Plasma and Fusion Research,ISSN 1880-6821

Plasma and Fusion Research: Regular Articles Volume 13, 3405062 (2018)
Performance Improvement of a Magnetized Coaxial Plasma Gun by Adopting Iron-Core Bias Coil and Pre-Ionization Systems∗)
Takahiro EDO, Tomohiko ASAI, Fumiyuki TANAKA, Shodai YAMADA, Akiyoshi HOSOZAWA, Yasuhiro KAMINOU1), Hiroshi GOTA2), Thomas ROCHE2), Ian ALLFREY2), Dmitry OSIN2), Roger SMITH2), Michl BINDERBAUER2), Tadafumi MATSUMOTO2,3) and Toshiki TAJIMA2,3)
College of Science and Technology, Nihon University, Tokyo 101-8308, Japan
1)Graduate School of Frontier Sciences, The University of Tokyo, Chiba 277-8561, Japan
2)TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610, USA
3)Department of Physics and Astronomy, University of California at Irvine, Irvine, CA 92697, USA (Received 28 December 2017 / Accepted 18 March 2018)
A magnetized coaxial plasma gun (MCPG) is utilized to generate a compact toroid (CT). An MCPG-type CT injector had been developed as a particle refueling system for C-2/C-2U field-reversed configuration (FRC) plasmas. To inject CTs repetitively for a long-lived plasma, the injector has been upgraded. Iron-core bias coil system has been adopted to generate stationary bias magnetic field. Typical MCPG systems use excess neutral gas to produce a breakdown; therefore, the excess gas tends to flow into the confinement vessel and cool off the edge plasma as well as the target plasma. This negative effect is more serious for repetitive CT injection so that a pre-ionization (PI) system is required to reduce initial gas amount. By injecting the initial plasma using the PI system, amount of the neutral gas for the injector can be reduced. The combination of these systems also expands operating range of the injector. By moving the iron-core bias coil, the radial magnetic field can be controlled. The PI system can easily produce breakdown; therefore, the MCPG can be operated at lower gas pressure, reduced by approximately 40 %. The optimum CT has higher velocity (>100 km/s) and ion temperature (>70 eV), increased by more than 40 %.
⃝c 2018 The Japan Society of Plasma Science and Nuclear Fusion Research
Keywords: magnetized coaxial plasma gun, compact toroid injection, stationary magnetic field, iron core, pre- ionization
DOI: 10.1585/pfr.13.3405062
1. Introduction
A magnetized coaxial plasma gun (MCPG) is utilized to generate a compact toroid (CT), which has a spheromak- like configuration. It typically consists of a set of coaxial cylindrical electrodes, a bias coil, and gas-puff valves. In a typical MCPG operation, bias magnetic field is firstly applied by the bias coil, and neutral gas is injected be- tween the electrodes. Then, a plasma is generated within the gap by applying a high voltage between electrodes and can be accelerated by J × B Lorenz self-force; where J is gun current and B is magnetic field which is generated by the gun current. The accelerated plasmoid captures inter- linkage bias flux, and the captured poloidal flux induces a toroidal current. Finally, a magnetized plasmoid that has toroidal and poloidal fields is ejected from the MCPG.
MCPG-type CT injectors had been developed as a par- ticle refueling system for C-2/C-2U field-reversed configu- ration (FRC) experiments [1–3]. The CT injection demon- strated successful refueling with a significant build-up of
author’s e-mail: asai.tomohiko@nihon-u.ac.jp
∗) This article is based on the presentation at the 26th International Toki Conference (ITC26).
20 - 30 % of the total particle number per single CT injec- tion without any disruptive effects on the target plasma [3]. To inject CTs repetitively for a long-lived plasma, a quasi-stationary bias magnetic field is required. By using bias coil with an iron-core, a stationary bias field, without capacitor discharge, is generated; i.e., it does not require a large current. By moving the iron-core bias coil, the dis- tribution of magnetic field between the electrodes can be easily controlled; therefore, a CT proceeds while gather- ing magnetic field so that the amount of poloidal flux can
be controlled.
The neutral gas necessary to produce a breakdown is
excessive for generating a CT; the excess neutral gas flows into the confinement vessel and cools off the edge plasma as well as the target plasma; the excess gas also remains inside the MCPG. The CT is generated with the bulk of the gas, while the remaining neutral gas cools the CT. These negative effects are more serious for repetitive CT injec- tion due to the inflow of neutral gas so it is necessary to reduce initial gas amount; thus, a pre-ionization (PI) sys- tem is required to solve this issue. By adopting the PI sys- tem, the injected neutral gas can be ionized efficiently, and
3405062-1 ⃝c 2018 The Japan Society of Plasma Science and Nuclear Fusion Research