Linear mode coupling of lower-hybrid waves in a tokamak

Abstract

The linear coupling of electromagnetic modes propagating in a toroidal device is studied in detail, using a global or full wave approach. This study is directly linked to the spectral energy gap observed in lower-hybrid current drive experiments, but it is formulated in quite general terms and can eventually be applied to other kinds of waves. Exact solutions for the mode amplitudes and mode energies are derived, in two distinct regimes. The first is the degenerate regime corresponding to a single eigenfrequency and is equivalent to the fixed phase approximation used in the theory of nonlinear wave interaction. The second is the nondegenerate regime corresponding to different eigenfrequencies and is equivalent to the random phase approximation of the nonlinear theory. The stochastic or diffusive regimes studied in previous publications are also obtained when the quasiclassical or quasicontinuum limit of the nondegenerate regime is considered.