. Plasmas 18 (2011) 070501.
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Steinhauer: Phys. Rev. Lett.92 (2004) 245001.
[3] M. W. Binderbauer, H. Y. Guo, M. Tuszewski et al.,Phys. Rev. Lett. 105, 045003 (2010).
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Takahashi, To.
Takahashi, et al., Fusion Science and Technology
55(2T) (2009) 76-81.
[6] T. Asai S. Akagawa, K. Akimoto, T. Tada, T.
Takahashi, and H. Tazawa, et al.: PFR Special Issue
on ITC20, (2010), in Press.
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