The generalized accessibility and spectral gap of lower hybrid waves in tokamaks

The generalized accessibility of lower hybrid waves, which may include shifting of the parallel refractive index (n∥), either upward or downward, is investigated, using the cold plasma dispersion relation and various geometrical constraint (GC) relations imposed on the behavior of n∥. It is shown that n∥ upshifting is bounded and can be insufficient to bridge a large spectral gap to cause wave damping, if the GC relation does not allow the oblique resonance to occur. The traditional n∥ upshift mechanism based upon the pitch angle of magnetic field lines is shown to lead to contradictions with experimental observations. An upshift mechanism brought about by the density gradient along field lines is proposed, which is not inconsistent with experimental observations, and provides plausible explanations to some unresolved issues of lower hybrid wave theory, including generation of ‘seed electrons.’