Collisional merging formation of a field-reversed configuration in the FAT-CM device

International Atomic Energy Agency Nuclear Fusion Nucl. Fusion 59 (2019) 056024 (6pp) https://doi.org/10.1088/1741-4326/ab0c45
Collisional merging formation of a field- reversed configuration in the FAT-CM device
T. Asai1 , T. Takahashi1, J. Sekiguchi1, D. Kobayashi1 , S. Okada1,
H. Gota2 , T. Roche2, M. Inomoto3, S. Dettrick2, Y. Mok2, M.W. Binderbauer2, T. Tajima2,4 and T. Takahashi5
1 Department of Physics, College of Science and Technology, Nihon University, Tokyo, Japan
2 TAE Technologies, Inc., Rancho Santa Margarita, CA, United States of America
3 Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Japan
4 Department of Physics and Astronomy, University of California, Irvine, CA, United States of America
5 Graduate School of Science and Technology, Gunma University, Kiryu, Japan
E-mail: asai.tomohiko@nihon-u.ac.jp
Received 6 January 2019, revised 20 February 2019 Accepted for publication 4 March 2019
Published 11 April 2019
Abstract
Collisional merging formation of field-reversed configurations (FRCs) at super Alfvénic velocity have been successfully initiated in the FAT-CM device at Nihon University. It is experimentally evidenced that the quiescent FRC profile is formed in a self-organizational manner after distructive disturbances by the super Alfvénic collision of two translated magnetized plasmoids. A drastic increase of the excluded flux compared to the field-reversed theta-pinch formation section has also been observed. This formation process has an important role in realizing an FRC based high-beta reactor core to capture high-energy beam ions, and
it has been clearly observed by magnetic diagnostics of excluded flux and internal probe
array. The experimental results are compared with two-dimensional magnetohydrodynamic (MHD) simulation results computed for the typical conditions of the FAT-CM experiments. Collisional merging of the two separately translated FRCs causes a conversion of the kinetic energy to mostly thermal ion energy, which contrasts with the spheromak merging dominated by magnetic energy in an MHD manner, resulting in an increase of the ion pressure and polodal flux that drastically expands the FRC volume.
Keywords: field-reversed configuration, collisional merging, super Alfvénic collision (Some figures may appear in colour only in the online journal)
1. Introduction
A field-reversed configuration (FRC) is a compact toroid which has the highest volume averaged beta value (〈β〉 ~ 1) among magnetically confined fusion systems [1–3]. Because of its high beta nature and simply-connected geometry, sus- tainment of FRC had not been realized because of the limi- tation of additional heating and current drive [4]. The FRC, therefore, has been studied mostly for the formation technique of high-density magnetized plasmoids as, for example, the target of magnetized target fusion [5] and formation of high- curret and high-flux magnetized plasmoid [6]. Recently, high confinement performance of FRCs have been achieved on a C-2/C-2U device via collisional-merging; that is, the kinetic
energy of translation is larger than the thermal and magnetic energy of the plasmoids being merged, process with large orbit high energy ions fed by neutral beam injection [7]. However, details of the collisional-merging process at super Alfvénic velocity, such as re-formation of field-reversal configuration, rethermalization from kinetic to ion thermal energy, have not been investigated.
In order to investigate the collisional-merging formation process of FRC at super Alfvénic velocity, the FAT (FRC Amplification via Translation) device at Nihon University [7] has recently been upgraded to a FAT-CM (collisional merging), consisting of two field-reversed theta-pinch (FRTP) formation sections and the central confinement section [8]. Collisional-merging of the two separately translated FRCs
1741-4326/19/056024+6$33.00 1
© 2019 IAEA, Vienna Printed in the UK