Waveguide array excitation of lower hybrid fields in a tokamak plasma

The launching of RF waves from a two- and four-waveguide array is studied. From Brambilla's (1976) grill theory, analytic expressions for the reflection coefficients, the power spectrum, and the fields in the slab model of a cold inhomogeneous plasma are derived. As a first approximation to the coupling problem, only the fundamental mode in the waveguides is considered, the fast wave in the plasma is neglected, and the density profile near the plasma edge is assumed to be linear. It is shown that for a four-waveguide array with alternate 0, pi phases, optimum coupling is obtained when the inside waveguides carry about four times the power of the outside ones. This reduces and equalizes the waveguide reflection coefficients and avoids excessive shifts of the penetrated power spectrum to low values of the wave number parallel to the magnetic field.