QUASI-LINEAR SATURATION OF DRIFT WAVES IN THE PRESENCE OF A BACKGROUND TRANSPORT PROCESS

The behaviour of drift waves localized in a narrow layer in a plasma w ith a density gradient is discussed. In zero order the plasma is assum ed to be in a steady state in which ionization is balanced by a plasma transport process independent of the drift waves, which leads to a fl ux of plasma through the drift wave layer. It is shown that this plasm a flux interacts with the drift wave to increase the damping rate by a n amount inversely proportional to the density gradient. Thus as the d rift wave amplitude increases, the density gradient flattens until the growth rate is reduced to zero. The theory leads to specific predicti ons of the saturated amplitude and the drift wave transport: the ampli tude rises until the plasma excursion is equal to DELTA, the half-thic kness of the drift wave layer, and the additional diffusivity is 1/2 g ammaDELTA2 where gamma is the linear growth rate at zero amplitude. Th is theory is shown to agree well with experimental results from the SH EILA heliac, while for comparison a fully nonlinear theory is shown to predict amplitudes much larger than those observed.