Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session BI3: Space & Astrophysical Plasmas, Laboratory AstrophysicsInvited
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Chair: Jason TenBarge, University of Maryland Room: 210 ABEF |
Monday, October 31, 2016 9:30AM - 10:00AM |
BI3.00001: Models of Dilute Relativistic Plasmas Around Black Holes Invited Speaker: Eliot Quataert In some regimes, mass flowing onto a central black hole can become sufficiently hot and low density that the collisional mean free path is appreciable compared to the size of the system. I describe new analytical and numerical models of these relativistically hot low collisionality plasmas around black holes. I also describe the application of these models to interpreting observations of the accreting black holes being observed by the Event Horizon Telescope. [Preview Abstract] |
Monday, October 31, 2016 10:00AM - 10:30AM |
BI3.00002: Generation of large-scale magnetic fields by small-scale dynamo in shear flows Invited Speaker: Jonathan Squire A novel large-scale dynamo mechanism, the magnetic shear-current effect, is discussed and explored. The effect relies on the interaction of magnetic fluctuations with a mean shear flow, meaning the saturated state of the small-scale dynamo can drive a large-scale dynamo -- in some sense the inverse of dynamo quenching. The dynamo is nonhelical, with the mean-field alpha coefficient zero, and is caused by the interaction between an off-diagonal component of the turbulent resistivity and stretching of the large-scale field by shear flow. In this talk, a variety of computational and analytic studies of this mechanism are discussed, which have been carried out both in regimes where magnetic fluctuations arise self-consistently through the small-scale dynamo and at lower Reynolds numbers. In addition, an heuristic description of the effect is presented, which illustrates the fundamental role played by the pressure response of the fluid and helps explain why the magnetic effect is stronger than its kinematic cousin. As well as being interesting for its applications to general high Reynolds number astrophysical turbulence, where strong small-scale magnetic fluctuations are expected to be prevalent, the magnetic shear-current effect is a likely candidate for large-scale dynamo in the unstratified regions of ionized accretion disks. [Preview Abstract] |
Monday, October 31, 2016 10:30AM - 11:00AM |
BI3.00003: A Field-Particle Correlation Technique to Explore the Collisionless Damping of Plasma Turbulence Invited Speaker: Kristopher Klein The nature of the dominant mechanisms which damp turbulent electromagnetic fluctuations remains an unanswered question in the study of a variety of collisionless plasma systems. Proposed damping mechanisms can be generally, but not exclusively, classified as resonant, e.g. Landau and cyclotron damping, non-resonant, e.g. stochastic ion heating, and intermittent, e.g. energization via current sheets or magnetic reconnection. To determine the role these mechanisms play in turbulent plasmas, we propose the application of field-particle correlations to time series of single spatial point observations of the type typically measured in the solar wind. This correlation, motivated by the form of the collisionless Vlasov equation, is the time averaged product of the factors comprising the nonlinear field-particle interaction term. The correlation both captures the secular transfer of energy between fields and perturbed plasma distributions by averaging out the conservative oscillatory energy transfer, and retains the velocity space structure of the secular transfer, allowing for observational characterization of the damping mechanism. Field-particle correlations are applied to a set of nonlinear kinetic numerical simulations of increasing complexity, including electrostatic, gyrokinetic, and hybrid Vlasov-Maxwell systems. These correlations are shown to capture the secular energy transfer between fields and particles and distinguish between the mechanisms accessible to the chosen system. We conclude with a discussion of the application of this general technique to data from current and upcoming spacecraft missions, including \textit{MMS}, \textit{DSCOVR}, \textit{Solar Probe Plus} and \textit{THOR}, which should help in determining which damping mechanisms operate in a variety of heliospheric plasmas. [Preview Abstract] |
Monday, October 31, 2016 11:00AM - 11:30AM |
BI3.00004: Laboratory astrophysics using differential rotation of unmagnetized plasma at large magnetic Reynolds number Invited Speaker: David Weisberg Differentially rotating plasma flow has been measured in the Madison Plasma Dynamo Experiment (MPDX). Spherical cusp-confined plasmas have been stirred both from the plasma boundary using electrostatic stirring in the magnetized edge {\sl and} in the plasma core using weak global fields and cross-field currents to impose a body-force torque. Laminar velocity profiles conducive to shear-driven MHD instabilities like the dynamo and the MRI are now being generated and controlled with magnetic Reynolds numbers of $Rm<250$ and fluid Reynolds numbers of $Re<200$. The measured plasma confinement contradicts existing theories for magnetic cusp confinement, and a new quasi-1D ambipolar diffusion model is presented to explain measurements of cusp loss widths that do not fit the classic hybrid gyroradius theory. Emissive electrode discharge is shown to be an efficient method for plasma heating, but