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This Week at Physics

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Events During the Week of March 24th through March 31st, 2019

Monday, March 25th, 2019

Plasma Physics (Physics/ECE/NE 922) Seminar
On The Interactions of Magnetic Fluctuations, Zonal Flows, and Microturbulence in Fusion Plasmas
Time: 12:05 pm
Place: 2241 Chamberlin Hall
Speaker: Zach Williams, University of Wisconsin-Madison, Department of Physics
Abstract: Analytic theory and gyrokinetic simulations show that turbulence-regulating zonal flows are weakened by radial magnetic field fluctuations as a consequence of particle streaming along radial fields and shorting out cross-flux-surface potential differences. Two prominent sources of radial magnetic field fluctuations are studied here, resonant magnetic perturbations (RMPs) in tokamaks and tearing modes in reversed-field pinches (RFPs). This work focuses on understanding the inherently multi-scale nature of the interplay of microturbulence, zonal flows, and large-scale magnetic fluctuations and its effect on transport. This interplay is studied with gyrokinetics to model DIII-D tokamak and MST RFP plasmas. An imposed magnetic perturbation that mimics a tearing mode increases the level of trapped-electron-mode turbulence to a level consistent with fluctuation and transport measurements in MST plasmas. This motivated a dedicated experiment on DIII-D to study the impact of varying RMP amplitude on turbulence in inboard-limited L-mode plasmas. Experimental observations demonstrate a clear dependence of microturbulence levels on RMP strength. Gyrokinetic simulations confirm this dependency, and show direct connections of this behavior to zonal flow degradation via magnetic fluctuations. To approach the underlying physics from a third angle, preliminary results from simulations which include a newly-incorporated current gradient drive that allow for the self-consistent generation of tearing modes and their interactions with microinstability are presented.
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Tuesday, March 26th, 2019

Chaos & Complex Systems Seminar
A form for the feeling of being alive and kicking”: Chaos and structure in Joyce’s Finnegans Wake
Time: 12:05 pm
Place: 4274 Chamberlin (refreshments will be served)
Speaker: Kevin Reilly, Former President UW System
Abstract: Right from the start, Finnegans Wake subverts the usual order by beginning at the end and ending at the beginning. Its very title is a spring-loaded word trick. The Wake is perhaps the most complex and unread of all the great works of world literature—and a great joke book too! We will read it out loud as Joyce intended, and hear how its fun chaos in language calls up profound speculation on life.
Host: Clint Sprott
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Council Meeting
CANCELED
Time: 3:30 pm
Place: 2314 Chamberlin Hall
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Wednesday, March 27th, 2019

No events scheduled

Thursday, March 28th, 2019

Cosmology Journal Club
Time: 12:00 pm
Place: 5242 Chamberlin Hall
Abstract: Please visit the following link for more details:
http://cmb.physics.wisc.edu/journal/index.html
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 Ross Cawthon (cawthon@wisc.edu) and Santanu Das (sdas33@wisc.edu).
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WISELI Seminar
The Role of Confidence-Building (By Self & Others) in the Recruitment and Retainment of Underrepresented Students in STEM
Time: 3:00 pm
Place: 4274 Chamberlin Hall
Speaker: Dr. Rellen Hardtke, UW River Falls
Host: Joelle Baer
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Astronomy Colloquium
"Clues to Globular Cluster Formation"
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee and cookies 3:30 PM, Talk begins 3:45 PM
Speaker: David Nataf, The Johns Hopkins University
Abstract: Globular clusters are now well-established to host “Second-generation” stars, which show anomalous abundances in some or all of He, C, N, O, Na, Al, Mg, etc. The simplest explanations for these phenomena typically require the globular clusters to have been ~20x more massive at birth, and to have been enriched by processes which are not consistent with the theoretical predictions of massive star chemical synthesis models. The library of observations is now a vast one, yet there has been comparatively little progress in understanding how globular clusters could have formed and evolved. In this talk I discuss two new insights into the matter. First, I report on a meta-analysis of globular cluster abundances that combined APOGEE and literature data for 42 globular clusters, new trends with globular cluster mass are identified. I discuss the chemical properties of former globular cluster stars that are now part of the field population, and what can be learned. Finally, use updated state-of-the-art massive star stellar evolution models from the Padova group to make predictions for the upcoming and inevitable JWST observations of young globular cluster progenitors in their starburst phase.
Host: Alexander Lazarian
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Friday, March 29th, 2019

Physics Department Colloquium
Ferromagnetic Josephson Junctions for Cryogenic Memory
Time: 3:30 pm
Place: 2241 Chamberlin Hall
Speaker: Norman O. Birge, Michigan State University
Abstract: Large-scale computing facilities and data centers are using electrical power at an ever increasing rate. Projections suggest that a future “exoscale” computer will require the power output of a typical nuclear power plant – clearly an untenable situation. One approach to addressing this problem is to build a computer out of all superconducting elements, which dissipate very little power. Such a computer would have to be cooled to cryogenic temperatures, so it must be extremely energy-efficient to justify the added complexity and cost associated with cooling. Superconducting logic circuits based on manipulating single flux quanta have existed for 30 years; what has been missing is a high-density, fast, and energy-efficient cryogenic memory. One approach is to use Josephson junctions containing ferromagnetic (F) materials as the memory element for such a memory. The basic building block is a Josephson junction containing two ferromagnetic layers whose magnetization directions can be switched between being parallel or antiparallel to each other, as in a conventional spin valve. We have demonstrated successful switching of such a junction between the “0” phase state and the “π” phase state, from measurements of two junctions in a SQUID geometry. An alternative approach is to use a Josephson junction containing three ferromagnetic layers, which is designed to carry spin-triplet supercurrent. We have also realized controllable 0 - π switching in such a spin-triplet junction. At the end of the talk I’ll mention what needs to be done to turn these results into a real superconducting computer.
Host: Alex Levchenko
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