Guided propagation of the Alfven wave in a Tokamak

Experimental observations are presented of the magnetically guided Alfven wave excited directly by a small dipole loop antenna located in the scrape-off layer of a tokamak plasma. This wave is excited most efficiently by antenna current elements aligned along the magnetic field and measurements indicate that, at all frequencies below the ion cyclotron frequency, the wave propagates for several transits around the machine with a high degree of localisation about magnetic field lines intersecting the antenna. Along the field, the wave has both a slowly varying amplitude and phase with predominantly radial electric and azimuthal magnetic field components. These experiments demonstrate that the Alfven wave can propagate as a magnetically guided TEM mode in plasmas which are highly inhomogeneous. A simplified mathematical description of the wave is also presented. 20 refs., 8 figs