Responses of a long-coil pulse-modulated induction plasma

Radio-frequency inductively coupled plasma in a pulse modulated approach was generated by a MOSFET inverter supply of high electric efficiency. The plasma torch has an extremely long coil region of 153 mm, which is an attractive feature for advanced materials processing, especially for better and more efficient vaporizing of solids. The operating conditions were: argon flow of 80 or 90 L/m at atmospheric pressure; supply power of 30 kW; and pulse on-time of 10 ms at 67% duty factor. Spectroscopic measurements were carried out to determine the temporal plasma properties, including the effects of shimmer current level (SCL) upon the spectral intensities. Additionally a time-dependent two-dimensional numerical model was solved for the same operating conditions employed in the experiment to predict and compare the plasma properties. Pulsed plasma dissipation sustained for a minimum SCL of 43% for 80 L/m gas flow-rate, and at any level below 43%, the plasma disappeared. Temporal variation of argon line intensities at 751 and 763.5 nm is similar, though the upper level intensity of the former one was significantly stronger than the latter. Intensified change of intensity is found at lower SCL because of higher change in the coil current and, in turn, in the plasma power. The predicted intensity of the 751-nm argon line showed similar behavior to the experimental intensity though the response around the instant of on-pulsation is somewhat slower.