Numerical Analysis on the Electrical and Thermal Flow Characteristics of Ar-N₂ Inductively Coupled Plasma Torch System

Numerical analysis on the electrical and thermal flow characteristics of Ar-N₂ inductively coupled plasma (ICP) were carried out for N₂ content from 0 to 50 mol% at plasma power level of 50 kW. Firstly, the computational results of thermal flow fields revealed that the addition of N₂ could reduce the radiation heat loss, together with increasing the exit enthalpy. For example, the radiation heat loss of Ar-N₂ ICP with N₂ content of 20 mol% is reduced by ∼ 33%, compared with that of Ar-only ICP, which are essential for the safe operation of an ICP system at the high power level of 50 kW. In addition, the increase of N₂ content was also found to increase the load resistance of a tank circuit for a vacuum tube oscillator. Equivalent circuit analysis using the numerical results shows that this increase of the load resistance comes from the increasing equivalent resistance and the decreasing equivalent inductance of Ar-N₂ ICP, corresponding to the changes of thermal flow fields with the increase of N₂ content. For a tank circuit consisting of a capacitor and an inductor with a capacitance of 6,500 pF, and an equivalent inductance of 0.84 μH, the load resistance was calculated to be increased from 81 Ω for Ar only ICP to 130 Ω for Ar-N₂ ICP with N₂ content of 20 mol%. Considering the internal resistances of high power vacuum tubes higher than 130 Ω, this increase of load resistance clearly shows that N₂ addition can improve the impedance mismatching of an ICP torch system with a vacuum tube oscillator having no variable part to tune the load resistance. By providing these basic data for the calculation of load resistance and thermal flow characteristics, the numerical analysis combined with equivalent circuit analysis used in this work can help in designing and operating a high-powered ICP torch system with a vacuum tube oscillator.