Characteristics of a microwave plasma torch with a coaxial field-structure at atmospheric pressure

An atmospheric microwave plasma torch is developed using a DC magnetron at 2.45 GHz. We introduce a waveguide-based coaxial field-structure in the plasma torch region, which consists of a waveguide hole boundary in the wide wall (H-plane) of a tapered and shorted WR-284 waveguide. The nozzle penetrates perpendicularly through the H-planes with its tip aligned at the outer plane. The flame-like plasma is formed above the nozzle according to the electric-field distribution radiated from the waveguide hole. The plasma flame appears to be the same as the flame of a burnable gas torch with high gas flow rate; the flame melts molybdenum easily with an average microwave power of 200 W and 5.0-liter per minute (lpm) air flow. The spectroscopic measurement for the gas temperature verified that the plasma flames are in the proximity of local thermodynamic equilibrium (LTE) based on the Griem criterion. An ion drift is also seen to exist in the plasma flame from the single Langmuir probe measurement. This plasma torch system is also applied to the treatment of benzene, a volatile organic compound. From the experimental results, 98 % of a high level of 900-ppm benzene is removed easily with an average microwave power of 250 W. For good microwave power coupling into the plasma flame, the hole diameter and the nozzle size are determined by using high frequency structure simulator (HFSS) program.