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This Week at Physics

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Events During the Week of April 6th through April 13th, 2014

Monday, April 7th, 2014

Cosmology Journal Club
An Informal discussion about a broad variety of arXiv papers related to Cosmology
Time: 12:00 pm
Place: 5242 Chamberlin Hall
Abstract: Please visit the following link for more details:
Please feel free to bring your lunch!
If you have questions or comments about this journal club, would like to propose a topic or volunteer to introduce a paper, please email Le Zhang (lzhang263@wisc.edu)
Host: Peter Timbie
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Plasma Physics (Physics/ECE/NE 922) Seminar
The Plasma Physics of Fusion Indirectly Driven with a Laser
Time: 12:00 pm
Place: 2241 Chamberlin
Speaker: Bob Kirkwood, Lawrence Livermore National Laboratory
Abstract: Igniting fusion fuel that is driven indirectly with a laser presents significant challenges for plasma physics because the intense beams required can drive instabilities in the plasma formed in the target. The instabilities in turn, can scatter light significantly and affect power flow and deposition. The National Ignition Facility (NIF) was constructed to study the physics of inertial fusion and ignition with a laser [1] and has now been in operation for over four years [2]. The design of the facility was based on an understanding of plasma instabilities derived from a series of experiments carried out with smaller lasers and their modeling. That work made it clear that the large hot plasmas that would be created when NIFs multiple intersecting beams entered a hohlraum target would open a new realm of plasma interactions, where stimulated scattering would not only limit power coupling but also provide control of power deposition profiles in the target interior [3].

The initial experiments at NIF [4] were also designed based on this understanding which allowed optimization of laser intensity, wavelength and spot size, as well as target dimensions and materials, and further indicated the areas of greatest uncertainty where there was need for final empirical tuning. The recent studies at NIF have now confirmed for the first time that under ignition relevant conditions plasma instabilities produce self-generated optical scattering cells that are not only controllable but also useful. The experiments have further demonstrated that deleterious plasma scatter that depletes power from one set of beams can be compensated for by inducing a plasma scattering cell that redirects power from another set of beams. This has allowed induced plasma scattering to become the primary means to control the power deposition profile and the resulting implosion symmetry via adjustments to the laser wavelengths [5]. These techniques have allowed enhanced target performance that is essential to the present experimental campaigns that study precision implosions [6], and were also an essential ingredient in the recent demonstration that net energy can be extracted from fusion fuel [7]. This talk will review the plasma physics studied in the first few years of NIC experiments in the context of the earlier work and highlight its importance for fusion ignition with a laser.

[1] G. H. Miller, E. I. Moses, and C. R. Wuest, Nucl. Fusion 44, S228 (2004).
[2] J. D. Lindl et al Phys. Plasmas 11, 339 (2004).
[3] R. K. Kirkwood et al Plasma Phys. Controlled Fusion 55, 103001 (2013).
[4] S. H. Glenzer et al Phys. Rev. Lett. 106, 085004 (2011).
[5] P. Michel et al Phys. Plasmas 17, 056305 (2010).
[6] M. J. Edwards Phys. Plasmas 20, 070501 (2013).
[7] O. A. Hurricane et al Nature 506, 343 (2014).
Host: Cary Forest
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Tuesday, April 8th, 2014

Chaos & Complex Systems Seminar
Algorithms, complex systems and chaos
Time: 12:05 pm
Place: 4274 Chamberlin (refreshments will be served)
Speaker: Jerry Tutsch, UW Department of Computer Sciences
Abstract: Computer programs consist of large collections of interacting algorithms. They are prime examples of complex systems. The recent advent of cheap, fast, small computers has led to an explosion of very complex programs that must remain easy to use. To a large extent, it is the ease-of-use constraint that is driving up the level of complexity. The programs generally have a direct-manipulation user interface, as opposed to a simple text-based interface. They are nonautonomous, that is event-driven.

