Development of a magnetized coaxial plasma gun for compact toroid injection into the C-2 field-reversed configuration device

REVIEW OF SCIENTIFIC INSTRUMENTS 87, 11D406 (2016)
Characterization of compact-toroid injection during formation, translation,
and field penetration
T. Matsumoto,1,a) T. Roche,2 I. Allfrey,2 J. Sekiguchi,1 T. Asai,1 H. Gota,2 M. Cordero,2
E. Garate,2 J. Kinley,2 T. Valentine,2 W. Waggoner,2 M. Binderbauer,2 and T. Tajima2,3
1Nihon University, Chiyoda-ku, Tokyo 101-8308, 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
(Presented 6 June 2016; received 4 June 2016; accepted 26 June 2016; published online 22 July 2016)
We have developed a compact toroid (CT) injector system for particle refueling of the advanced beam- driven C-2U field-reversed configuration (FRC) plasma. The CT injector is a magnetized coaxial plasma gun (MCPG), and the produced CT must cross the perpendicular magnetic field surrounding the FRC for the refueling of C-2U. To simulate this environment, an experimental test stand has been constructed. A transverse magnetic field of ⇠1 kG is established, which is comparable to the C-2U axial magnetic field in the confinement section, and CTs are fired across it. On the test stand we have been characterizing and studying CT formation, ejection/translation from the MCPG, and penetration into transverse magnetic fields. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4959571]
I. INTRODUCTION
A compact toroid (CT) injector has been developed1 for a particle fueling of the advanced beam-driven field-reversed configuration (FRC), which is formed by C-2U at Tri Alpha Energy.2 On tokamaks and other relevant large fusion devices, CT injector systems have been used for decades,3–5 and in- jected CTs are typically generated by a magnetized coaxial plasma gun (MCPG). The MCPG generates plasma between electrodes, and it can be easily accelerated by Lorentz force J ⇥ B, where J is the gun current through the electrodes and B is the self-formed magnetic field by the gun current. After that, a spheromak-like plasmoid is formed with high speed during interlink with bias flux. To inject the CT into the FRC for particle refueling, the CT needs to penetrate perpendicular external magnetic field surrounding the FRC. Therefore, we built a test stand with transverse field coils that generate a field similar to the perpendicular external magnetic field of C-2U to simulate CT penetration of the FRC. The test stand also included a measurement region for observing the behavior of the CT inside the magnetic field and measuring their typical plasma parameters. In this paper, we describe the diagnostic system setup on the test stand, which is used to measure the CT performance, as well as describe its typical waveforms on the drift tube and behavior of penetrated CT inside the transverse magnetic field. This paper provides an overview of the test-stand experiment to observe and characterize the ejected CTs.
Note: Contributed paper, published as part of the Proceedings of the 21st Topical Conference on High-Temperature Plasma Diagnostics, Madison, Wisconsin, USA, June 2016.
a)Author to whom correspondence should be addressed. Electronic mail:
cstd14003@g.nihon-u.ac.jp
II. DIAGNOSTICS SETUP AND TYPICAL WAVEFORMS
The CT-injector test stand consists of three sections: MCPG, drift tube, and glass tube regions. Figure 1 shows the schematic of test stand with various diagnostics. To charac- terize the behavior of ejected CTs, the following diagnostics have been set up on the test stand: magnetic probes, colli- mated fibers, dispersion interferometer, and triple Langmuir probe. In the drift tube region, we can measure the typical parameters of the ejected plasmoid such as velocity, density, and temperature. In the glass tube region, we measure the trajectory of the penetrated plasmoid/CT inside the transverse field. The diameter of this glass tube is more than three times as large as the drift tube, and transverse magnetic field is similar to the external magnetic field inside the vessel of the C-2U confinement region. Therefore, we can measure the behavior and trajectory of the injected CT inside the magnetic field by the magnetic probes and collimated fibers as illustrated in Fig. 1. In addition, we had a fast-framing camera to mea- sure the behavior/trajectory of penetrated CT inside the glass tube. Subsections II A–II E describe the diagnostics that are deployed on each section of the test stand as well as typical CT plasma parameters during translation and penetration into the transverse field.
A. Gun voltage and current
The MCPG power supply has a capacitance of 125 μF with an optimum operating voltage at 10 kV, which corre- sponds to the stored energy at 6.25 kJ. Typical waveform of the gun current is shown in Fig. 2(a). The working gas is deuterium. To measure the gun current, we use a Rogowski coil mounted around the ceramic break in between inner/outer- electrode flanges. This gun current is, therefore, used for an acceleration of the plasmoid before interlink with bias flux. The rise-time of the gun current is ⇠10 μs and its peaked
0034-6748/2016/87(11)/11D406/4/$30.00 87, 11D406-1 Published by AIP Publishing.