NONLINEAR STABILITY AND EVOLUTION OF DRIFT-TEARING MODES (FLUID INSTABILITIES, NUMERICAL SIMULATION)

The question of the nonlinear stability and subsequent evolution of drift-tearing modes in tokamak-like environments is considered. The tearing mode and the drift physics are introduced, and a brief review of previous work given. A set of reduced equations for the drift-tearing mode is derived from two-fluid magnetohydrodynamics. The equations are specialised for small, nonlinear magnetic islands in slab geometry. These are used to scrutinise the results of linear stability theory in light of nonlinear physics arising from the effect of the mode on the equilibrium density and temperature profiles. It is shown that linearly growing drift-tearing modes are rendered stable at a very small island width by quasilinear thermal effects. However, both linearly and quasilinearly stabilised modes grow to large amplitude if the initial island width is larger than the linear tearing layer, demonstrating the importance of nonlinear considerations in predictions of stability.;Having concluded that drift-tearing modes will in fact be seen in present and near-future thermal regimes, their evolution is addressed. Observations of rotating m = 2 magnetic fluctuations in tokamak discharges are often attributed to diamagnetically propagating drift-tearing modes. It is shown, however, that this propagation ceases at small island width as the local density profile is flattened by sound waves. The critical width for density flattening is small compared to island widths typically inferred from the observed fluctuations. The rotation of these fluctuations must therefore result from radial electric fields, implying that observed rotation rates can be used as a local diagnostic for these fields.