Results of the PROMISE experiment on helical magnetorotational instability

The PROMISE experiment relies on the fact that the critical Reynolds number for the appearance
of the magnetorotational instability (MRI) in liquid metal flows drastically decreases when the the purely axial magnetic field is replaced by a helical one. We report the results of two series of experiments which differ slightly in the radial electric boundary conditions. It is shown that the helical MRI emerges as a travelling wave with a frequency that is in good correspondence with numerical predictions. Special focus is laid on the role of the radial jet region where the two Ekman vortices from the top and the bottom meet each other.