Effect of toroidicity during lower-hybrid mode conversion

Abstract

The effect of toroidicity during lower-hybrid mode conversion is examined by treating the wave propagation in an inhomogeneous medium as an eigenvalue problem for ω2 (m, n), m, n poloidal and toroidal wavenumbers. Since the fre-quency regime near ω = ω2LH is an accumulation point for the eigenvalue spectrum, the degenerate perturbation technique must be applied. The toroidal eigenmodes are constructed by a zeroth-order superposition of monochromatic solutions with different poloidal dependence m; thus they generically exhibit a wide spectrum in k‖ for given fixed ω2 even for small inverse aspect ratio є. When the average 〈k‖〉 is in the neighbourhood of kmin, the minimum wave-number for accessibility of the mode conversion regime, it is possible that excitation of toroidal modes rather than geometrie optics may determine the wave coupling to the plasma. Our results are not changed significantly by a small amount of dissipation. The level of density fluctuations in modem tokamaks, on the other hand, may cause enough k‖ scattering to mask the toroidicity effects. Nevertheless, it is shown that a wide k‖ spectrum excited by a monochromatic pump will persist even with vanishing fluctuation level.