Control of Compact-Toroid Characteristics by External Copper Shell

A collaborative research project by Tri Alpha Energy and Nihon University has been conducted for several years, which led to the development of a new compact toroid (CT) injector [1] for efficient FRC particle refueling in the C-2U experiment [2]. The CT is formed by a magnetized coaxial plasma gun (MCPG), consisting of coaxial cylindrical electrodes. In CT formation via MCPG, the magnetic helicity content of the generated CT is one of the critical parameters. A bias coil is inserted into the inner electrode to generate a poloidal flux. The resultant bias magnetic field is spread out of MCPG with time due to its low-frequency bias current. To obtain a more effectively distributed bias magnetic field as well as to improve the voltage breakdown between electrodes, the MCPG incorporates a novel $\sim$ 1 mm thick copper shell mounted outside of the outer electrode. This allows for reliable and controlled operation and more robust CT generation. A detailed discussion of the copper shell and experimental test results will be presented.\\[4pt] [1] T. Matsumoto \textit{et al}., Bull. Am. Phys. Soc. \textbf{59}, UP8.00008 (2014) \newline [2] M. Binderbauer \textit{et al}., Phys. Plasmas \textbf{22}, 056110 (2015)