Temperature control of Ar induction thermal plasma with diatomic molecular gases by pulse amplitude modulation of the coil current
A high-pressure inductively coupled thermal plasma (ICTP) system was developed; it can modulate the amplitude of the induction coil current periodically with a cycle of several hundred hertz by switching a metal-oxide semiconductor field effect transistor inverter power supply. This type of ICTP was named a `pulse-modulated induction thermal plasma (PMITP)'. The PMITP in different gases of Ar, Ar-H2, Ar-N2 and Ar-O2, was successfully generated at an input power of 30 kW under atmospheric pressure conditions. Shimmer current level (SCL), which is the ratio of lower to higher levels of coil current in modulation, can be reduced from 100% to 40% to sustain the Ar PMITP. The influence of SCL and diatomic molecular gas-inclusion on the PMITP behaviour was investigated. Temperature variation of PMITP was evaluated by the two-line method. The minimum value of the temperature measured during a modulation cycle proved to decrease drastically with a reduction of SCL by over 1000 K, especially with inclusion of the diatomic molecular gas, with the maximum temperature value almost unchanged. The inherent response time of the PMITP was found to have a magnitude of several milliseconds; it increases with decreasing SCL and with the inclusion of a diatomic molecular gas.