Instabilités bidimensionnelles d'une couche libre cisaillée d'origine électromagnétique

The stability of a free shear layer, with one-directional velocity field, is addressed. The studied configuration is a special case among the quasi-two-dimensional motions, induced in an electro-conducting and incompressible fluid by a strong magnetic field, superimposed to electric current. The limit of unconditional stability of the shear flow is computed. When the electric current is under the threshold, every deviation from the basic state is monotonically damped. The result holds without any restriction, connected with the size of the perturbations. Patterns set on just above the unconditional stability limit, in the form of small vortices, on the slow side of the shear layer.