Place: 2241 Chamberlin Hall (Coffee and cookies at 3.30 pm)
Speaker: Hantao Ji, Center for Magnetic Self-Organization in Laboratory and Astrophysical Plasmas, Princeton Plasma Physics Laboratory, Princeton University
Abstract: Rapid angular momentum transport in accretion disk plasmas has been a longstanding astrophysical puzzle. Since Keplerian flows are linearly stable in hydrodynamics, there exist only two viable mechanisms for the required turbulence: nonlinear hydrodynamic instability or linear magnetorotational instability (MRI). The latter is considered operating in hot disks ranging from quasars and X-ray binaries to cataclysmic variables. The former has been proposed mainly for cooler protoplanetary disk plasmas, whose Reynolds numbers are enormous. Despite their popularity, however, both candidate mechanisms have been rarely demonstrated and studied in the laboratory. In this talk, I will describe a novel laboratory experiment in a short Taylor-Couette flow geometry intended for such purposes. The experiments in water have shown, rather surprisingly, that quasi-Keplerian flows at Reynolds numbers as large as two millions are essentially laminar, effectively ruling out hydrodynamic turbulence as a candidate mechanism. The experiments in liquid gallium eutectic by imposing an axial magnetic field have shown the emergence of nonaxisymmetric modes identified as magnetocoriolis waves, one branch of which should become the MRI at higher speeds.