Bulletin of the American Physical Society
2007 APS April Meeting
Volume 52, Number 3
Saturday–Tuesday, April 14–17, 2007; Jacksonville, Florida
Session J10: Momentum Transport in Astrophysical and Fusion Plasmas |
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Sponsoring Units: GPAP DPP Chair: Philipp Kronberg, Los Alamos National Laboratory Room: Hyatt Regency Jacksonville Riverfront City Terrace 6 |
Sunday, April 15, 2007 10:30AM - 11:06AM |
J10.00001: Spontaneous Rotation in Tokamak Plasmas Invited Speaker: Spontaneous toroidal rotation has been observed in Alcator C-Mod tokamak plasmas with no external momentum input. The magnitude of the rotation ranges from --60 km/s in discharges with low energy confinement (L-mode) to +140 km/s in plasmas with good energy confinement (H-mode). The rotation in L-mode plasmas is found to depend strongly and in a complicated fashion on the electron density, the plasma current and the magnetic topology, and is typically in the counter-current direction. In contrast, the rotation velocity in H-mode discharges is observed to scale linearly with the plasma stored energy (or plasma pressure) normalized to the plasma current, a relatively simple dependence, and is directed co-current. Immediately following the abrupt transition from L-mode to H-mode, the co-current rotation appears near the plasma edge and propagates to the center on a time scale similar to the energy confinement time, but anomalously fast compared to the classical (collisional) momentum diffusion time. Very similar scalings in H-mode plasmas have been made on many tokamaks worldwide in a variety of operating conditions, indicating the fundamental nature of spontaneous rotation. A universal scaling is beginning to emerge with an eye toward prediction of the level of rotation expected in future devices such as ITER. At present there is no comprehensive theory which explains this phenomenon. [Preview Abstract] |
Sunday, April 15, 2007 11:06AM - 11:42AM |
J10.00002: Laboratory Studies of Angular Momentum Transport in Astrophysically Relevant Flows Invited Speaker: Rapid angular momentum transport in accretion disks has been a longstanding astrophysical puzzle. Molecular viscosity is inadequate to explain observationally inferred accretion rates. Since Keplerian flow profiles are linearly stable in hydrodynamics, there exist only two viable mechanisms for the required turbulence: nonlinear hydrodynamic instability or magnetorotational instability (MRI). The latter is regarded as a dominant mechanism for rapid angular momentum transport in hot accretion disks ranging from quasars and X-ray binaries to cataclysmic variables. The former has been proposed mainly for cooler protoplanetary disks, whose Reynolds numbers are typically large. Despite their popularity, however, both candidate mechanisms have been rarely demonstrated and studied in the laboratory. In this paper, I will describe a laboratory experiment at Princeton in a short Taylor-Couette flow geometry intended for such purposes. Based on the results from prototype experiments and simulations, the apparatus contains novel features for better controls of the boundary-driven secondary flows. The experiments in water have shown that nonmagnetic quasi-Keplerian flows at Reynolds numbers as large as $2 \times 10^6$ are essentially laminar [1]. Scaled to accretion disks, rates of angular momentum transport lie far below astrophysical requirements. By ruling out hydrodynamic turbulence, our results indirectly support MRI as the likely cause of turbulence even in cool disks. Initial results on MRI, using a liquid gallium eutectic, will be also discussed if available. \\ $[1]$ H. Ji, M. Burin, E. Schartman, \& J. Goodman, Nature 444, 343-346 (2006). \\ [Preview Abstract] |
Sunday, April 15, 2007 11:42AM - 12:18PM |
J10.00003: Angular Momentum Transport in Astrophysical Accretion Flows Invited Speaker: Accretion of plasma onto a central object is responsible for many of the most energetic phenomena observed in astrophysics; stars, planets, and galaxies are also formed via such accretion disks. I summarize the physics of angular momentum and energy transport in astrophysical disks. A linear instability of differentially rotating plasmas -- the magnetorotational instability (MRI) -- amplifies magnetic fields and gives rise to MHD turbulence in accretion disks. Magnetic stresses due to MHD turbulence transport angular momentum, allowing plasma to accrete. In addition, the gravitational potential energy of the inflowing plasma is converted into heat via the action of MHD turbulence -- powering the radiation we see from accretion flows. I highlight recent work on the physics of a particular class of accretion flows onto black holes and neutron stars, in which the inflowing plasma is macroscopically collisionless and kinetic effects are crucial for the angular momentum and energy evolution of the accretion flow. [Preview Abstract] |
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