Comparison of coupling to 5 GHz lower hybrid waves and 0.5 GHz helicon waves

We compare, qualitatively and with a simple semi-quantitative model, the difficulty of coupling to fast waves at 0.5 GHz (’whistlers’ or ’helicons’) and to slow waves (’lower hybrid waves’) at 5 GHz and n||=3 under conditions relevant to the DIII-D tokamak. Though the density at which the slow wave begins to propagate is much lower than the fast wave, the stronger evanescence in vacuum for the higher frequency wave makes excitation of the SWs more difficult than for the FW case. Enforcing a minimum density at the coupler face, as might be produced by local gas puffing, is shown to likely be more effective for the SW case. The fact that the surface admittance for the slow wave at 0.5 GHz is much larger than for the FW at the same frequency has possibly significant implications for attempts to excite the FW.