Performance Improvement of a Magnetized Coaxial Plasma Gun by adopting Iron-core Bias Coil and New Pre-Ionization System

A magnetized coaxial plasma gun (MCPG) is a device used to generate a compact toroid (CT), which has a spheromak-like configuration. A typical MCPG consists of a set of axisymmetric cylindrical electrodes, bias coil, and gas-puff valves. In order to expand the CT operating range, the distributions of the bias magnetic field and neutral gas have been investigated. We have developed a new means of generating stuffing flux. By inserting an iron core into the bias coil, the magnetic field increases dramatically; even a small current of a few Amps produces a sufficient bias field. According to a simulation result, it was also suggested that the radial distribution of the bias field is easily controlled. The ejected CT and the target FRC are cooled by excess neutral gas that typical MCPGs require to initiate a breakdown [1,2]; therefore, we have adopted a miniature gun [3] as a new pre-ionization (PI) system. By introducing this PI system, the breakdown occurs at lower neutral gas density so that the amount of excess neutral gas can be reduced. [1] T. Matsumoto\textit{ et al}., Bull. Am. Phys. Soc. \textbf{61}, CP10.00083 (2016) [2] I. Allfrey\textit{ et al}., Bull. Am. Phys. Soc. \textbf{61}, CP10.00085 (2016) [3] T. Asai\textit{ et al}., Rev. Sci. Instrum. \textbf{81}, 10E119 (2010)