Nonlinear Evolution of Drift-Tearing Modes

It is shown that diamagnetic propagation of drift‐tearing modes ceases at small island widths 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 observed magnetic fluctuations of m=2 in tokamak discharges. The rotation of these fluctuations must therefore result from radial electric fields.