Increased ionization efficiency for high performance CT injection

A magnetized coaxial plasma gun (MCPG) is one of the devices used to generate and accelerate a compact toroid (CT) with a spheromak-like magnetic configuration. A typical MCPG consists of a set of axisymmetric cylindrical electrodes, a bias coil, and gas-puff valves; and it requires excessive neutral gas to obtain breakdown. The excess gas degrades the performance not only of the generated CT but also of the target plasma. By using a pre-ionization (PI) system (a miniature gun that is installed radially at the same cross section as the gas-puffing ports) we have succeeded in reducing the amount of neutral gas required (~40%) for breakdown, which results in improved performance of ejected CTs [1]. For further improvement of the CT performance, we focus on the gas flow and its spatial distribution in the miniature gun as well as MCPG. The breakdown condition of the miniature gun is optimized by changing the distance between electrodes and varying the amount of initial enclosed gas. A simulation of the gas flow is conducted to optimize the geometry of gas inlets on the miniature gun. We will present an updated PI system and effects on the ejected CT. [1] T. Edo et al., J. Plasma Fusion Res. 13, 3405062 (2018).