Performance improvement of a magnetized coaxial plasma gun by adopting iron-core bias coil and pre-ionization systems

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%. (author)