Electronic excitation temperature profiles in an air microwave plasma torch

A 0.9- to 1.5-kW 2.45-GHz atmospheric pressure air microwave plasma torch has been operated efficiently with less than 1% reflected power. The plasma is sustained in a 28-mm internal diameter fused quartz tube, which penetrates perpendicularly through the wide walls of a tapered and shorted WR-284 (72×17-mm cross section) waveguide. A study has been made of the effects of power and airflow on the electronic excitation temperature, T_exc. Abel inversion of radial profile chord averaged Fe I emission lines in the 370-377-nm range have been used to obtain localized profile measurements of T_exc inside the waveguide excitation region. In general, temperature profiles peak on axis with no evidence of a skin effect in the large diameter (10-mm full width at half maximum emission intensity) plasmas. A maximum central T_exc of 6550 K±350 K is observed at an airflow rate of 28 Ipm. When maintaining a constant flow rate of 14 Ipm, a 55% increase in microwave power from 0.9 to 1.4 kW causes a ~100% increase in plasma volume without any noticeable effect on the central T_exc value. At a constant microwave power of 1.4 kW, an increase in total flow rate from 11 to 28 Ipm decreases the volume of the plasma by ~25% and increases the central T_exc by ~13%. The axially peaked temperature profiles are consistent with an electron density of ~10¹³ cm^-3