The paper is devoted to the nonlinear evolution of the single tearing type perturbation in a plasma configuration with magnetic shear. The primary effect in the study of the nonlinear dynamics of the magnetic perturbation is the modification of the orbits of resonant particles caused by the chain of magnetic islands, moving along the magnetic surface due to drift effects. The structure of a parallel electric field and its influence on ion dynamics, including the so-called 'integral' effects dealing with the finiteness of the Larmor radius, are incorporated in the nonlinear analysis. Nonlinear growth rates and saturation widths of the magnetic islands are evaluated. The drift tearing mode is found to have metastable properties: the perturbations of finite amplitude which are damped in the linear approximation appear to be nonlinearly unstable. The authors discuss the energetics of such a hard regime of instability onset and the nonlinear stabilization mechanisms. The influence of a uniform stationary plasma background (BG) is considered. The presence of the cold dense BG slows down the growth of aperiodical instability and decreases the threshold of its nonlinear saturation. The field-aligned oscillation of BG electrons has a strong stabilizing effect on the drift tearing mode, even if their density is relatively small. When the density of the BG plasma exceeds the density of the current-carrying plasma, drift tearing modes cannot exist. The presence of the BG plasma strengthens the metastable properties of configurations with magnetic shear. The nonlinear growth rates and thresholds of linear and nonlinear stabilization are evaluated for a wide range of parameters (densities, temperatures) of current-carrying and BG plasmas.
Nonlinear evolution and deformation of driven magnetic islands in rotating plasmas