Multiple states of nonlinear drift-tearing islands

Localized modulation of rotating waves in Taylor-Couette flow

We report the results of an experimental study on the multiplicity of states in Taylor-Couette flow as a result of axial localization of azimuthally rotating waves. Localized states have been found to appear hysteretically from time-dependent Taylor-Couette flow at Reynolds numbers significantly above the onset of wavy Taylor vortices. These localized states have the shape of a modulated rotating wave and differ significantly from global modulated wavy Taylor vortex states in their spatial characteristics. Axial localization of rotating waves is accompanied with a significant increase in size of the underlying pair of Taylor vortices. Our work reveals that localization provides a mechanism for the appearance of multiple time-dependent states in Taylor-Couette flow.

Generation and amplification of magnetic islands by drift interchange turbulence

We investigate the multiscale nonlinear dynamics of a linearly stable or unstable tearing mode with small-scale interchange turbulence using 2D MHD numerical simulations. For a stable tearing mode, the nonlinear beating of the fastest growing small-scale interchange modes drives a magnetic island with an enhanced growth rate to a saturated size that is proportional to the turbulence generated anomalous diffusion. For a linearly unstable tearing mode the island saturation size scales inversely as one-fourth power of the linear tearing growth rate in accordance with weak turbulence theory predictions. Turbulence is also seen to introduce significant modifications in the flow patterns surrounding the magnetic island.

Nonlinear dynamics of magnetic islands imbedded in small-scale turbulence

The nonlinear dynamics of magnetic tearing islands imbedded in a pressure gradient driven turbulence is investigated numerically in a reduced magnetohydrodynamic model. The study reveals regimes where the linear and nonlinear phases of the tearing instability are controlled by the properties of the pressure gradient. In these regimes, the interplay between the pressure and the magnetic flux determines the dynamics of the saturated state. A secondary instability can occur and strongly modify the magnetic island dynamics by triggering a poloidal rotation. It is shown that the complex nonlinear interaction between the islands and turbulence is nonlocal and involves small scales.

Natural velocity of magnetic islands

The phase velocity of magnetic islands is calculated in the semicollisional regime with cold ions. Two solution branches arise, corresponding to islands propagating with the ions and with the electrons. For the ion branch the phase velocity and the polarization current are small. For the electron branch, the phase velocity depends on the ratio of W, the half-width of the island, and rho(s), the Larmor radius calculated with the electron temperature. For W >>rho(s) the phase velocity is larger than the electron drift velocity and the polarization current is destabilizing. For W << rho(s), the situation is reversed provided that the density and temperature gradients point in the same direction.