Extension of Measurable Density Ranges in Microwave Plasma Diagnostics

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Abstract
In order to apply microwaves for measuring the time-varying electron densities down to their low values in gaseous plasmas, a technique is developed, which can increase the small phase difference between the microwaves transmitted through a path with and without plasma by means of a dielectric plate of 1/4 wave length thickness. By using two dielectric plates, the electron density lower by at least one order of magnitude than that measured by the previous method is measured in a decaying afterglow plasma in a waveguide. The sensitivity of this method is comparable to that by a cavity technique which is sensitive particularly in measuring low plasma densities but is not always convenient in probing plasmas in free space. The present system, however, has a wide application even in plasmas in free space, if a doubled path is established with a reflector. In addition, this technique has another advantage that the sensitivity can be changed by a very simple procedure. The electron density from 108 to 1018 cm-3 for a plasma of one wave length thickness is detectable by the present technique at X-band (9000 Mc) microwave frequency.