Far-forward scattering diagnostics of an Alfven wave heating experiment

Long-wavelength density perturbations in a plasma present a particularly difficult diagnostic problem. A number of related techniques have been proposed and used by various authors. These include homodyne scattering, small k scatterring far-forward scattering, Fraunhofer diffraction, phase scintillation and phase contrast imaging. In this paper we describe measurements of driven density fluctuations during Alfven wave heating in the small tokamak TORTUS using the far-forward scattering technique. With the aid of a model of the interaction of the laser beam with the plasma it has been possible to distinguish between two resonance conditions in the plasma: The global discrete Alfven wave resonance of the slow Alfv n wave and the localised resonance where the slow wave mode converts to a kinetic Alfven wave. It is the latter resonance which is of interest for heating tokamak plasmas. (orig.)