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Terahertz sensing of non-equilibrium microplasmas

The characterization of plasma by microwaves has been performed for decades. Generally, all known microwave approaches can be classified as analysis of either the wave interference within a quasi-transparent plasma or the absorption near the so-called critical point. These approaches can be generalized for the study of media containing absorbing space-localized irregularities. Essentially, the problem is to identify the power transmittance dependence on the shape, size and refraction or absorption of these irregularities. Moreover, the probing field should have a duration sufficiently short compared to the time scale of the irregularity evolution in order to clearly image the medium. The generalized analysis involves a diffraction theory development for typical plasma formations. The Khizhnyak method of integral equations is exploited here for this purpose. Solutions are obtained in a robust analytical form that allows constructive inversion to determine the particular plasma properties that produce the observed diffraction pattern. The specific cases of a flat plasma layer, plasma cylinder and plasma sphere are described in this manner. The proposed analysis can be implemented experimentally with a compact, tunable, pulsed terahertz source. This development is discussed as well.