Laboratory studies of magnetic vortices. II. Helicity reversal during reflection of a magnetic vortex at a conducting boundary

The reflection of a magnetic vortex from a conducting boundary is studied e xperimentally in a large laboratory plasma. The parameter regime is that of electron magnetohydrodynamics and the vortex consists of a spheromak-like magnetic field perturbation propagating in the whistler mode along a unifor m background magnetic field. In this work we focus on the helicity properti es of the vortex magnetic field, electron velocity, and vorticity. The refl ection conserves magnetic energy but reverses the sign of all helicities. T he change in topology arises from a self-consistent reversal of one linked vector field without involving helicity injection, reconnection, or dissipa tion processes. The breakdown of helicity conservation and the frozen-in co ncept is explained by the presence of a vacuum-like sheath at the plasma-bo undary interface. (C) 1999 American Institute of Physics. [S1070-664X(99)03 108-0].