Daniel R. Sisan

Experimental observation and characterization of the magnetorotational instability

Differential rotation occurs in conducting flows in accretion disks and planetary cores. In such systems, the magnetorotational instability can arise from coupling Lorentz and centrifugal forces to cause large radial angular momentum fluxes. We present the first experimental observation of the magnetorotational instability. Our system consists of liquid sodium between differentially rotating spheres, with an imposed coaxial magnetic field. We characterize the observed patterns, dynamics, and torque increases, and establish that this instability can occur from a hydrodynamic turbulent background.

Lorentz force effects in magneto-turbulence

Abstract We experimentally characterize magnetic field fluctuations in a strongly turbulent flow of liquid sodium in the presence of a large externally applied field. We reach high interaction parameter (up to N =17) for moderate magnetic Reynolds number (up to Re m =18), a previously unexplored parameter range for liquid metal flows. As the interaction parameter (i.e. the ratio of Lorentz to inertial forces) is increased, the system passes through distinct regimes, which we classify. We find that for certain ranges of the applied magnetic field, particularly at high values, the induced magnetic field exhibits large, coherent oscillations. Spatial structure in these induced field oscillations suggests the formation of non-axisymmetric vortices that precess at a fraction of the impeller rotation rate. We also investigate the effect of rough versus smooth boundaries and relate these results to topographic core–mantle coupling in the Earth.

Hunting For Dynamos: Eight Different Liquid Sodium Flows

In attempting to create a laboratory scale dynamo, an experimentalist is faced with a daunting question: What sort of flow can I produce that will yield a dynamo? We present eight variations of a flow motivated by the s2t2 flow numerically studied by Dudley and James [1]. Pulse decay measurements of an externally applied magnetic field are used to quantify the approach to transition to dynamo action.

Characterization of the magnetorotational instability from a turbulent background state

Experiments in spherical Couette flow (flow between concentric rotating spheres) with an imposed magnetic field have yielded induced magnetic fields consistent with the magnetorotational instability. This might be expected due to the decreasing rotation rate profile in the base state. The observation is at odds though with existing theory, in that the base state has a significant turbulent component. We characterize the observed induced magnetic fields, as well as the velocity disturbance underlying the instability. The saturated state shows a variety of patterns and dynamics depending on applied magnetic field strength and rotation rate. The observed phase diagram is in qualitative agreement with linear stability theory. We also compare the observed stability diagram with that of MHD instabilities calculated by Hollerbach and Skinner.

Liquid Sodium Experiments: The Effect of Turbulence and Lorentz Forces

We are pursuing several liquid sodium experiments in order to understand magnetic field generation in astrophysical bodies. In one experiment, pulse decay techniques using small externally applied magnetic fields quantify the system’s approach toward a dynamo. Using the same apparatus, a large constant external magnetic field was applied modifying the flow through the Lorentz force. The induced field fluctuations indicate four distinct regimes that were uniquely quantified by the Elasasser number, the ratio of the Lorentz force to the advective force. Two future experiments are also described briefly.