Direct antenna excitation of the shear Alfven wave at finite frequency in the boundary of a fusion plasma

The problem of direct excitation of the shear Alfven wave by antennas in the boundary of a fusion plasma is considered. In the plasma scrape-off layer the physics of this wave is determined by finite electron inertia. Expressions for the wave electric and magnetic field are derived without any assumption on the smallness of the ratio of frequency to ion cyclotron frequency. It is shown that the shear Alfven wave propagates as a guided beam along the steady magnetic field over a wide range of frequencies below the ion cyclotron frequency, and is predominantly excited by current elements parallel to the steady magnetic field. A simple transmission-line analogy suggests that guidance of the Alfven wave is also expected in the presence of strong plasma inhomogeneity.