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Events During the Week of November 27th through December 4th, 2022

Sunday, November 27th, 2022

Academic Calendar
Thanksgiving recess
Abstract: *Note: actual end time may vary.* University offices closed. URL:
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Monday, November 28th, 2022

Plasma Physics (Physics/ECE/NE 922) Seminar
Advanced Approaches in Stellarator Optimization
Time: 12:00 pm - 6:00 pm
Place: 1610 Engineering Hall
Speaker: Sophia Henneberg, IPP Greifswald
Abstract: Stellarators possess three-dimensional magnetic fields to generate external rotational transform -- rotational transform solely generated by the coils’ magnetic field. This reduces or even eliminates the need for generating toroidal plasma currents, which can lead to instabilities such as disruptions. However, the three-dimensionality can in general involve some drawbacks, e.g., more complicated coils are typically needed compared to the axisymmetric case. Nonetheless, with careful exploitation of the large design space via optimization, the apparent disadvantages can be diminished.
In stellarator optimization studies, the boundary of the plasma is usually described by Fourier series that are not unique: several sets of Fourier coefficients describe approximately the same boundary shape. A simple method for eliminating this arbitrariness is proposed and shown to work well in practice. Additionally, we investigate the mathematical structure of the various inter-related calculations that underpin the integrated stellarator optimization problem to better understand how the equilibrium calculation, the coil calculation, and the optimization calculation communicate with each other.

Dr. Sophia Henneberg is a researcher in the stellarator theory group at IPP Greifswald. Her current research focuses on analytic and numeric approaches to stellarator optimization. She obtained her PhD at the University of York in 2016. Before her PhD she completed her Fulbright year with a master’s degree in Madison, Wisconsin.
Host: ECE/NE/PHYSICS 922 Seminar in Plasma Physics
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Tuesday, November 29th, 2022

Thesis Defense
Asymptotic series and gravitational particle production
Time: 2:00 pm
Place: 4274 Chamberlin
Speaker: Edward Basso, Physics Graduate Student
Abstract: The dynamics of the coherent oscillation period of inflation are solved using a formalism of asymptotic expansions reminiscent of renormalization in gauge theory. This yields an analytic description of the fluctuations observed in the spectral density of gravitationally produced particles. These spectral fluctuations can be interpreted as quantum interference between gravitational scattering amplitudes due to the coherent oscillatory state of the inflaton field.

Host: Dan Chung
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Theory Seminar (High Energy/Cosmology)
Dark Matter Substructure: simulations, semi-analytic models and observations
Time: 4:00 pm - 5:00 pm
Place: Chamberlin 5280
Speaker: Xiaolong Du, Carnegie Observatories & UCLA
Abstract: Roughly 85% of the matter in the Universe is in the form of invisible matter, i.e. dark matter. Under gravitational interactions, dark matter clusters and forms hierarchical structure. Over the last decades, simulations with higher and higher resolutions have greatly improved our understanding of the formation of dark matter halos and the properties of their substructure (subhalo). Simulations of the standard cold dark matter (CDM) show that CDM halos have cuspy density profiles and contain a large number of small subhalos, which are not fully consistent with the current observations of dwarf galaxies. Thus different dark matter models, such as warm dark matter, self-interacting dark matter, fuzzy dark matter, have been proposed, which predict very different phenomena on small scales (~kpc). Running numerical simulations under different dark matter assumptions requires a large amount of computing resource. On the hand, even the state-of-art simulations suffer from numerical artifacts, limiting their predicting power on small scales. In this talk, I will talk about some of our latest progresses in modeling the evolution of subhalos using the semi-analytic code Galacticus and how we can model and control the numerical artifacts. Using our semi-analytic models, we can get accurate predictions for the abundance of subhalos and the evolution of their density profiles due to tidal effects. Our semi-analytic models are ~10^4 times faster than direct simulations, making it possible to generate a large number of realizations of dark matter halo systems with correct substructure, which are useful for getting robust constraint on different dark matter models from observations such as strong gravitational lensing.
Host: George Wojcik
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Wednesday, November 30th, 2022

