Testing a very good microwave interferometer

A 4 mm microwave interferometer is described, together with a working system for numerical evaluation of the recorded signals. The interferometer signals are processed in a computer to provide immediate display of the plasma column density. The numerical evaluation makes it possible to eliminate the influence of also rather large imperfections in the components, and to take into account large and rapid variations in the strength of the transmitted microwave signal. The limits of performance of the system are tested in a pulsed plasma source. Column plasma densities are measured from 1015m-2 to 3x1018m-2, corresponding to phase swings from less than one degree up to several times 360 degrees. The upper limit corresponds to densities approaching the cutoff, 6x1019m-3, where reflection and refraction of the transmitted beam become severe. Measurements were possible even when the power in the transmitted branch was reduced (due to the plasma) by more than a factor 30, and when the transmitted power varied as rapidly as a factor 4 in 200 ns. (author)