title: Helical magnetorotational instability in a Taylor-Couette flow with strongly reduced Ekman pumping.
abstract: 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 disks. 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.
authors.affiliation: Forschungszentrum Dresden-Rossendorf, Dresden, Germany.
authors.forename: Frank
authors.initials: F
authors.lastname: Stefani
authors.forename: Gunter
authors.initials: G
authors.lastname: Gerbeth
authors.forename: Thomas
authors.initials: T
authors.lastname: Gundrum
authors.forename: Rainer
authors.initials: R
authors.lastname: Hollerbach
authors.forename: Jānis
authors.initials: J
authors.lastname: Priede
authors.forename: Günther
authors.initials: G
authors.lastname: Rüdiger
authors.forename: Jacek
authors.initials: J
authors.lastname: Szklarski