  The magnetorotational instability (MRI) is thought to play a key role in the
formation of stars and black holes by sustaining the turbulence in
hydrodynamically stable Keplerian accretion discs. In previous experiments the
MRI was observed in a liquid metal Taylor-Couette flow at moderate Reynolds
numbers by applying a helical magnetic field. The observation of this helical
MRI (HMRI) was interfered with a significant Ekman pumping driven by solid
end-caps that confined the instability only to a part of the Taylor-Couette
cell. This paper describes the observation of the HMRI in an improved
Taylor-Couette setup with the Ekman pumping significantly reduced by using
split end-caps. The HMRI, which now spreads over the whole height of the cell,
appears much sharper and in better agreement with numerical predictions. By
analyzing various parameter dependencies we conclude that the observed HMRI
represents a self-sustained global instability rather than a noise-sustained
convective one.
