Abstract
A simple theory is developed to study the polarization of high-frequency electromagnetic waves in plasmas. The elliptical birefringence is separated into the effects of linear birefringence and optical activity for frequencies much above the plasma frequency. It is shown that linear birefringence and optical activity are essentially independent phenomena related respectively to the transversal and longitudinal magnetic fields. The theory is applied to study experimental results on the measurement of electron density and plasma current in Tokamaks. The conditions for pure linear birefringence or pure optical activity are derived. An equation is obtained for the polarization evolution on the Poincare sphere including collisional attenuation to first order.