Effect of surface modes on coupling to fast waves in the lower hybrid range of frequencies

The effect of surface modes of propagation on coupling to fast waves in the lower hybrid range of frequencies is studied theoretically and experimentally. The previously reported 'up-down' poloidal phasing asymmetry for coupling to a uniform plasma is shown to be due to the properties of a mode which carries energy along the interface between the plasma and the conducting wall. Comparison of the theory with coupling experiments performed on the PLT tokamak with a phased array of twelve dielectric loaded waveguides at 800 MHz shows that the observed dependence of the net reflection coefficient on toroidal phase angle can be explained only if the surface wave is taken into account. (author). 39 refs, 10 figs