Events at Physics |
Events During the Week of January 26th through February 2nd, 2025
Monday, January 27th, 2025
- Plasma Physics (Physics/ECE/NE 922) Seminar
- "Optimizing a quasi-helically-symmetric stellarator for reduced TEM-driven transport"
- Time: 12:00 pm - 1:15 pm
- Place: 1227 Engineering Hall
- Speaker: Michael Gerard, University of Wisconsin - Madison
- Abstract: Magnetic confinement fusion (MCF) aims to deliver a carbon-free energy source to meet global demands. Turbulent heat and particle transport across magnetic fields remains a key challenge for MCF, limiting confinement and hence reactor performance. The 3D magnetic fields of stellarators offer a promising path forward by enabling optimized configurations for enhanced plasma confinement. To identify novel turbulence-optimization experiments that can be performed in the Helically Symmetric eXperiment (HSX) stellarator, the operational space of HSX has been explored by varying individual coil currents to produce a database of over 106 magnetohydrodynamic equilibria. From this database, we identify ~103 configurations that preserve the neoclassical properties of the standard quasi-helically symmetric (QHS) configuration while exhibiting a broad spectrum of modified magnetic field geometries. Using the gyrokinetic code GENE, we demonstrate that increased flux-surface elongation reduces the growth rates of the trapped-electron mode (TEM)—the dominant microinstability in HSX—provided quasi-helical symmetry is maintained. This stabilization is attributed to geometric effects that suppress destabilizing particle drifts. Quasilinear modeling and nonlinear simulations, which compare the QHS configuration with a high-elongation QHS (HE-QHS) configuration, reveal that changes in the linear growth rates fail to capture changes in the nonlinear heat-flux between the two configurations. This discrepancy is attributed to large-scale quasi-coherent fluctuations that appear in both configurations. Evidence is presented that suggests these large-scale modes are driven by a set of tearing-parity TEMs (TTEMs), which couple nonlinearly through the zonal flow. Moreover, the TTEMs are shown to be correlated with the appearance of micro-tearing modes, despite such modes being linearly stable. How these dynamics relate to the underlying magnetic field geometry is discussed, providing new insights into TEM-driven turbulence in quasi-helically symmetric magnetic fields. Moreover, preliminary results comparing the simulation data with experimental interferometry data are discussed.
- Host: Prof. Adelle Wright
Tuesday, January 28th, 2025
- Physics Education Innovation Seminar
- Spring Brown Bag/ Planning Meeting
- Time: 1:00 pm - 2:00 pm
- Place: B343 Sterling Hall
- Speaker: Physics Education Innovation Forum Steering Committee, University of Wisconsin - Madison
- Host: Josh Weber
- Wisconsin Quantum Institute
- Quantum Coffee Hour
- Time: 3:00 pm - 4:00 pm
- Place: Rm.5294, Chamberlin Hall
- Abstract: Please join us for the WQI Quantum Coffee today at 3PM in the Physics Faculty Lounge (Rm.5294 in Chamberlin Hall). This series, which takes place approximately every other Tuesday, aims to foster a casual and collaborative atmosphere where faculty, post-docs, students, and anyone with an interest in quantum information sciences can come together. There will be coffee and treats.
Wednesday, January 29th, 2025
- Preliminary Exam
- Interpolation in Cell-Based Coaddition Images and Impacts on Shear Measurements
- Time: 10:00 am - 12:00 pm
- Place: B343 Sterling
- Speaker: Miranda Gorsuch, Physics PhD Graduate Student
- Abstract: Weak gravitational lensing encodes information of the structure of matter as well as the expansion history of the observable universe. However, the weak lensing signal is small, and requires measurements of the signal across a large sample of galaxies. In the era of the Vera C. Rubin Observatory and the Legacy Survey of Space and Time, billions of shape measurements will be detected across 18,000 square degrees. A galaxy sample of this scale affirms that we are in the era of precision cosmology; now considerable care must be taken to accurately measure the shapes of galaxies in order to preserve the weak lensing signal against systematics. Part of the work for maintaining accurate shape measurements includes a recent method for stacking images - or coadding - referred to as cell-based coadds. This coaddition scheme better preserves the shape measurements of galaxies by neglecting input images that do not fully cover the region of the coadd, which if included introduce systematic biases in the shape measurements. I will present my current work that focuses on interpolated pixels within cell-based coadd images. Allowing images with a large fraction of masked pixels into the final coadd may impact shear measurements; on the other hand, discarding too many input images may affect final coadd depth and object detection. I have developed a pipeline to determine the average fraction of input images lost due to the images going over the allowed fraction of masked pixels. The next stage of this project will use the metadetection algorithm to measure the shear response on simulated image data. The purpose of this stage is to characterize how shear measurements are impacted by changes in the maximum mask fraction of input images. Finally, an object detection stage will be done to understand impacts in the variation of coadded image depths.
