Atomic UV emission temperature profiles in an air microwave plasma torch

Abstract

Summary form only given, as follows. An atmospheric pressure microwave plasma torch has been shown to be a sensitive and accurate atomic excitation medium for stack exhaust compliance monitoring of trace metals air pollution. High electron excitation energies are needed for minimizing the detection limits of some important metal pollutants such as mercury and arsenic. A study has been made of the effects of power and airflow on the electronic excitation temperature, T/sub exc/, of a 0.9 - 1.5 kW, 2.45 GHz plasma torch. The plasma is sustained in a 28 mm internal diameter fused quartz tube which penetrates through the wide walls of a tapered and shorted WR-284 (72/spl times/17 mm cross-section) waveguide. Abel inversion of radial profile chord averaged Fe I emission lines in the 370 to 377 nm range have been used to obtain localized T/sub exc/ profiles inside the waveguide excitation region. Microwave power has little effect on temperature, but increased airflow from 14 lpm to 28 lpm constricts the plasma and increases temperature by about 13 %. Temperature profiles are relatively flat with a maximum central T/sub exc/ observed of 6550 K +/- 350 K at 28 lpm. The axially peaked temperature profiles and almost no microwave reflection (< 1%) are consistent with skin depth of order 10 mm and an electron density of /spl sim/ 10/sup 13/ cm/sup -3/.