The behaviour of drift waves localized in a narrow layer in a plasma with a density gradient is discussed. In zero order the plasma is assumed 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 flux of plasma through the drift wave layer. It is shown that this plasma flux interacts with the drift wave to increase the damping rate by an amount inversely proportional to the density gradient. Thus as the drift wave amplitude increases, the density gradient flattens until the growth rate is reduced to zero. The theory leads to specific predictions of the saturated amplitude and the drift wave transport: the amplitude rises until the plasma excursion is equal to Delta , the half-thickness of the drift wave layer, and the additional diffusivity is 1/2 gamma Delta 2 where gamma is the linear growth rate at zero amplitude. This theory is shown to agree well with experimental results from the SHEILA heliac, while for comparison a fully nonlinear theory is shown to predict amplitudes much larger than those observed.
Evolution of the wavenumber range during the development of the drift instability