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 h as been operated efficiently with less than 1% reflected power. The plasma is sustained in a 28-mm internal diameter fused quartz tube, which penetrat es 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 o f power and airflow on the electronic excitation temperature, T-exc. Abel i nversion of radial profile chord averaged Fe I emission lines in the 370-37 7-nm range have been used to obtain localized profile measurements of T-exc inside the waveguide excitation region. In general, temperature profiles p eak 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 mai ntaining a constant flow rate of 14 Ipm, a 55% increase in microwave power from 0.9 to 1.4 kW causes a similar to 100% increase in plasma volume witho ut any noticeable effect on the central T,,, value. At a constant microwave power of 1.4 kW, an increase in total flow rate from 11 to 28 Ipm decrease s the volume of the plasma by similar to 25% and increases the central T-ex c by similar to 13%. The axially peaked temperature profiles are consistent with an electron density of similar to 10(13) cm(-3).