Density and temperature measurements of a torch plasma

A diagnostic method based on microwave absorption is being developed to measure the density and temperature of torch plasma. It uses an arrangement, which streams torch plasma through aligned holes on the bottom and top walls of a rectangular waveguide to act as a dielectric post. The dielectric post has a complex dielectric constant ε = ε′ - j ε″ and can be represented by an equivalent two-port T-circuit with specified shunt and series impedances. Marcuvitz has analyzed this problem in the Waveguide Handbook. The analytical solution is the basis of the diagnostic technique. A 10 GHz microwave and a rectangular waveguide of dimensions 1 in. × 1/2 in. are used in the experiment. The holes on the waveguide top and bottom walls for torch plasma to stream through have a diameter of 1/2 in.. A network analyzer is used to measure the TE₁₀ mode transmission coefficient of this two-port network. Since the amplitude and phase of the transmission coefficient depend on the dielectric property of torch plasma, the measured results establish two independent relations for the two unknowns, the density and temperature, in the dielectric function of torch plasma. The experiment and results will be presented.