Magnetic field dynamos and magnetically triggered flow instabilities

Abstract

Magnetic fields of planets, stars and galaxies are generated by self-excitation in moving electrically conducting fluids. Once produced, magnetic fields can play an active role in cosmic structure formation by destabilizing rotational flows that would be otherwise hydrodynamically stable. For a long time, both hydromagnetic dynamo action as well as magnetically triggered flow instabilities had been the subject of purely theoretical research. Meanwhile, however, the dynamo effect has been observed in large-scale liquid sodium experiments in Riga, Karlsruhe and Cadarache. In this paper, we summarize the results of liquid metal experiments devoted to the dynamo effect and various magnetic instabilities such as the helical and the azimuthal magnetorotational instability and the Tayler instability. We discuss in detail our plans for a precession-driven dynamo experiment and a large-scale Tayler-Couette experiment using liquid sodium, and on the prospects to observe magnetically triggered instabilities of flows with positive shear.

Self-excitation in a helical liquid metal flow: the Riga dynamo experiments

“…Important new results were, among others, the observation of magnetic field reversals in the French VKS experiment (Berhanu et al 2010), and the experimental demonstration of the helical (Stefani et al 2006) and azimuthal (Seilmayer et al 2014) magnetorotational instability as well as the current-driven Tayler instability (Seilmayer et al 2012). A comparative survey of these, and further dynamo related experiments, can be found in the review papers by and Stefani et al (2017).…”

“…Important new results were, among others, the observation of magnetic field reversals in the French VKS experiment (Berhanu et al 2007;Gallet et al 2012;Miralles et al 2014), the observation of an unexpected axisymmetric induced field at the dynamo experiment in Madison (Spence et al 2006), the accurate determination of a turbulence-enhanced resistivity in Perm (Frick et al 2010), the experimental demonstration of the helical (Stefani et al 2006) and azimuthal (Seilmayer et al 2014) magnetorotational instability (MRI) and the current-driven Tayler instability (Seilmayer et al 2012) and the identification of an MRI-like mode in a turbulent spherical Couette flow in Maryland (Sisan et al 2004). A comparative survey of these, and further dynamo related experiments, can be found in the review papers by , Verhille et al (2010) and Stefani et al (2017).…”

MHD-Computersimulationen zur Begleitung des Projektes DRESDyn

“…After this, the time was right for a renewed attempt for a purely homogeneous dynamo without any assisting measures to guide the flow except the precessing container similar to the work of Gans. This is currently done at the facility of DRESDyn next to other experiments examining magnetohydrodynamics effects like the magnetorotational instability (MRI) and Taylor instability (TI) , Stefani et al, 2017. In comparison to Gans' experiment at DRES-Dyn a sodium filled cylinder of 2 m in height and diameter will rotate around its main axis with a frequency of 10 Hz and 1 Hz around its precession axis trying to reach a magnetic Reynolds number of Rm = µ 0 σω D R 2 ≈ 700 with the magnetic permeability µ 0 , electric conductivity of sodium σ, radius R and a rotation rate ω D while achieving an Ekman number of Ek = ν ω D (2R) 2 ≈ 10 −8 with viscosity ν.…”