Anomalous behaviour of the group velocity for ordinary waves in the electron-cyclotron region

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Abstract
In a recent paper, Tanaka et al. (1979) have shown that the group velocity nu g for ordinary modes (OM), calculated along the geometric optics rays in a plasma column with parameters typical of the present Tokamak generation, assumes values greater than c, the velocity of light in vacuum, inside the electron-cyclotron resonance layer (ECR). The ray-tracing is based on a purely classical dispersion relation, taking finite temperature effects into account. The angle between the wave vector and the toroidal magnetic field at the injection point has the value of 80 degrees . With this choice, the refractive index n|| parallel to the magnetic field satisfies the condition allowing a classical description, everywhere along the ray and, therefore, also within the ECR region. This condition requires that n|| >> nut/c where nut=(T/m)1/2 is the electron thermal velocity, T and m being the electron temperature and rest mass, respectively. The author extends the preceding analysis to the relativistic range of values of n||, i.e. n||t/c, in order to treat in particular, the case of exactly perpendicular injection (n|| = 0).