Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session CM10: Mini-Conference: The Magnetic Universe -- A Mini-Conference in Honor of Stirling Colgate I |
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Chair: Hui Li, Los Alamos National Laboratory Room: Salon FGH |
Monday, October 27, 2014 2:00PM - 2:30PM |
CM10.00001: Is Transport in Accretion Disks Primarily Local or Non-local? Eric G. Blackman Accretion disks likely involve some combination of local and non-local angular momentum transport. Coronae and jets provide evidence for large scale transport and disk thermal emission may provide evidence for local transport. Identifying the principles that determine the relative local vs. nonlocal fraction poses a set of challenges and highlights a significant gap between numerical simulation results and improved, practical mean field accretion theory. The dominant mechanisms of transport may in fact be non-local and non-viscous. Even the magneto-rotational instability (MRI) for example, often invoked as a source of local turbulence, may produce predominantly non-local transport. I will overview progress and open issues on these themes, drawing in concepts from disk theory, dynamo theory, and corona formation. [Preview Abstract] |
Monday, October 27, 2014 2:30PM - 2:45PM |
CM10.00002: Magnetorotational dynamo instability in statistical models of shearing box turbulence Jonathan Squire, Amitava Bhattacharjee A large scale dynamo generating a strong azimuthal field is a fundamental component of the turbulence induced by the magnetorotational instability (MRI). The dynamo appears to be inherently time-dependent, producing well-defined butterfly diagrams, and is never kinematic even in its earliest stages, since without the magnetic field the MRI does not exist. In this talk we consider the dynamo in MRI turbulence in its simplest possible form, studying the zero net-flux unstratified shearing box. With the aim of isolating the core dynamo process, we remove as much of the nonlinearity as possible from the system, studying the statistics of driven linear fluctuations in a vertically dependent mean-field that evolves self-consistently due to Reynolds and Maxwell stresses. We find that homogeneous background turbulence becomes unstable above some critical parameter to a mean-field dynamo instability with a strong dependence on magnetic Prandtl number. This instability saturates to either time-independent or time-periodic states with characteristics that strongly resemble features of fully developed MRI turbulence. We discuss the driving and saturation terms in this MRI dynamo and the relation of these to the underlying nonmodal linear dynamics. [Preview Abstract] |
Monday, October 27, 2014 2:45PM - 3:05PM |
CM10.00003: Superbubble Explosions and the Galactic Dynamo Russell Kulsrud The alpha-omega dynamo appears to be the most likely origin for the galactic magnetic field. However, it has a major problem in that to complete the dynamo operation, flux of the wrong sign must be expelled. For normal situations this is no problem. However, in the case of the galactic disc, the combination of almost perfect flux freezing and a strong gravitational field strongly inhibit this expulsion. It is energetically impossible to expel straight magnetic lines from the disc because they would carry all their ISM with them and their gravitational binding energy is much too large. I propose that the lines can be expelled in a topological manner. This can be done by massive superbubble explosions that can expel a tiny piece of each line leading to a situation where the lines in the disc are broken and act like lines of finite length. Such lines can be random turned in the disc and cause the disappearance of any negative flux. If this proposal should be valid then the alpha-omega dynamo can work to amplify the a very weak field to the present galactic value. [Preview Abstract] |
Monday, October 27, 2014 3:05PM - 3:35PM |
CM10.00004: Experimental optimization of high magnetic Reynolds number, two-vortex flows on the Madison plasma dynamo experiment David Weisberg Laminar counter-rotating two-vortex flows, predicted to excite a large-scale dynamo, are generated and optimized in the Madison plasma dynamo experiment (MPDX). Numerical simulations show that the topology of these simply-connected flows may be optimal for creating a plasma dynamo in the lab and predict a critical threshold of $Rm_{crit}=\mu_0\sigma LV=250$ for optimal flows. MPDX plasmas are shown to exceed this critical $Rm$, generating large ($L=1.4$\,m), hot ($T_e>10$\,eV) plasmas where $Rm=600$. Plasma flow is driven using eight thermally emissive LaB$_6$ cathodes which generate a $J\times B$ torque at the magnetized edge of spherical He plasmas. Mach probe measurements show counter-rotating flows at speeds of $V>10$\,km/s; the driven flow at the plasma edge viscously couples inward to the unmagnetized core via ion-ion collisions, and flow measurements along radial chords compare favorably to hydrodynamic calculations using Braginskii viscosity. To optimize flow for dynamo generation, cathode bias and positioning are varied along with plasma viscosity ($\nu\sim T_i^{5/2}/n_i$) and the frictional neutral-ion drag force ($\mu=L^2/(\nu\tau_{in})$). Prospects for observing a dynamo, hydrodynamic transitions to turbulence, and eventual large Rm fast dynamos will be presented. [Preview Abstract] |
Monday, October 27, 2014 3:35PM - 3:55PM |
