Bulletin of the American Physical Society
2006 APS April Meeting
Saturday–Tuesday, April 22–25, 2006; Dallas, TX
Session J6: Laboratory Astrophysics: Dynamo and the Magnetorotational Instability |
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Sponsoring Units: DPP DAP Chair: Steven Cowley, University of California, Los Angeles Room: Hyatt Regency Dallas Cumberland J |
Sunday, April 23, 2006 1:15PM - 1:51PM |
J6.00001: Angular Momentum Transport in Accretion Flows Invited Speaker: Accretion disks are central to astrophysical phenomena ranging from planets to black holes and quasars. They can be powerful energy sources: quasar disks convert rest mass to radiant energy more efficiently than fusion. Accretion requires a mechanism to dissipate orbital energy. Evidence that the mechanism is MHD turbulence driven by magnetorotational instability (MRI) is summarized, and the drawbacks of alternative mechanisms are briefly discussed. Despite theoretical and numerical advances, fundamental questions about MRI remain open: in particular, mechanisms of saturation, tolerance of non-ideal MHD effects, and relevance to magnetic dynamos. The prospects for addressing these questions through laboratory experiments is assessed. [Preview Abstract] |
Sunday, April 23, 2006 1:51PM - 2:27PM |
J6.00002: Magnetic Field Generation in the Madison Dynamo Experiment Invited Speaker: The Madison dynamo experiment is designed to self-generate magnetic fields from flows of conducting metal in a simply connected spherical geometry. Thus far, the experiment has been operated at $\sim$60\% of its design speed (1/3 of its maximum power) achieving a Magnetic Reynolds numbers (based on propellor tip speed) of 130 (comparable to the core of the Earth). The technical operation of the experiment (transfers of liquid sodium, rotating seals, etc) has been demonstrated. The experimental approach to understanding the electromagnetic properties of the flowing sodium involves comparisons between experimental measurements of the magnetic field in the sodium experiment, measurements of the velocity field in a dimensionally identical water experiment, and predictive MHD codes that model the currents induced in the turbulent flows by externally applied fields. Self-excitation has not yet been observed, rather experiments have been performed using externally applied magnetic fields. Initial experimental results include: direct observation of the production of a toroidal (an east-west component inside the sphere) magnetic field from a dipolar magnetic field; the expulsion of an applied magnetic field by vortical fluid motion; generation of a dipole magnetic field by a mean-field EMF; measurement of the turbulent shredding of a large scale magnetic field by small scale turbulence in the velocity field; and intermittent self-excitation of the dynamo eigenmode. Future plans will be discussed, including the strategy for observing self-excitation. [Preview Abstract] |
Sunday, April 23, 2006 2:27PM - 3:03PM |
J6.00003: Magnetorotational and dynamo instabilities in liquid sodium experiments Invited Speaker: Experiments conducted in liquid sodium confined between a rotating inner sphere and a concentric outer sphere show a host of MHD behavior. With a sufficiently strong external coaxial magnetic field, this system exhibits the magnetorotational instability. The primary instability shows a continuous bifurcation to a rotating $m=1$ pattern in the magnetic field, and concomitant oscillations in the velocity field. By varying both the rotation rate and the external magnetic field (made dimensionless as the magnetic Reynolds number and the Lundquist number respectively), we have navigated the parameter plane to observe a number of states with distinct dominant wavenumber and parity. The onset conditions compare favorably with expectations from linear stability calculations. This is remarkable considering the significant background turbulence (15\% to 25\% turbulent intensity) in the base state. Finally we have explored the suitability of spherical Couette flow to show dynamo action. [Preview Abstract] |
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