Plasma and Fusion Research,ISSN 1880-6821

Plasma and Fusion Research: Regular Articles Volume 13, 3402098 (2018)
Collisional Merging Process of Field-Reversed Configuration Plasmas in the FAT-CM Device∗)
Fumiyuki TANAKA, Tomohiko ASAI, Junichi SEKIGUCHI, Tsutomu TAKAHASHI, Junpei ISHIWATA, Takahiro EDO, Naoto ONO, Keisuke MATSUI, Shintarou WATANABE, Daiki HISHIDA, Daichi KOBAYASHI, Yousuke HIROSE, Akiyoshi HOSOZAWA, Yung MOK1), Sean DETTRICK1), Thomas ROCHE1), Hiroshi GOTA1), Michl W. BINDERBAUER1)
and Toshiki TAJIMA1,2)
College of Science and Technology, Nihon University, Tokyo 101-8308, Japan
1)TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610, USA
2)Department of Physics and Astronomy, University of California at Irvine, Irvine, CA 92697, USA (Received 28 December 2017 / Accepted 8 July 2018)
In order to investigate the collisional merging process of field-reversed configurations (FRCs), the FAT device has recently been upgraded to FAT-CM, consisting of two field-reversed theta-pinch (FRTP) formation sections and the confinement section. Collisional merging of the two FRCs causes a conversion of the kinetic energy to mostly thermal ion energy, resulting in an increase of the ion pressure that greatly expands the FRC size/volume. This increase of the FRC size is observed by magnetic diagnostics in the confinement region, leading to an increase in the excluded flux; on a side note, these characteristics/phenomena have also been observed in C- 2/C-2U experiments at TAE Technologies. The process of FRC formation, translation and collisional merging in FAT-CM has been simulated by Lamy Ridge, 2D resistive magnetohydrodynamics code, in which the same phenomenon of the excluded-flux increase via FRC collisional merging has been observed. Simulation results also indicate that there is an importance of the external magnetic field structure/profile in the confinement region, clearly affecting the FRC merging. Steeper magnetic field gradient by a strong mirror field appears to suppress the axial expansion of collided FRCs and lead a merged FRC to higher temperature.
⃝c 2018 The Japan Society of Plasma Science and Nuclear Fusion Research
Keywords: magnetically confined plasma, field-reversed configuration, high beta plasma, FRC merging, 2D resistive MHD simulation
DOI: 10.1585/pfr.13.3402098
1. Introduction
A field-reversed configuration (FRC) is a compact toroid (CT) that has predominantly poloidal magnetic field with zero or small amount of self-generated toroidal field [1, 2]. FRC has a potential as a feasible fusion reactor because of its simple geometry, ease of translation, natu- ral diverter, and extremely high beta value. The averaged beta of FRCs is near unity: ⟨β⟩ = 2μ0⟨p⟩/B2e ∼ 0.9 (p is plasma pressure and Be is the external magnetic field, re- spectively).
Recently, the C-2/C-2U device at TAE successfully demonstrated a quasi-static sustainment of merged FRCs by fast ions which are introduced via ∼ 10 MW neutral- beam injection [3]; also, the collisional merging process clearly exhibited an increase of the diamagnetic signals [4, 5]. However, the collisional merging process itself and its effect on FRC performance have not yet fully been stud- ied in detail.
Field-reversed theta-pinch (FRTP) is the conventional
author’s e-mail: asai.tomohiko@nihon-u.ac.jp
∗) This article is based on the presentation at the 26th International Toki Conference (ITC26).
method to produce hot and high-density plasma [6]. The FAT (FRC Amplification via Translation) device at Ni- hon University has been upgraded to FAT-CM to have two FRTP formation regions in order to investigate the colli- sional merging process of FRCs. In parallel with FAT-CM experiments, a simulation research has started to better un- derstand the dynamics of formation, translation and col- lisional merging processes of FRC’s. We have employed Lamy Ridge [7] that is 2D resistive magnetohydrodynam- ics (MHD) code with real experimental boundary and ini- tial conditions.
In this paper, we report initial results of the FAT-CM experiments and numerical simulations.
2. FAT-CM Device
Figure 1 illustrates a schematic of the FAT-CM de- vice. It consists of the central confinement vessel and two FRTP formation sections, called “V-formation” and “R- formation”. Formation tubes are made of transparent fused quartz, and the confinement chamber is made of stainless steel (inner wall radius 0.39 m; skin time ∼ 5 ms). It serves as a flux conserver in the timescale of the translation pro-
3402098-1 ⃝c 2018 The Japan Society of Plasma Science and Nuclear Fusion Research