CM10.00005: Soft-iron impellers in the Madison Sodium Dynamo Experiment Mark Nornberg, M.M. Clark, C.B. Forest, N. Plihon In an attempt to increase the magnetic flux amplification of the two-vortex flow in the Madison Sodium Dynamo Experiment, the stainless steel impellers were replaced with soft-iron disks similar in design to the VKS dynamo experiment. Past attempts at creating a homogeneous dynamo in the Madison Sodium Dynamo Experiment relied on stainless steel impellers to drive a two-vortex flow predicted to be unstable to dynamo excitation. The resulting induction process was much weaker than laminar predictions due to the turbulent enhancement of the resistivity. The measured amplification and pulse-decay times with the soft-iron disks show an improvement in the flux amplification, but not sufficient for self-excitation. Despite the similarities in the impeller design with the VKS experiment, the differences in geometry still play a significant role in determining the threshold conditions for dynamo action. [Preview Abstract] |
Monday, October 27, 2014 3:55PM - 4:15PM |
CM10.00006: The New Mexico dynamo: the past, the present, and the future Jiahe Si, Stirling Colgate, Art Colgate, Richard Sonnenfeld, David Westpfahl, Joe Martinic, Mark Nornberg, Hui Li The New Mexico dynamo experiment was designed to simulate a star-disk collision. It consists of two co-axial cylinders to make Taylor-Couette (TC) flows simulating differential rotation of accretion disks. In response to a radial seed field of 10 Gauss, the $\omega$-effect wound up the field lines to produce an 80-Gauss toroidal field. This is, to date, the largest gain obtained by any experiment in the world. We attribute this success to the largely coherent TC flow field in the instrument. Turbulence dissipates magnetic energy by increasing the effective resistivity of the fluid (the ``$\beta$-effect'') and has been observed by the Madison group. We will study this effect in our geometry by applying an external B-field pulse and observing its penetration into the liquid sodium flows vs time for varying levels of turbulence. In addition, we will revisit the $\omega$-effect at varying levels of turbulence. The final challenge for the New Mexico dynamo is the pursuit of the $\alpha$-effect. A plume injection apparatus has been devised and instrumentation for the full simulation of a star-disk collision is being developed. [Preview Abstract] |
Monday, October 27, 2014 4:15PM - 4:30PM |
CM10.00007: The role of magnetic helicity flux on the alpha dynamo effect Fatima Ebrahimi, A. Bhattacharjee, H. Ji Self-organized plasmas are common throughout the universe. Examples include self-organized plasmas of flow-dominated astrophysical disks and magnetically-dominated star surfaces. We will treat the dynamo problem in both laboratory (magnetically dominated) and astrophysical (flow-dominated) plasmas from a common perspective. The constraint imposed by magnetic helicity conservation on the alpha effect, the correlated flow and magnetic field fluctuations, is considered for the two important, and very different, examples of tearing instability in laboratory plasmas and magneto-rotational instability in flow-driven astrophysical disks. Through direct numerical simulations, the role of magnetic helicity fluxes on the alpha effect and the final sustainment of large-scale magnetic field will be examined. For the two examples of an unstratified Keplerian cylinder and a reversed-field pinch, a dominant contribution to the alpha effect, in the functional form of a total divergence of an averaged helicity flux, called the helicity-flux-driven alpha effect, will be demonstrated. For the second example the results will be compared with MST data. The effect of averaging (both temporal and spatial) on the results for the helicity fluxes will be discussed. Supported by CMSO and DE-FG02-12ER55142. [Preview Abstract] |
Monday, October 27, 2014 4:30PM - 4:45PM |
CM10.00008: Laboratory Study of Angular Momentum Transport in Astrophysical Accretion Disks Hantao Ji Studying astrophysical processes in the lab becomes increasingly possible and exciting, as one of Stirling's favorite subjects throughout his scientific career. In this talk, I will describe experimental efforts to study mechanisms of rapid angular momentum transport required to occur in accretion disks to explain a wide range of phenomena from star formation, energetic activity of cataclysmic variables, to powering quasars, the most luminous steady sources in the Universe. By carefully isolating effects due to artificial boundaries, which are inherent to terrestrial experiments, certain astrophysical questions regarding hydrodynamic and magnetohydrodynamic stabilities are being addressed in the laboratory. Inspirations from Stirling as well as scientific exchanges with him will be mentioned during this talk as part of my scientific journey on this subject. [Preview Abstract] |
Monday, October 27, 2014 4:45PM - 5:00PM |
CM10.00009: A spherical Couette experiment to observe inductionless MHD instabilities at medium Reynolds numbers Elliot Kaplan, Benjamin Gohl, Thomas Gundrum, Martin Seilmayer, Frank Stefani Turbulent spherical Couette flows in a strong axial magnetic field (Re $ \in (10^4, 10^6)$, Ha $\in (0,3000)$) have given rise to an interesting set of instabilities. Like the, long sought after, magnetorotational instability (MRI) they transport angular momentum outward. Unlike the MRI they are azimuthally nonaxisymmetric and change their equatorial symmetry as the applied field is increased [Sisan (2004)]. Subsequent theoretical and numerical investigations found a set of inductionless (Rm=0) instabilities that replicate both these properties [Hollerbach (2009), Gissinger (2011)]. A liquid metal (GaInSn) spherical Couette flow is being carried out at the Helmholtz-Zentrum Dresden-Rossendorf to explore a region of Reynolds-Hartmann space (Re $\in (10^3,10^4)$, Ha $\in (0,160)$) between the simulations and the experiments. The diagnostic coverage in the new experiment is also much denser (ultrasound Doppler velocimeter array for m $\leq$ 3, electric potential probes for m $\leq 12$ ) than that of the 2004 experiment. Data from the initial runs of the experiment and results from the predictive simulations are discussed here. [Preview Abstract] |
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