Internal magnetic field measurements of translated and merged field-reversed configuration plasmas in the FAT-CM device

REVIEW OF SCIENTIFIC INSTRUMENTS 89, 10J114 (2018)
Internal magnetic field measurements of translated and merged
field-reversed configuration plasmas in the FAT-CM device
H. Gota,1,a) J. Ishiwata,2 F. Tanaka,2 A. Hosozawa,2 T. Asai,2 Ts. Takahashi,2 J. Sekiguchi,2 T. Roche,1 T. Matsumoto,1,3 S. Dettrick,1 Y. Mok,1 M. W. Binderbauer,1 and T. Tajima1,3
1TAE Technologies, Inc., Foothill Ranch, California 92610, USA
2College of Science and Technology, Nihon University, Tokyo 101-8308, Japan 3Department of Physics and Astronomy, UCI, Irvine, California 92697, USA
(Presented 17 April 2018; received 20 April 2018; accepted 4 June 2018; published online 13 August 2018)
Field-reversed configuration (FRC) Amplification via Translation–Collisional Merging (FAT-CM) experiments have recently commenced to study physics phenomena of colliding and merged FRC plasma states. Two independently formed FRCs are translated into the confinement region of the FAT-CM device, collided near the mid-plane of the device with a relative speed of up to ∼400 km/s, and a final merged FRC plasma state is achieved. To measure internal magnetic field profiles of the translated and merged FRC plasmas as well as to understand its collisional-merging process, an internal magnetic probe array, developed by TAE Technologies, has been installed in the mid-plane of the FAT- CM device. Initial magnetic field measurements indicate that both the translated and the merged FRC plasma states exhibit a clear field-reversed structure, which is qualitatively in good agreement with 2D MHD simulation. It is found and verified that a sufficient mirror field in the confinement region is required for colliding FRCs to be fully merged into a single FRC plasma state. Published by AIP Publishing. https://doi.org/10.1063/1.5036997
I. INTRODUCTION
Magnetic probes are widely used in measurements of current-carrying plasmas, in particular, the area inside the sep- aratrix of magnetically confined plasma is of great interest to investigate. However, internal magnetic probes are often too disturbing to a plasma configuration in the high tem- perature and high-density regime, so the development of a robust and minimally disturbing probe is a requirement. In past compact toroid (CT) experimental research, a number of good, miniature internal probe arrays have been developed and used to characterize magnetic field structures of field-reversed configuration (FRC) plasmas in STX1 and TCSU2,3 and of spheromak plasmas in Caltech4 and SSX.5 In C-2 FRC exper- iments6 at TAE Technologies, Inc., internal magnetic probe arrays were also newly designed and developed to investigate the FRC translation and collisional-merging process and its characteristics.7
Recently, one of the internal magnetic probe arrays devel- oped by TAE Technologies has been transferred to Nihon University as part of collaborative research in order to study and characterize the internal magnetic field structure of trans- lated and merged FRCs in the mid-plane of the experimental device at Nihon University, named FAT-CM: FRC Amplifica- tion via Translation–Collisional Merging. This is the first time to measure and characterize such FRC structure on FAT-CM as well as to report its experimental result in this paper.
Note: Paper published as part of the Proceedings of the 22nd Topical Confer- ence on High-Temperature Plasma Diagnostics, San Diego, California, April 2018.
a)Electronic mail: hgota@tae.com.
FRCsarehigh-betaCTsthathavepredominantlypoloidal magnetic field with zero or small self-generated toroidal mag- netic field.8,9 In FAT-CM, FRCs are produced by colliding and merging two-oppositely translated CTs in the confine- ment section of the device, in which initial CTs are formed via field-reversed theta-pinch (FRTP) technique in the forma- tion regions and translated into the confinement chamber by a gradient of the external magnetic field.10,11 This FRC for- mation scheme is very similar to C-2.6 Like C-2 experiments, global performance of the collided and merged FRC plasma appears to be better than that of a single-sided translated FRC. These FRC translation, collision, and merging characteristics have recently been investigated by using the newly installed internal magnetic probe array in the mid-plane of the FAT-CM device.
In this paper, the experimental apparatus, FAT-CM, as well as the internal magnetic probe array is briefly described in Sec. II. Experimental results of single-sided/translated FRC as well as of collisional-merging FRCs with and without the probe array inserted are presented in Sec. III. The poloidal flux of the FRC plasma is estimated from the internal magnetic field profile and compared with the rigid-rotor (RR) model in Sec. IV. The effect of confinement mirror field strength on the FRC collisional-merging processes is also described.
II. EXPERIMENTAL APPARATUS A. FAT-CM device
The FAT-CM device, illustrated in Fig. 1, consists of the central confinement chamber and two FRTP formation sec- tions, called “R-formation” and “V-formation,” respectively.
0034-6748/2018/89(10)/10J114/5/$30.00 89, 10J114-1 Published by AIP Publishing.