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Monday, March 19, 2012
3:00 PM - 4:00 PM
CNLS Conference Room (TA-3, Bldg 1690)

Colloquium

Magnetic Dynamos in the Lab: Progressing from Liquid Metal to Plasmas

Cary Forest
University of Wisconsin - Madison

Every astrophysical plasma is, as far as we can measure, magnetized and turbulent. How these magnetic fields spontaneously self-generate, through a process called the dynamo, and then act back on their surroundings is a central question in plasma astrophysics. Dynamos occur in plasmas that range from relatively small and dense stellar plasmas to diffuse plasmas in galaxy clusters, and can have impacts that range from making life possible on Earth to controlling accretion onto blackholes. Laboratory experiments on dynamos have been pursued for the last decade using large volumes of fast flowing, electrically neutral liquid metals to create conditions in which magnetic induction dominates resistive dissipation of electrical currents. Some of these experiments are self-exciting magnetic dynamos: configurations that spontaneously self-generate magnetic fields at the expense of the energy in the flow. This talk begins with an overview of the basics of dynamos, also demonstrating how dynamos in the lab relate to natural dynamos in the Earth (liquid metal), and the Sun (plasma). Laboratory experiments can help explain dynamo processes through direct measurement and help validate theoretical and numerical models. As one example of this, our group has recently measured the turbulent electromotive force (through correlated fluctuations of velocity and magnetic field) in a 200 horsepower, 1 meter diameter liquid sodium experiment. We have directly observed how turbulence, on average, enhances the effective resistivity of the liquid metal and more rapidly transports magnetic flux. Recent experiments on a novel plasma device will then be described that establish the feasibility for creating a large, steady-state, fast flowing, weakly magnetized, hot plasma, exhibiting all of the critical parameters for dynamo studies. Remarkably, by changing plasma composition and density, the viscosity (and therefore the fluid Reynolds number) can be independently controlled. Finally, we describe a much larger device, the Madison Plasma Dynamo Experiment (MPDX), now under construction, which is projected to extend the accessible parameter space to a regime much closer to astrophysical dynamos than liquid metals.

Host: Jonathan Graham