No events scheduled

Thursday, December 1st, 2022

R. G. Herb Condensed Matter Seminar
Imaging the Breaking of Electrostatic Dams in Graphene for Ballistic and Viscous Fluids
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Zachary Krebs , UW-Madison
Abstract: Under special conditions the charge carriers in a material can behave as a viscous fluid. In this work, we investigate such behavior by using scanning tunneling potentiometry to probe the nm-scale flow dynamics of electron fluids in graphene as they pass through constrictions defined by smooth and tunable in-plane p-n junction barriers. We observe that as the sample temperature and channel widths are increased, the electronic fluid flow undergoes a Knudsen-to-Gurzhi transition from ballistic to viscous flow that is characterized by a channel conductance that exceeds the ballistic limit, and suppressed charge accumulation against the barriers. Our results are well-modeled by finite-element simulations of 2D viscous current flow, and they illustrate how Fermi liquid flow evolves with carrier density, channel width, and temperature.
Host: Alex Levchenko
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Preliminary Exam
Exploration of problems in space physics using the Magnetospheric Multiscale Mission
Time: 10:00 am
Place: B343 Sterling or
Speaker: Jack Schroeder, Physics Graduate Student
Abstract: The main topic of discussion will be on recent work on magnetic reconnection in the Earth’s magnetotail. Models for collisionless magnetic reconnection in near-Earth space are distinctly characterized as 2D or 3D. In 2D kinetic models, the frozen-in law for the electron fluid is usually broken by laminar dynamics involving structures set by the electron orbit size, while in 3D models the width of the electron diffusion region is broadened by turbulent effects. We present an analysis of in situ spacecraft observations from the Earth's magnetotail of a fortuitous encounter with an active reconnection region, mapping the observations onto a 2D spatial domain. While the event likely was perturbed by low-frequency 3D dynamics, the structure of the electron diffusion region remains consistent with results from a 2D kinetic simulation. As such, the event represents a unique validation of 2D kinetic, and laminar reconnection models.

Additionally, future work regarding magnetic pumping at Earth’s bow shock will be discussed.
Host: Jan Egedal
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
The Changing Landscape of Astrophysical Searches and Indirect Detection of Dark Matter
Time: 2:30 pm - 3:30 pm
Place: CH 4274
Speaker: Eric Charles, SLAC
Abstract: Ten years ago, astrophysical searches and indirect detection of dark matter largely meant “searches for WIMP annihilation signatures in specific targets such as Dwarf Spheroidal galaxies or the Galactic Center with the Fermi Large Area Telescope”. Since then the landscape has expanded dramatically; the techniques we use, the places we look, the sort of information that we hope to extract from our searches, all of these have evolved as we have explored the sky in new ways with the advent of new observing facilities such as GAIA, DES, IceCube, and will continue to evolve as JWST, Euclid, Roman and Rubin revolutionize the way we see the sky. In this talk I will describe those changes, the motivations behind them, and the implications for how we need to work together going forward to make the best use of the facilities that will be coming on-line
Host: Ke Fang
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Astronomy Colloquium
Why do massive stars have "inflated" cores?
Time: 3:30 pm - 4:30 pm
Place: 4421 Sterling Hall
Speaker: Dr. Evan Henry Anders, Northwestern University
Abstract: Stars with masses greater than about 1.1 M_sun have turbulent convection in their cores. Standard stellar evolution models fail to reproduce many observations, but models and observations can be brought into agreement by "inflating" the core with excess mixing beyond the boundary of the convection zone. In this colloquium, I will present a review of observations of excess mixing in the cores of massive stars. I will discuss how excess mixing affects stellar evolution and the populations of stars and compact objects which are being characterized by space-based missions like Gaia and ground-based gravitational wave interferometers like LIGO. I will then discuss different forms of convective boundary mixing from a fluid dynamics perspective, talking about three processes which likely occur in stellar interiors. I will discuss one or two sets of simulations which we are using to shed light on this tricky problem in modern stellar astrophysics.
Host: Ke Zhang
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Friday, December 2nd, 2022

Preliminary Exam
First Steps for a Joint Analysis of the MINOS and NOvA Experiments
Time: 9:00 am
Place: 4274 Chamberlin
Speaker: Anna Cooleybeck, Physics Graduate Student
Abstract: The MINOS and NOvA experiments are both long-baseline neutrino oscillation experiments, with a focus on measuring 3-flavor oscillation parameters from a muon neutrino beam. While they utilize the same neutrino beam, MINOS is directly on the beam axis and NOvA is 14.6 mrad off axis, giving us the opportunity to constrain beam-related systematic uncertainties. By analyzing data from both these experiments together, we can better understand the uncertainties present while still retaining the unique advantages of each experiment. This talk will discuss this novel analysis technique and present the first steps towards developing a framework for the complete analysis
Host: Brian Rebel
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Graduate Program Event
Physicists in Finance. A Personal Journey
Time: 11:00 am - 6:00 pm
Place: 4274 Chamberlin Hall
Speaker: Dr. Senthil Sundaram, KULA Investments, LLC
Abstract: The speaker will discuss: 1) Evolution of career prospects for physicists in the finance industry over the last 25 years, and 2) Skillsets that physicists possess that make them eminently suitable for many roles in finance.

Host: Shivani Lomte /PGSC
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Physics Department Colloquium
Quantitative Trading and Portfolio Management
Time: 3:30 pm - 5:00 pm
Place: 2241 Chamberlin Hall -
Speaker: Senthil Sundaram
Abstract: Quantitative methods and the attendant systems architecture used to generate higher risk-adjusted returns. (How the heck do companies like Renaissance and Two Sigma make so much money using scientists?)
Host: Mark Eriksson
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Academic Calendar
Graduate School Fall 2022: Request for all Master's and Doctoral Degree Warrants
Time: 4:00 pm - 5:00 pm
Abstract: CONTACT: 262-2433,
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