GUIDED PROPAGATION OF THE ALFVEN-WAVE IN A TOKAMAK

Experimental observations are presented of the magnetically guided Alf ven wave excited directly by a small dipole loop antenna located in th e scrape-off layer of a tokamak plasma. This wave is excited most effi ciently by antenna current elements aligned along the magnetic field a nd measurements indicate that, at all frequencies below the ion cyclot ron frequency. the wave propagates for several transits around the mac hine with a high degree of localisation about magnetic field lines int ersecting the antenna. Along the field, the wave has both a slowly var ying amplitude and phase with predominantly radial electric and azimut hal magnetic field components. These experiments demonstrate that the Alfven wave can propagate as a magnetically guided TEM mode in plasmas which are highly inhomogeneous. We also present a simplified mathemat ical description of the wave.