Additional technical constraints drive up the complexity. The program's component parts may be distributed over various computers and they may be executing at different times. Furthermore the programs may need to be able execute on different platforms which themselves change over time as operating systems and hardware is updated. Even with the help of software development tools, an increasingly difficult task facing the software engineer is that of controlling and managing the complexity of a program over its lifetime as it inevitable grows in functionality and size.

In the parlance of dynamical systems, as perhaps best defined in Math and Physics, computer programs in general are extremely complex nonautonomous t-advancing iterative maps defined on discrete phase spaces of high dimension.

Over the past several years, as an experiment to learn more about controlling and managing the complexity of computer programs, I have been writing a program designed to help students visualize the complexity and chaos that emerges when small nonlinear dynamical systems, in the form of differential equations and iterative maps, evolve in time. In the course of writing the program, I came to the realization that the program itself was a meta dynamical system.

I will discuss how a program's complexity is controlled and managed in theory. Time permitting, I will also demonstrate how the complexity arises and is managed in practice.&lt;br&gt;<br>
Host: Clint Sprott
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"Physics Today" Undergrad Colloquium (Physics 301)
South Pole Ultra-high-energy neutrinos with ARA
Time: 1:20 pm
Place: 2223 Chamberlin
Speaker: John Kelly, UW Madison Department of Physics
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Wednesday, April 9th, 2014

Department Meeting
Time: 12:15 pm
Place: 5310 Chamberlin Hall
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Thursday, April 10th, 2014

Astronomy Colloquium
Supermassive black hole binaries: The Search Continues
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Tamara Bogdanovvic, Georgia Tech
Abstract: Gravitationally bound supermassive black hole binaries are thought to be a natural product of galactic mergers and growth of the large scale structure in the universe. They however remain observationally elusive, thus raising a question about characteristic observational signatures associated with these systems. In my talk I will discuss current theoretical understanding and latest advances made in observational searches for supermassive black hole binaries.
Host: Audra Hernandez
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Friday, April 11th, 2014

Theory Seminar (High Energy/Cosmology)
Light from Dark Strings
Time: 2:15 pm
Place: 5280 Chamberlin Hall
Speaker: Andrew Long, Arizona State University
Abstract: I will suppose that there is new physics above the TeV scale that is sequestered from the Standard Model in a hidden sector. One may want to invoke such a scenario to develop a viable model of dark matter, to motivate Z’ searches at colliders, or to ameliorate electroweak baryogenesis. It has not been generally recognized that if the hidden sector contains a spontaneously broken Abelian gauge symmetry, then our universe is permeated by a network of cosmic “dark strings.” With a mass scale of, say, 10 TeV these strings are far too light to be exposed by the standard gravitational probes. However, with a coupling to the (comparatively light) Standard Model fields, the strings can produce SM particles in abundance, and their decay products may be observed on Earth. Over the recent years, there has been significant success in constraining empirical models of cosmic strings via astrophysical probes, particularly the diffuse gamma ray flux measured by EGRET and Fermi-LAT. In this talk, I will focus on a simple model that allows only two points of interaction with the SM: the so-called gauge kinetic mixing and the Higgs portal, which have been probed up to the TeV scale by a variety of terrestrial tests. I will discuss whether bounds on the diffuse gamma ray flux that originates from cosmic dark strings provides a complimentary test at or above the TeV scale.
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Physics Department Colloquium
Liquid Metal Dynamos : GEO/Astrophysics in the Lab
Time: 3:30 pm
Place: 2241 Chamberlin Hall (coffee at 4:30 pm)
Speaker: Jean-François Pinton, École normale supérieure de Lyon, CNRS
Abstract: The dynamo, a conversion of kinetic energy into magnetic energy in planets and stars is a fascinating puzzle, full of surprises. I will review some progress (and many more questions!) after over 15 years of lab experiments using liquid metals.
Host: Forest
Poster: http://www.physics.wisc.edu/twap/posters/2014/3028.pdf
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"This Week at Physics" poster: http://www.physics.wisc.edu/twap/posters/2014/2014-04-07.pdf

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