Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Plasma Physics
Volume 65, Number 11
Monday–Friday, November 9–13, 2020; Remote; Time Zone: Central Standard Time, USA
Session QI02: Invited: Fundamental Plasma PhysicsLive
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Chair: Jeremiah Williams, Wittenberg University |
Wednesday, November 11, 2020 3:00PM - 3:30PM |
QI02.00001: Invited:Capturing the First Image of a Black Hole Invited Speaker: Katherine Bouman This talk will present the methods and procedures used to produce the first image of a black hole from the Event Horizon Telescope, as well as discuss future developments. It had been theorized for decades that a black hole would leave a "shadow" on a background of hot plasma. Taking a picture of this black hole shadow would help to address a number of important scientific questions, both on the nature of black holes and the validity of general relativity. Unfortunately, due to its small size, traditional imaging approaches require an Earth-sized radio telescope. In this talk, I discuss techniques the Event Horizon Telescope Collaboration has developed to photograph a black hole using the Event Horizon Telescope, a network of telescopes scattered across the globe. Imaging a black hole’s structure with this computational telescope required us to reconstruct images from sparse measurements, heavily corrupted by atmospheric error. This talk will summarize how the data from the 2017 observations were calibrated and imaged, and explain some of the challenges that arise with a heterogeneous telescope array like the EHT. The talk will also discuss future developments, including how we are developing machine learning methods to help design future telescope arrays. [Preview Abstract] |
Wednesday, November 11, 2020 3:30PM - 4:00PM Live |
QI02.00002: Instability of an electron-plasma shear layer in a strain flow. Invited Speaker: J. R. Danielson The $E\times B$ shear instability of a two-dimensional ($2D$) filament (i.e., a thin, rectangular strip) of a magnetized pure electron plasma is studied in the presence of an externally imposed strain flow.\footnote{N. C. Hurst, J. R. Danielson, D. H. E. Dubin, and C. M. Surko, {\it Phys. Plasmas} {\bf 27}, 042101 (2020).} Experiments are conducted using a specially designed Penning-Malmberg trap in which such flows can be imposed in $2D$ by biasing segmented electrodes surrounding the plasma. Electron density, which is the analog of fluid vorticity, is measured directly with a CCD camera. The situation studied corresponds to the Rayleigh instability of a finite-width shear layer in a $2D$ incompressible fluid. Theory predicts that neutrally stable traveling waves on opposite surfaces of the filament will phase lock and go unstable. The experimentally observed phase locking and the time-evolution of the wavenumber spectrum are in quantitative agreement with a linear model\footnote{D. G. Dritschel, P. H. Haynes, M. N. Juckes, and T. G. Shepherd, {\it J. Fluid Mech.} {\bf 230}, 647 (1991).} that extends Rayleigh's work to account for the imposed strain flow. For weak strain, the system maintains a phase relationship that corresponds to an instantaneous (though evolving) Rayleigh eigenmode. A nonlinear regime is observed at later times that includes wave breaking, vortex formation, a vortex-pairing instability, and vorticity transport perpendicular to the filament. This evolution is suppressed, but not quenched as the strain rate is increased. Remaining open questions will be discussed. [Preview Abstract] |
Wednesday, November 11, 2020 4:00PM - 4:30PM Live |
QI02.00003: Laser-driven and Magnetized Ultracold Neutral Plasmas Invited Speaker: Thomas Killian Ultracold neutral plasmas (UNPs), created by photoionizing laser-cooled atoms just above threshold, stretch the boundaries of neutral plasma physics towards low energies and strong Coulomb coupling. They also offer precise diagnostics and control over plasma conditions, making them useful for validating plasma theory and discovering new phenomena. In this talk, I will describe several experiments with UNPs formed by photoionizing an ultracold gas of neutral strontium atoms. Laser cooling of ions in the UNP [1] yields ion temperatures as low as 50 mK and Coulomb coupling parameters as high as $\Gamma=11$. This opens new possibilities for studying transport phenomena in strongly coupled systems. Electrons and Sr$^+$ ions can be magnetized with experimentally accessible fields ($\sim 100$G). I will describe our use of laser-induced fluorescence to study the expansion of an UNP created in a quadrupole magnetic field. Density and velocity maps provide evidence for trapping of the plasma. Laser forces, in conjunction with magnetic fields, also offer opportunities for controlling plasma hydrodynamic flow and cooling to lower plasma temperatures. [1]``Laser Cooling of Ions in a Neutral Plasma," T. K. Langin, G. M. Gorman, and T. C. Killian, \textit{Science} \textbf{363}, 61 (2019). [Preview Abstract] |
Wednesday, November 11, 2020 4:30PM - 5:00PM Live |
QI02.00004: Experiments on the dynamics and scaling of spontaneous-magnetic-field saturation in laser-produced plasmas Invited Speaker: Graeme Sutcliffe Saturation of spontaneous magnetic fields in plasmas results from the balancing of various physical processes. Understanding the saturation mechanisms is a challenging undertaking essential to basic plasma physics. In high-energy-density (HED) plasmas produced with high-power lasers, large-scale strong magnetic fields are generated by the Biermann-battery effect when temperature and density gradients are misaligned. Saturation takes place when advection and dissipation balance the field generation. While theoretical and numerical modelling provide useful insight into the saturation mechanisms, experimental demonstration remains elusive. To quantitatively study these dynamical processes, new experiments were performed at the OMEGA laser. Illuminating a plastic foil with a laser of energy $\sim$ kJ in a $\sim$ ns pulse, $\sim$ megagauss Biermann fields were generated, which subsequently advected and dissipated in an expanding plasma plume. With time-resolved proton radiography and Thomson scattering, the evolution of the magnetic field structure and plasma conditions were measured. The experiments and resulting data were modeled with hydrodynamic simulations. For the first time at these conditions, the spatially resolved magnetic fields were reconstructed, leading to a picture of field saturation with a scaling of B $\sim$ 1/L$_{T}$ for a convectively dominated plasma, a regime where the temperature gradient scale (L$_{T}$) exceeds the ion skin depth (d$_{i}$). This work not only quantifies the saturation mechanism and scaling with underlying physics in a typical laser-plasma experimental regime, but also more broadly provides new physical insight into the dynamics of spontaneous magnetic fields in HED plasmas. [Preview Abstract] |
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