limits on input heating power have been observed (believed to be caused by the formation of double-layers at anodes). These confinement studies have culminated in large ($R=1.4$\,m), warm ($T_e<20$\,eV), dense ($n_e<5\times10^{18}$\,m$^{-3}$), unmagnetized ($M_A>1$), steady-state plasmas. Results of the ambipolar transport model are good fits to measurements of pressure gradients and fluid drifts in the cusp, and offer a predictive tool for future cusp-confined devices. Hydrodynamic modeling is shown to be a good description for measured plasma flows, where ion viscosity proves to be an efficient mechanism for transporting momentum from the magnetized edge into the unmagnetized core. In addition, the body-force stirring technique produces velocity profiles conducive to MRI experiments where $d\Omega/dr<0$. Measured values of $Rm$ and $Re$ are significantly higher than previous flow experiments in cusp-confined plasmas, setting the stage for future progress in laboratory research of flow-driven astrophysical MHD instabilities. [Preview Abstract] |
Monday, October 31, 2016 11:30AM - 12:00PM |
BI3.00005: Statistical Physics Experiments Using Dusty Plasmas Invited Speaker: John Goree Compared to other areas of physics research, Statistical Physics is heavily dominated by theory, with comparatively little experiment. One reason for the lack of experiments is the impracticality of tracking of individual atoms and molecules within a substance. Thus, there is a need for a different kind of experimental system, one where individual particles not only move stochastically as they collide with one another, but also are large enough to allow tracking. A dusty plasma can meet this need. A dusty plasma is a partially ionized gas containing small particles of solid matter. These micron-size particles gain thousands of electronic charges by collecting more electrons than ions. Their motions are dominated by Coulomb collisions with neighboring particles. In this so-called strongly coupled plasma, the dust particles self-organize in much the same way as atoms in a liquid or solid. Unlike atoms, however, these particles are large and slow, so that they can be tracked easily by video microscopy. Advantages of dusty plasma for experimental statistical physics research include particle tracking, lack of frictional contact with solid surfaces, and avoidance of overdamped motion. Moreover, the motion of a collection of dust particles can mimic an equilibrium system with a Maxwellian velocity distribution, even though the dust particles themselves are not truly in thermal equilibrium. Nonequilibrium statistical physics can be studied by applying gradients, for example by imposing a shear flow. In this talk I will review some of our recent experiments with shear flow. First, we performed the first experimental test to verify the Fluctuation Theorem for a shear flow, showing that brief violations of the Second Law of Thermodynamics occur with the predicted probabilities, for a small system. Second, we discovered a skewness of a shear-stress distribution in a shear flow. This skewness is a phenomenon that likely has wide applicability in nonequilibrium steady states. Third, we performed the first experimental test of a statistical physics theory (the Green-Kubo model) that is widely used by physical chemists to compute viscosity coefficients, and we found that it fails. [Preview Abstract] |
Monday, October 31, 2016 12:00PM - 12:30PM |
BI3.00006: Collisionless Coupling between Explosive Debris Plasma and Magnetized Ambient Plasma Invited Speaker: Anton Bondarenko The explosive expansion of a dense debris plasma cloud into relatively tenuous, magnetized, ambient plasma characterizes a wide variety of astrophysical and space phenomena, including supernova remnants, interplanetary coronal mass ejections, and ionospheric explosions. In these rarified environments, collective electromagnetic processes rather than Coulomb collisions typically mediate the transfer of momentum and energy from the debris plasma to the ambient plasma. In an effort to better understand the detailed physics of collisionless coupling mechanisms in a reproducible laboratory setting, the present research jointly utilizes the Large Plasma Device (LAPD) and the Phoenix laser facility at UCLA to study the super-Alfv\'{e}nic, quasi-perpendicular expansion of laser-produced carbon (C) and hydrogen (H) debris plasma through preformed, magnetized helium (He) ambient plasma via a variety of diagnostics, including emission spectroscopy, wavelength-filtered imaging, and magnetic field induction probes. Large Doppler shifts detected in a He II ion spectral line directly indicate initial ambient ion acceleration transverse to both the debris plasma flow and the background magnetic field, indicative of a fundamental process known as Larmor coupling. Characterization of the laser-produced debris plasma via a radiation-hydrodynamics code permits an explicit calculation of the laminar electric field in the framework of a ``hybrid'' model (kinetic ions, charge-neutralizing massless fluid electrons), thus allowing for a simulation of the initial response of a distribution of He II test ions. A synthetic Doppler-shifted spectrum constructed from the simulated velocity distribution of the accelerated test ions excellently reproduces the spectroscopic measurements, confirming the role of Larmor coupling in the debris-ambient interaction. [Preview Abstract] |
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