Basic equations of quasi-isotropic approximation (QIA) of geometrical optics method are presented, which describe electromagnetic waves propagation in weakly inhomogeneous and weakly anisotropic media. It is shown that in submillimiter range of electromagnetic spectrum plasma in all modern thermonuclear reactors, both acting and under construction, manifest properties of weakly inhomogeneous and weakly anisotropic medium, even for extreme electron density Ne ~ 1014 cm-3 and magnetic field B0 ~ 5 T accepted for project ITER. In these conditions QIA serves as natural theoretical basis for plasma polarimetry in tokamaks and stallarators. It is pointed out that Stokes vector formalism (SVF), widely used in polarimetry, can be derived from QIA in a generalized form, admitting the rays to be curvilinear and torsiened. Other important result of QIA is development of angular variables technique (AVT), which deals directly with angular parameters of polarization ellipse and operates with the system of two differential equations against three equations in form of SVF.
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