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Physics & Astronomy Colloquium
Thursdays, 3:30 p.m.
Call us at: (323) 343-2100
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Spring Quarter 2013 Schedule

Biological Sciences Building, BIOS 335
Refreshments - 3:15 p.m., Colloquium - 3:30 p.m


Professor Paul S. Nerenberg
W.M. Keck Science Department
Claremont McKenna, Pitzer, and Scripps Colleges

"Balancing the Energy Scale of Biomolecular Simulations"

Abstract: Molecular dynamics (MD) simulations enable us to understand the physical behavior of biological and chemical systems at the atomic scale. These simulations are now being used as a primary tool to investigate biomolecules that do not lend themselves to study with conventional structural biology techniques. One such category of biomolecules are intrinsically disordered proteins (IDPs) proteins that do not fold to single low energy structure or contain a substantial region that does not fold which comprise nearly half of the proteins in the human body. The structural ensembles of IDPs are dictated by small differences in energies between states, which in turn demand ever more accurate potential energy functions for the MD simulations to yield meaningful results. In this talk I will discuss recent improvements that my group has made to a popular MD force field to increase the accuracy of both protein-water and protein-protein interactions. Achieving the appropriate energetic balance between these interactions will be critical to understanding how IDPs play a role in both regular physiology and diseases such as Alzheimer's, Parkinson's, and even many cancers.


Dr. James Lamb
Assistant Professor, Department of Radiation Oncology, UCLA

"Physics in Radiation Oncology: Respiratory-Correlated Imaging,
Lung Motion Modeling, and Beyond"

Abstract: Physics and physicists play an integral role in clinical radiation therapy treatments and in the development of new techniques for radiation therapy for cancer. The ability to deliver large radiation doses to small volumes inside the body has been achieved with the recent development of intensity modulated radiotherapy and stereotactic body radiotherapy. A remaining challenge has been imaging and targeting lung and upper-abdominal tumors that move with respiration, the so-called motion management problem. I will discuss various technological approaches to the motion management problem, focusing in particular on a technique for imaging the lungs during free breathing that has been developed by the lung motion modeling group at UCLA. Potential applications of the technique to the characterization of non-cancer lung disease, as well as other selected areas of active research in radiation oncology physics at UCLA, will also be discussed.


Dr. Brian Kappus
Department of Physics & Astronomy, UCLA

"Sonoluminescence as an Ultra-Dense Microplasma"

Abstract: Sonoluminescence is the nonlinear interaction of a standing sound wave with a bubble which leads to pulsations that are so enormous that light is emitted at the moment of maximum compression. A Xenon bubble is compressed to near-liquid densities (10 21 /cc) and a temperature of 10,000K.  Under these conditions a simple application of equilibrium statistical mechanics predicts very low levels of ionization and a photon mean free path four orders of magnitude larger than the plasma (50 microns).  Instead, recent findings show opaque behavior.  We conclude that this previously overlooked region of parameter space contains a first-order screening phase transition to a highly-ionized, dense plasma. 


Professor Brian Siana
Department of Physics and Astronomy, UC Riverside

"Dwarf Galaxies and the Reionization of the Universe"

Abstract: About 13 billion years ago, most of the atoms in the universe became ionized, and have remained that way ever since. The culprit is thought to be massive stars, which can produce light with sufficient energy to ionize hydrogen. However, there are two questions which must be answered: Were there enough stars in the early universe? and did a large enough fraction of the ionizing photons escape the galaxies? I will discuss our recent efforts to answer these questions with deep observations with the Keck Observatory and Hubble Space Telescope.


