Electronic Excitation Temperature Profiles in an Air Microwave 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 x 17 mm cross-section) waveguide. A study has been made of the effects of power and airflow on the electronic excitation temperature, Texc. Abel inversion of radial profile chord averaged Fe I emission lines in the 370 to 377 nm range have been used to obtain localized profile measurements of Texc 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 FWHM emission intensity) plasmas. A maximum central Texc of 6550 K ± 350 K is observed at an airflow rate of 28 lpm. When maintaining a constant flow rate of 14 lpm, 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 Texc value. At a constant microwave power of 1.4 kW, an increase in total flow rate from 11 to 28 lpm decreases the volume of the plasma by ~25 % and increases the central Texc by ~13 %. The axially peaked temperature profiles are consistent with an electron density of ~ 10 cm.