Effects of Radius, Microwave Power, and Gas Flow Rate on the Electronic Excitation Temperature in an Atmospheric Pressure Microwave Plasma Torch

The microwave plasma continuous emissions monitor (MP-CEM) utilizes atomic emis-sion spectroscopy to study trace quantities of hazardous metals such as mercury and lead. With the aim of improving the MP-CEM sensitivity to these metals, a series of experiments are performed to characterize this microwave plasma operating at atmospheric pressure. All experiments use air as the working gas. Assuming local thermal equilibrium, a comparison of the relative emissivity val-ues of single-ionized iron lines yields the electronic excitation temperature, Texc, of the plasma. While maintaining a constant flow rate of 0.5 cfm, a 55 % increase in microwave power from 900 W to 1400 W leads to ∼100 % increase in plasma volume. The same increase in microwave power has no effect on the electronic excitation tem-perature. With a microwave power of 1400 W, an increase in total flow rate from 0.4 cfm to 1.0 cfm decreases the volume of the plasma by ∼25 % and increases the Texc by ∼10 %