Professor Christoph A. Haselwandter
Department of Physics and Astronomy, USC

"Mechanics of Bacterial Cell Membranes"

Abstract: While modern structural biology has provided us with a rich and diverse picture of membrane proteins, the biological function of membrane proteins is often influenced by the mechanical properties of the surrounding lipid bilayer. In particular, the bilayer hydrophobic core couples to the hydrophobic regions of membrane proteins, yielding membrane deformations which can be described quantitatively by the continuum elasticity theory of membranes. Employing mechanosensitive ion channels as a model system we show that, in addition to the hydrophobic mismatch between membrane proteins and the surrounding lipid bilayer, the symmetry and shape of the hydrophobic surfaces of membrane proteins play an important role in the regulation of protein function by bilayer membranes. Moreover, we find that, for a given protein shape, the sign and strength of elastic interactions, and associated cooperative function of membrane proteins, can depend on the relative protein orientation. The approach developed here represents a first step towards a physical theory of how elastic interactions affect the molecular structure, organization, and biological function of proteins in the crowded membrane environment provided by living cells.


Professor Kyle Stewart
Department of Natural and Mathematical Sciences
California Baptist University

"Angular Momentum Acquisition in Milky Way Sized Galaxy Halos"

Abstract: Using high-resolution cosmological hydrodynamic simulations, we study the angular momentum acquisition of Milky Way sized galaxies, as well as the gaseous halos surrounding them. We find that "cold flow" gas accretion (gas that never shock-heats as it falls onto galaxies from the cosmic web) enters galaxy halos with roughly 70% more angular momentum than dark matter, when averaged over cosmic time. In fact, we find that all matter has more specific angular momentum when measured at first accretion, rather than averaged over the galaxy's lifetime. Combined with the fact that cold flow gas spends a relatively short time in the galaxy halo (1-2 dynamical times) before sinking to the center, this naturally explains why it has a higher spin parameter than the dark matter halo, and often forms extended "cold flow disks" of material that extend far beyond the visible portion of the galaxy. We demonstrate that the higher angular momentum associated with cold flow gas is related to the fat that it tends to be preferentially accreted along cosmic filaments.


Professor Naveen Reddy
Department of Physics and Astronomy, UC Riverside

" How Dust Affects Our View of the Distant Universe "

Abstract: Dust obscuration has a profound influence on our view of distant galaxies, and can shed light on star formation processes at high redshift.  I will review recent efforts to examine the dust attenuation and bolometric star-formation rates of typical star-forming galaxies at a time when the Universe was just 20% of its current age.  I will discuss the trend of dust obscuration with bolometric  luminosity and redshift, and show how multi-wavelength indicators of star formation can be used to understand how stars produce  heavy elements and enrich the interstellar medium of galaxies.


Dr. David C. Pace
DIII-D National Fusion Facility, General Atomics

"The Fast and the Furious:
Energetic Ion Transport in Magnetic Fusion Devices"

Abstract: Nuclear fusion is an ideal energy source that is capable of powering society without generating greenhouse gases or high-level radioactive waste. The tokamak approach to controlled nuclear fusion employs a toroidally-shaped magnetic field configuration to confine plasmas at temperatures beyond 200 million K (20 keV). Present devices aim to utilize the deuterium-tritium fusion reaction due to its favorable cross-section, where this DT reaction produces a 14 MeV neutron and a 3.5 MeV alpha particle. While the neutron is collected by the surrounding structure to allow for energy extraction, the alpha particle must be confined such that it transfers its energy to plasma, thereby maintaining fusion temperatures. This self-heated scenario is known as a burning plasma state. The fusion-alpha population is suprathermal, however, and is therefore able to excite plasma instabilities (e.g., Alfven eigenmodes) that cause detrimental alpha transport and reduce fusion power output. Existing tokamaks study these phenomena using energetic ion populations produced by auxiliary heating with particle beams or injected radio-frequency waves. The fundamentals of Alfven eigenmodes will be presented, along with the advanced diagnostics and theoretical treatments applied to the study of energetic ion transport and losses in modern tokamaks. A thorough understanding of this physics is essential for the ITER tokamak (an international reactor designed to maintain a burning plasma that generates 500 MW of fusion power), which will feature an energetic ion population capable of damaging plasma-facing surfaces in cases of enhanced transport.

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