- Host: Keith Bechtol
- Thesis Defense
- A high-spectral resolution study of the galactic center soft X-ray bulge with the X-ray Quantum Calorimeter
- Time: 12:00 pm - 2:00 pm
- Place: 6242 CH or
- Speaker: Conjeepuram V Ambarish, Physics PhD Graduate Student
- Abstract: The galactic center soft X-ray bulge, clearly seen in the ROSAT all-sky survey map in the 0.44-1.2 keV band, is one of the brightest diffuse X-ray features in the sky in this energy range. The extended nature of the emission suggests that the source is likely hot gas shock heated by supernovae. Deep shadows from molecular clouds at 2-4 kpc show that the soft X-ray bulge is not a local feature and is likely associated with either the galactic center or an inner spiral arm. The soft X-ray bulge has been studied previously with ROSAT and Suzaku, but with their energy resolution of ~ 300 eV and 60 eV respectively, they are only able to separate bulge emission from at least four foreground and background emission components without resorting to highly oversimplified models. The X-ray quantum calorimeter (XQC) is a sounding rocket instrument with silicon thermistor microcalorimeters operated at 50 mK. With a detector area of 1.44 cm2 mechanically collimated to a 60 degree field of view, XQC has a high throughput (~ 1 cm2 sr) and an energy resolution of ~ 8 eV FWHM below 1 keV. Since the soft X-ray bulge is too far south to observe from NASA’s usual launch site at White Sands Missile Range (WSMR), XQC was part of NASA’s long-awaited campaign to Australia, where we had a successful flight to observe the bulge. I present here a high resolution spectrum with clearly resolved lines from multiple ionization states of carbon, nitrogen, oxygen, neon and iron. A model-independent analysis of ratios of fluxes from these lines requires emission from a minimum of three thermal components at ~ 1,2, and 8 MK. I show that our spectrum is consistent with emission from either an adiabatic polytrope filling the inner galaxy or an isothermal halo at ~ 2 MK, both with additional X-ray emission from unresolved stars. An alternative explanation for the soft X-ray bulge could be a series of blowouts from star-forming regions in the inner spiral arm. The relative abundance estimates from the observation, particularly the Fe/O ratio, are used to distinguish between a galactic bulge or a spiral arm origin for the hot gas since these two regions should be dominated by different types of supernovae, type Ia and type II respectively. The metal yields of the two are distinct with Type Ia producing mostly iron and Type II producing oxygen and other alpha elements. Our fits indicate an Fe/O ratio of about twice the solar value, favoring enrichment by Type Ia supernova and a galactic bulge origin for the hot gas.
- Host: Dan McCammon
Thursday, January 30th, 2025
- Astronomy Colloquium
- Shake, rattle, and roll: waves, turbulence, and tides in stellar and planetary systems
- Time: 3:30 pm - 4:30 pm
- Place: 4421 Sterling Hall
- Speaker: Janosz Dewberry, Canadian Institute for Theoretical Astrophysics (CITA)
- Abstract: Today, modern telescopes, planetary science missions, and gravitational
wave experiments are collecting an unprecedented volume of precise data.
Insights from these data are in turn accelerating advances toward
accurate and efficient fluid dynamical models of stellar and planetary
systems: seismic signatures in Saturn’s rings are changing our picture
of the deep interiors of giant planets. Measurements of tides in Jupiter
and Saturn tell us about the structure and dynamics of rapidly rotating
gaseous bodies. Outside of our solar system, the interactions of
exoplanets and stars both with each other and with their environments
leave imprints in astronomical survey data. In this talk, I will
describe research focused on developing improved, multidimensional
models for planets and stars that rise to the challenge and opportunity
presented by modern astronomical observations. - Host: Melinda Soares-Furtado
Friday, January 31st, 2025
- Physics Department Colloquium
- TBA (CHIME and FRBs)
- Time: 3:30 pm - 4:30 pm
- Place: 2241 Chamberlin Hall
- Speaker: Kendrick Smith, Perimeter Institute for Theoretical Physics
- Host: Moritz Münchmeyer