Transient Response of the Induction Coil Loading Impedance and rf Inverter System in the Initial Startup Phase of rf Induction Thermal Plasmas

Abstract

Generation and sustainment of rf induction thermal plasmas in atmospheric pressure range require a high voltage in the initial startup phase and a high rf power during the steady state sustainment phase. The high rf electrostatic filed induced between induction coil conductor wound around a glass discharge tube generates surface electrostatic discharges in which the discharge current flows through the dielectric glass tube. After several hundreds microseconds the electrostatic discharges grow sufficiently to transfer to a volumetric electromagnetic discharge which is maintained by the inductive electric field. During this discharge mode transition, the loading impedance of the induction coil changes significantly. This loading impedance change of the induction coil leads a deviation from a series resonance condition of the LCR output circuit including the induction coil, which strongly affects the output characteristics of a semiconductor rf inverter power supply. In this paper transient response of the induction coil impedance in the initial startup phase of the rf thermal plasma generation is studied experimentally. Dynamic interactions of the rf inverter power supply with generated rf plasmas are also investigated and strong deterioration of the inverter output efficiency is found experimentally. Preliminary frequency tracking experiments show a successful improvement of the inverter output efficiency and absorbed rf power in the generated plasma.