Equivalent circuit and numerical analyses of an inductively coupled plasma torch with a tapped induction coil

Equivalent circuit and numerical analyses of an Ar-N₂ inductively coupled plasma (ICP) were conducted in order to clarify the effect of the induction coil with a grounded tap on the electrical characteristics of an ICP torch system. First, from the computational results, it was revealed that the load resistance of tank circuit for a free running radio-frequency (RF) oscillator can be reduced to about 25% with a center-tap of the induction coil grounded. Despite the asymmetric distributions of the electric fields in the inside of the ICP torch, this effect was found due to the equivalent resistance and inductance of ICP that were divided in approximately half into each part of the center-tapped induction coil. The reduction of the load resistance by a grounded tap was also observed in generation experiments of Ar-N₂ ICPs using a vacuum tube oscillator for various N₂ contents ranging from 6.3 % to 25.0 %. By providing a way to reduce the load resistance of the tank circuit, the induction coil with a grounded tap can be used to improve the impedance matching condition of ICP systems with the load resistances higher than the internal resistance of a free running RF oscillator.