This Week at Physics

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Events on Thursday, February 23rd, 2012

R. G. Herb Condensed Matter Seminar
Seeing and Sculpting Nematic Liquid Crystal Textures with the Thom construction
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Bryan Chen, University of Pennsylvania
Abstract: Nematic liquid crystals are the foundation for modern display technology and also exhibit topological defects that can readily be seen under a microscope. Recently, experimentalists have been able to create and control more and more interesting defect textures, including controllably knotted defect lines around colloids (Ljubljana) and the "toron", a pair of hedgehogs bound together with a ring of double-twist between them (CU Boulder). I will discuss recent work with Gareth Alexander (Warwick) applying the Thom construction from algebraic topology which allows us to visualize 3 dimensional molecular orientation fields as certain colored surfaces in the sample. These surfaces turn out to be a generalization to 3 dimensions of the dark brushes seen in Schlieren textures of two-dimensional samples of nematics. Manipulations of these surfaces correspond to deformations of the nematic orientation fields, giving a hands-on way to classify liquid crystal textures which is also easily computable from data and robust to noise.
Host: Coppersmith
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Plasma Physics (Physics/ECE/NE 922) Seminar
Studying Fast Magnetic Reconnection with Laboratory Plasmas
Time: 12:00 pm
Place: 4274 Chamberlin
Speaker: William Fox, University of New Hampshire
Abstract: Magnetic reconnection is a ubiquitous plasma process which controls the dynamics of magnetic fields a number of plasma systems, allowing the release of magnetic energy which powers solar flares and magnetospheric substorms, and also which allows instabilities in magnetized fusion devices to quickly transport plasma out of the core of the device. In this talk I will discuss two sets of experiments studying magnetic reconnection in laboratory plasmas. First, I will discuss the Versatile Toroidal Facility (VTF) experiment at MIT, a basic laboratory experiment which studies magnetic reconnection in the regime of a strong toroidal "guide" field, and specifically my work studying current-driven instabilities driven by reconnection events. In the second set of experiments, I will discuss my recent work to understand magnetic reconnection experiments conducted on inertial-fusion-class laser-facilities. This is a novel regime for magnetic reconnection study, characterized by extremely high magnetic fields, high plasma beta and strong, super-Alfvenic plasma inflow. Work to-date with particle-in-cell simulations has identified two key ingredients for explaining the fast observed rates of reconnection: two-fluid reconnection mediated by collisionless effects (the Hall current and electron pressure tensor), and strong flux-pileup of the inflowing magnetic field. I will close with a discussion of our new proposals for upcoming laser-driven reconnection experiments using this new platform.
Host: Cary Forest
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Astronomy Colloquium
"Star Formation, gas loss, and the molecular gas in simulations of evolving galaxies"
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Charlotte Christensen, University of Arizona
Abstract: The physics of the interstellar media (ISM) affects both the location of star formation and the efficiency of supernova feedback by changing the properties of the star forming gas. In most previous galaxy formation simulations, though, the cold, molecular phase of the ISM has been neglected. In this talk, I present a method for integrating the non-equilibrium molecular hydrogen (H2) abundance throughout a simulation, including such processes as dissociation by Lyman-Werner radiation, shielding of molecular gas, and H2-based star formation. I apply this model to high-resolution cosmological simulations of galaxies ranging from 10^9 to 10^12 solar masses and compare it to simulations with different ISM models. I find that the inclusion of H2 results in galaxies with clumpier ISMs and more dispersed star formation. The increased clumpiness of the gas leads to greater efficiency of supernova at removing of low-angular momentum material from the galaxy. The result is spiral galaxies with smaller bulges and more realistic rotation curve. I discuss how the these interaction between the ISM structure, star formation, and supernova feedback result in changes to the mass distribution and compare the formation histories of galaxies of different masses.
Host: Alyson Brooks
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
The Pierre Auger Observatory: the highest energy frontier
Time: 4:00 pm
Place: 4274 Chamberlin Hall
Speaker: Ines Valino Rielo, University of Santiago de Compostela
Abstract: A century after the discovery of cosmic rays, the nature and origin of the highest energy particles in the Universe, the Ultra-High Energy Cosmic Rays (UHECR), remain enigmatic. Thanks to experiments dedicated to probing the extreme end of the energy spectrum, exciting progress is being made in solving these puzzles.The Pierre Auger Observatory is currently the World`s largest detector for UHECR. The goal is to measure the cosmic ray energy spectrum, arrival directions, and the properties of the extensive air showers induced by the UHECRs in the atmosphere with the objective of unveiling cosmic ray elemental composition, origins and propagation effects. The observatory has now collected more data than all previous experiments combined, and employs multiple detection (hybrid) detection techniques allowing for a large exposure and excellent control of systematic uncertainties. The original design, optimized for the energy range from 1018 eV to the end of the spectrum, has recently been enhanced to cover energies down to almost 1017 eV, allowing us to view additional interesting features of the spectrum. I will give an overview of the latest results with a focus on the current status of the search for ultra-high energy neutrinos and photons and the promising prospect to use cosmic rays to study hadronic interactions at energies beyond the reach of the LHC.
Host: Halzen
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