23P114-P
Control of Field-Reversed Configuration via Translation Process
移送法によるFRCプラズマのパラメータ制御	 
Junichi Sekiguchi, Hirotoshi Uriu, Michiaki Inomoto, Toshiki Takahashi,Tomohiko Asai and Tsutomu Takahashi
関口純一1), 瓜生博俊1)，井	  通暁2)，高橋俊樹3),高橋	  努1), 浅井朋彦1)
1)

College of Science and Technology, Nihon University
1-8-14, Kandasurugadai, Chiyoda-ku, Tokyo 101-8303, Japan
2)
Graduate school of Frontier Sciences, The University of Tokyo
5-1-5, Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan3)
Faculty of Engineering, Gunma University
1-5-1 Tenjinchou, Kiryuu-shi, Gunma, Japan1)
日本大学理工学部 〒101-8303	  東京都千代田区神田駿河台1-8-142)
東京大学大学院新領域 〒277-8561 千葉県柏市柏の葉5-1-53)
群馬大学大学院工学研究科 〒376-8515 群馬県桐生市天神町1-5-1
A field-reversed configuration (FRC) can be easily translated along the axial guide field, because of theirsimply connected structure, and the translation speed, which can be controlled by the profile of guidemagnetic field, reaches the range of Alfven speed. On the other hand, additional heating and current
driving technique are not established yet, therefore the temperature and density of the FRC have beenattempted to control by injecting the FRC into a neutral particle atmosphere. Also the aspect of FRC has
been tried to modify to oblate shape via translation into large-bore translation chamber.

1. Introduction
A field-reversed configuration (FRC) plasmas
formed by field-reversed theta-pinch (FRTP)
method has a limited confinement lifetime of theorder of micro-seconds. Therefore, formed FRC
is translated into a confinement region with static
field in the most of scenarios of FRC based fusion
reactor to apply additional heating and current
drive by neutral beam injection (NBI). In thiswork, this translation process has been tried toutilize to control FRC parameters and aspect.
2. Experimental Device
Nihon University Compact Torus Experiment(NUCTE)-III is a theta-pinch type FRC device[1].
It consists of a set of theta-pinch coils and quartz
discharge tube that the external diameter is 256mm
and the length is 2.0m. The FRC is formed with thebias magnetic field of -0.032T and the main
magnetic field of 0.5T with the rise time is 4µs.
Translation region with a quasi-steady state
confinement field named NUCTE-T has recently
been constructed and connected to theNUCTE-III[1]. It consists of a quartz discharge
tube of 0.4m in O.D. and 1.4m in length. The
quasi-steady magnetic field is 0.06T with the risetime of 13ms. The schematic view of NUCTE-III/Tis shown in Fig 1.

Quasi-steady coil

Metal chamber

θ- pinch coil

Discharge tube!"#$%&$-4.0-3.0!"#$%&'''-2.0-1.00z[m]1.0

Fig.1. A schematic view of NUCTE-III/TThe discharge tubes are made of transparent
quartz. Therefore optical measurements can bearranged flexibly from all angles and positions on
the chamber. Basic diagnostics on the translation
region consist of magnetic probes, flux loops,
optical collimater arrays and a He-Ne laserinterferometer.
3. Translation into Background Neutral Gas
Translation experiments of FRC into a translation	 chamber filled with neutral gas has been performed
on NUCTE-III/T facility. This experiments has
been tried to demonstrate “equivalent neutral beam
injection (NBI)” effect of background gas onto thetranslated FRC. The filled background gas is
injected into the translated FRC with relativevelocity. Therefore, it makes large current low
energy axial NBI effect on the FRC[1].
For background gases, deuterium, helium andargon have been chosen in this series ofexperiments. Figure 2 shows the time evolution ofmaximum separatrix radius rs. Here the curves of rs
are tracing maximum radius of translated FRC.
Figure 3 shows the time evolution ofbremsstrahlung observed along a chord passing
through the z axis indicating toridal deformation byrotational mode instability.
In the case of helium and deuterium, theconfiguration lifetimes estimated from the time
evolution of rs are prolonged compared to thestandard operation case from 100 to 110µs.
As show in Fig. 3, onset time of toroidal
deformation is also delayed by existence ofbackground neutral gas. In the case with helium gas
puffing, the onset is delayed 60µs compared to thecase without gas fill.

In the case of translation experiment into
NUCTE-T, the prolonged confinement lifetime iswithin the order of a few hundred micro-seconds. In
order to extend confinement lifetime, additional
current drive is necessary. In the concept of D-3Hefusion reactor “ARTEMIS”[2], Neutral Beam
Injection (NBI) is schemed as additional heating
method, but the current stage of experimentallyformed FRC plasmas have not achieved enoughpoloidal flux to apply NBI. For example, 20-30keVhydrogen ions require flux on order of 20mWb for
NBI [3]. In the translated FRC plasmas into
NUCTE-T have less than 0.1mWb of poloidal flux
at total temperature 40keV[3]. Therefore a center
solenoid is employed to build-up poloidal flux on
FAT FRC experiments.

Fig.4. A schematic view of FAT.
Fig.2. Time evolution of maximum value of separatrix
radius.

Fig.3. Time evolution of bremsstrahlung.

4. New Confinement Device
For faster translation velocity and to achieve highaspect ratio (i.e. oblate) FRC, new confinement
region named FAT (FRC Amplification via
Translation/Transformer)
has
newly
been
constructed. The FAT has a quartz discharge tube
of 0.8m in O.D. and the 1.0m in length. The
quasi-steady magnetic field is 0.08T with the risetime of 49ms One of the experimental purpose ofFAT is modification of separatrix shape oftheta-pinch based high density FRC into an oblate
one through the translation process. The global
stabilities of FRC plasmas depend on their aspectratio, therefore the global instabilities are verified
on FAT FRC experiments.

Acknowledgments
This work was partially supported by
Grant-in-Aid for Scientific Research (KAKENHI)
21740403 and 19560829, Nihon University
Symbolic project and Nihon University Strategic
Projects for Academic Research.
References[1] H. Tazawa: “Dependence on equivalent neutralbeam injection effect for a translated FRC plasma ongas species”, Master’s thesis, College of Science andTechnology, Nihon University (in Japanese) (2011).
[2] H. Momota, A. Ishida, Y. Kohzaki, G. Miley, S. Ohi,
M. Ohnishi, K. Sato, L. Steinhauer, Y. Tomita, andM. Tuszewki: Fusion Technol. 21, (1992) 2307.
[3] E.V Belova, M. Yamada, H. Ji, S. Gerhardt: Phys.
Plasmas 8, (2001) 1267.
[4] K. Sakuraba: “Dynamics of field-reversed
configuration plasma in translation process”,
Master’s thesis, College of Science and Technology,
Nihon University (in Japanese) (2008).