Comparison of Plasma Densities Measured by the Microwave Reflection Probe and those from Stark Broadening

Abstract

When microwaves are incident perpendicularly upon plasmas through a dielectric plate, the plasma densities, which are much higher than the cutoff density, can be sensitively measured. It is shown that, when the dielectric plate has 1/4 wave length thickness, the small differences in the phase angles of the reflection coefficients are most effectively expanded. This means that the range of the electron density measureable by the microwave reflection probe previously proposed can be much expanded by such a modification. Using an 8mm wave reflection probe with the dielectric plate at its end, one can detect the plasma density up to 1019 cm-3. The reflection probe is used to measure the plasma density behind the reflected shock wave in a T-shaped electromagnetically driven shock tube filled with hydrogen or helium of a few mmHg. The results are compared with those measured from Stark broadening of the H β line or HeI (4471Å). They show a good agreement within experimental errors.In addition, these measured densities are compared with the theoretical values predicted from Rankine-Hugoniot and Saha equations. It is confirmed that the measured densities agree with the theoretical values for higher Mach numbers and that the measured densities, which are much less than the theoretical values for lower Mach numbers, are not in the errors at the microwave reflection measurements.