Suppressed ion-scale turbulence in a hot high-beta plasma

REVIEW OF SCIENTIFIC INSTRUMENTS 87, 053512 (2016) Development of a magnetized coaxial plasma gun for compact toroid
injection into the C-2 field-reversed configuration device
T. Matsumoto,1,a) J. Sekiguchi,1 T. Asai,1 H. Gota,2 E. Garate,2 I. Allfrey,2 T. Valentine,2 M. Morehouse,2 T. Roche,2 J. Kinley,2 S. Aefsky,2 M. Cordero,2 W. Waggoner,2
M. Binderbauer,2 and T. Tajima2,3
1College of Science and Technology, Nihon University, 1-8-14 Kanda, Chiyoda-ku, Tokyo 1018308, Japan 2Tri Alpha Energy, Inc., P.O. Box 7010 Rancho Santa Margarita, California 92688, USA
3Department of Physics and Astronomy, University of California, Irvine, California 92697, USA
(Received 5 January 2016; accepted 12 May 2016; published online 31 May 2016)
A compact toroid (CT) injector was developed for the C-2 device, primarily for refueling of field- reversed configurations. The CTs are formed by a magnetized coaxial plasma gun (MCPG), which consists of coaxial cylindrical electrodes and a bias coil for creating a magnetic field. First, a plasma ring is generated by a discharge between the electrodes and is accelerated by Lorenz self-force. Then, the plasma ring is captured by an interlinkage flux (poloidal flux). Finally, the fully formed CT is ejected from the MCPG. The MCPG described herein has two gas injection ports that are arranged tangentially on the outer electrode. A tungsten-coated inner electrode has a head which can be replaced with a longer one to extend the length of the acceleration region for the CT. The developed MCPG has achieved supersonic CT velocities of ⇠100 km/s. Plasma parameters for electron density, electron temperature, and the number of particles are ⇠5 ⇥ 1021 m 3, ⇠40 eV, and 0.5–1.0 ⇥ 1019, respectively. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4952581]
I. INTRODUCTION
Lifetime of a field-reversed configuration (FRC) has been limited by instabilities (e.g., n = 1, n = 2 mode) until recently. Therefore, FRC refueling has only recently been required. The FRCs formed by C-2 have achieved long-lifetime1 by control of these instabilities.2,3 In order for the plasma to be sustained, the FRC will need a particle refueling source.
On tokamak devices, various refueling techniques (e.g., gas pu ng,4 pellet injection,5 and compact toroid (CT) injection) have been adopted and investigated. In the case of gas pu ng and pellet injection, the speed of the fuel particles is relatively slow (i.e., thermal velocity of around 1 km/s for gas injection, <2 km/s for pellet injection) and fuel temperature (equal or lower than room temperature) is quite low compared with target plasma (⇠keV). On the other hand, a CT formed by a magnetized coaxial plasma gun (MCPG) has relatively high-temperature (tens of eVs) and high-speed (>100 km/s). The first CT injection experiment was carried out on TdeV device, which is a tokamak reactor.6 Subsequently, several MCPGs have been developed for particle refueling in tokamak devices (e.g., STOR-M, JFT-2M).7,8 To further this work, we are developing a CT injector for particle refueling of the C-2 FRC.
A spheromak-like plasmoid is produced by the MCPG which consists of coaxial cylindrical electrodes and a bias coil for creating a magnetic field. First, the biasing poloidal magnetic field is applied and the working gas is injected between electrodes. Then, the MCPG formation bank is triggered to begin breakdown between the electrodes and to ionize the gas. Then, the voltage Vg applied between the
a)Electronic mail: cstd14003@g.nihon-u.ac.jp
coaxial electrodes drives a radial current Jr through the plasma which generates an azimuthal, magnetic field B✓ around itself as illustrated in Fig. 1. Finally, the plasma ring is accelerated by Jr ⇥ B✓ Lorenz axial force, captured by an interlinkage poloidal flux Bbias, and ejected as a spheromak-like plasmoid away from the MCPG. The main role of interlinkage poloidal flux which is radial field Br is to lead the toroidal current across the accelerated plasmas by Lorentz axial force.
In this paper, we describe an overview of the design of the MCPG in Sec. II. Typical experimental results on a test stand are presented in Sec. III.
II. DESIGN OF THE CT INJECTOR A. MCPG electrodes
The MCPG, newly designed for C-2 FRCs, has coaxial cylindrical electrodes as illustrated in Fig. 2. The stainless- steel (SS 304L) electrodes consist of inner and outer electrodes. The lengths of the inner and outer electrodes are 516 mm and 428 mm, respectively. The outer diameter of the inner electrode and inner diameter of the outer electrode are ⇠ 54.0 mm and ⇠ 83.1 mm, respectively. The inductance between electrodes is 86 nH/m. Thicknesses of these electrodes are 3 mm. The length of the formation region for the plasma ring is approximately 190 mm as measured between the gas injection port and the head of the inner electrode. The potentials of electrodes are the following: the inner electrode is the cathode, the outer electrode is the anode, which is grounded. The diameter of outer electrode was selected to be compatible with the available interface on the C-2 vessel. A magnetic energy density (B2/2μ0) of 4 kJ/m3 is required to penetrate the C-2 external field of ⇠0.1 T. The velocity of typical MCPG is over 100 km/s.
0034-6748/2016/87(5)/053512/6/$30.00 87, 053512-1 Published by AIP Publishing.