Effects of Different Coolants on the Properties of Sensitively Coupled Plasma

The Inductive Coupled Plasma （ICP） source, a crucial component of the fusion Neutral Beam Injection （NBI） heating system, generates significant heat during operation. This heat not only affects the vacuum of the device but also alters the plasma impedance, thereby impacting the RF power feed. A double-layer glass cylinder discharge chamber is designed to actively cool the discharge chamber. The effects of different coolants （deionized water, purified water, fluorinated liquid, etc） on ICP discharge are quantitatively studied. The experimental results demonstrate that actively injecting coolant into the double-layer glass cylinder could effectively mitigate the influence of the plasma heat load on the vacuum chamber. Moreover, when fluorinated liquid is used as a coolant, the plasma impedance value is minimized, resulting in the highest increase in plasma electron density, approximately 15.3% and 18% higher than that of deionized water and pure water, respectively. By comparing the volume resistivity and relative dielectric constant of the coolants, it can be concluded that the relative dielectric constant is inversely correlated with changes in plasma electron density and equivalent impedance. In other words, lower relative dielectric constants correspond to lower equivalent impedance and higher plasma density. The experimental results provide valuable guidance for the design of sense-coupled plasma discharge cavities and the selection of coolants.