Plasma interferometry at high pressures

Abstract

A general formulation for the measurement of plasma density and effective collision frequency for lowly as well as highly, collisional plasmas using millimeter wave interferometry is presented. In the presence of high density and collisionality at high gas pressures where the collision frequency (ν) is of the order of both the plasma (ωp) and the wave frequency of the millimeter wave (ω) (ν∼ω,ωp), the measured line-average plasma density has a complex dependence on phase shift as well as the amplitude change of the millimeter wave signal. The measurement scheme and analysis presented in this article show that for collisional plasmas, simultaneous measurement of the phase change and the amplitude change data is required to uniquely determine the plasma density and collision frequency. The treatment allows the application of millimeter wave interferometry to a wide range of relative collision frequency, wave frequency and plasma frequency since it uniquely determines the line-average plasma density and effective collision frequency using the phase and amplitude change data. This diagnostic is particularly valuable at plasma densities (ne<1014/cm3) and high pressures (P>10 Torr) where Langmuir probe and optical emission diagnostics including the Stark effect are not accurate for density measurements.