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This Week at Physics

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Events During the Week of October 27th through November 3rd, 2019

Monday, October 28th, 2019

Plasma Physics (Physics/ECE/NE 922) Seminar
Efficient Numerical Methods for Stellarator Optimization
Time: 12:05 pm
Place: 2241 Chamberlin Hall
Speaker: Antoine Cerfon, NYU
Abstract: Stellarators can be designed in such a way as to provide plasma confinement without any contribution from the plasma itself. They are inherently steady-state, and mostly free from disruptions. However, these advantages as compared to other confinement devices come at the cost of increased technological complexity, and a design parameter space which has a much higher dimension. The high dimensionality of the design parameter space can be viewed as an opportunity to rely predominantly on numerical optimization to find particularly attractive reactor designs. But it can only be a practical strategy for the rapid development of commercial fusion if one possesses high performance numerical tools to efficiently explore the parameter space, and optimize over it. In this talk, I will present new solvers developed with that goal in mind, as part of the ongoing Simons collaboration on Hidden Symmetries and Fusion Energy. Specifically, I will introduce new methods for computing stellarator equilibria more efficiently and accurately, and also present the first results of a new approach for stellarator optimization based on direct coil-plasma optimization with analytically computed derivatives. Although the presentation focuses on new numerical solvers, it is intended for a physics audience, and will highlight the central physics ideas in our work rather than algorithmic and mathematical details.
Host: John Sarff
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Tuesday, October 29th, 2019

Chaos & Complex Systems Seminar
The role of auditory hair cells in frequency discrimination
Time: 12:05 pm
Place: 4274 Chamberlin (refreshments will be served)
Speaker: Robert Fettiplace, UW Department of Neuroscience
Abstract: Hair cells, the sensory receptors of the vertebrate inner ear, convert incident sound stimuli into electrical signals. They also separate the sound frequency components along the cochlea behaving like an acoustic prism. Frequency analysis underlies the ability to identify environmental sounds and categorize conspecific calls, and is implemented by two distinct mechanisms. In all vertebrates except mammals, the hair cell receptor potential is electrically tuned by voltage-dependent membrane ion channels, but this process has a limited upper frequency range of a few kHz. In mammals, broad mechanical tuning is augmented by contractions of outer hair cells, underpinned by the piezoelectric protein prestin. Both mechanisms are supported by gradients in hair cell properties along the cochlea. Examples of such gradients include the numbers and types of voltage-dependent potassium channel and of the mechanically-sensitive transducer channel. An important clinical correlate is that hair cells tuned to high frequencies are much more vulnerable to damage by noise and ototoxic agents, and are the first to be lost with aging.
Host: Clint Sprott
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Council Meeting
Physics Council Meeting
Time: 3:00 pm
Place: 2314 Chamberlin Hall
Speaker: Sridhara Dasu, UW-Madison
Host: Sridhara Dasu, Department Chair
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Wednesday, October 30th, 2019

Department Meeting
Department Meeting
Time: 12:15 pm
Place: B343 Sterling Hall
Speaker: Sridhara Dasu, UW-Madison
Host: Department Chair
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Thursday, October 31st, 2019

R. G. Herb Condensed Matter Seminar
Wrestling with ultrastrongly coupled parasitic modes in circuit quantum electrodynamics
Time: 11:00 am
Place: 5310 Chamberlin Hall
Speaker: Dr. Ivan Pechenezhskiy , University of Maryland
Abstract: Several promising superconducting qubit designs, including the protected ones, require superconductors operating as perfect inductors at microwave frequencies. As the value of the inductance increases with the inductor length, so does the parasitic capacitance due to the spatial extension of the inductor. Parasitic self-resonances formed this way impede the potential applications of extremely large inductances. Even in the simplest superconducting qubit circuit, in which a Josephson junction is shunted by a hyperinductor, the distributed nature of parasitic capacitance leads to an ultrastrong coupling of the parasitic modes to the qubit. This ultrastrong coupling of the parasitic modes prevents a perturbation theory treatment of the qubit excitation spectra. While a complete quantum description of the underlying qubit circuit is computationally prohibitive, diagonalization of an effective multi-mode Hamiltonian reproduces the experimental data both below and above the lowest parasitic modes. On the experimental side, the effects of the parasitic modes can be partially mitigated by releasing the qubit circuit off the substrate. This trick enables the realization of a new qubit design — blochnium.
Host: McDermott
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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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Astronomy Colloquium
"Suppressed Superwinds: A New Paradigm for Extreme Massive-Star Feedback”
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee and cookies 3:30 PM, Talk begins 3:45 PM
Speaker: Sally Oey, University of Michigan
Abstract: Feedback from young, super star clusters (SSCs) is a major driver for galaxy evolution at all cosmic epochs, but may be especially relevant for cosmic reionization. The standard scenario for massive-star feedback has been that superwinds from SSCs clear pathways for Lyman continuum (LyC) and Ly-alpha radiation to escape from host galaxies. However, theoretical predictions indicate that for the most massive and compact SSCs, superwinds are actually suppressed by catastrophic cooling and pressure confinement. These extreme conditions are rare, but observational evidence from local starbursts increasingly supports this scenario. I will present our work in establishing this new paradigm and its consequences for LyC and Ly-alpha escape.
Host: Professor Amy Barger
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Friday, November 1st, 2019

Thesis Defense
Justin Walker Thesis Defense
Time: 10:00 am
Place: B343 Sterling Hall
Speaker: Justin Walker, Physics PhD Graduate Student
Abstract: Instability-driven Magnetohydrodynamic Turbulence
Host: Stas Boldyrev, Faculty Advisor
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