Improved measurement of the electron temperature of a low density shock-heated argon plasma by means of microwaves

Abstract

The determination of electron temperature of a low density shock-heated argon plasma (3 Torr ≤p1≤ 10 Torr, 5500 °K ≤ 9500 °K) by means of microwave diagnostics is improved with a test section which permits the simultaneous transmission of two microwave beams at different frequencies, and with two corrections applied to the measured attenuation of transmitted waves. The purpose of these corrections is to obtain the true attenuation due to wave power dissipation into the plasma, which is dependent on the electron temperature and density. The first correction takes into account the diffraction of waves by apertures of the test section and the wave defocusing by the plasma. The second correction considers the effect of wave beam bending due to the electron density gradient in the flow direction of the plasma. Both corrections diminish the measured wave attenuation. Consequently a lower ratio of attenuation to phase shift of transmitted waves is determined, which in turn yields lower values of electron collision frequency and electron temperature. This report describes the electron temperature evaluation technique in detail, and presents results. The electron temperature values obtained have a range of uncertainty of – 20 % and +10% only, with exceptions at the beginning of the test time. In most tests, the electron temperature is equal or lower by approximately 1000 °K in comparison with the heavy particle temperature.