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Events on Friday, September 17th, 2021

Thesis Defense
ITG Turbulence Saturation and Near-Resonant Heat Flux Reduction in Gyrokinetic Dimits-Shift Analysis
Time: 9:00 am
Place: zoom link below
Speaker: Ping-Yu Li , Physics PhD Graduate Student
Abstract: Microturbulence is caused by gyroradius-scale instabilities such as the Ion-Temperature-Gradient-driven (ITG) instability, Trapped Electron Mode (TEM), Kinetic Ballooning Mode (KBM), etc. Understanding how these instabilities saturate and form turbulence is important for the optimization of magnetic confinement fusion devices in the quest for sustained fusion energy. The objective of this thesis is to understand the important factors and mechanisms that saturate ITG turbulence and to utilize said understanding to build reduced models that capture key physical behavior as described by full-physics approach. Zonal-flow-catalyzed interactions that involve large-scale stable and unstable modes are crucial for the saturation of curvature-driven ITG turbulence. A corresponding saturation theory is built based on a fluid model and implemented and tested numerically. The crudest saturation theory drops the non-zonal interactions and also the nonlinear corrections to frequencies, it also truncates the wavenumber space to obtain scalings for the saturation level with the triplet correlation times with linear frequencies and coupling coefficients. It is then discovered that nonlinear interactions can cause nonnegligible modifications on the mode oscillations for systems with higher turbulence level. Furthermore, the kx direction in wavenumber space needs to be resolved in order to break the symmetry between modes and build up the zonal flow, which is shown in both time-dependent and time-independent research. Constructing a two-predator-prey model with no free parameter inputted base on the saturation theory is also demonstrated. This provides an idea how to build a predator-prey model from the first principle, which has the potential to help understanding the limit-cycle oscillations observed in L-H transition. The importance of large-scale stable modes and the triplet correlation time derived from the saturation theory are tested in gyrokinetics. Numerical results show that the resonance between the stable and unstable modes through the coupling of zonal flow corresponds to long nonlinear interaction life times, or large triplet correlation times, which increases nonlinear energy transfer and leads to strong turbulence suppression beyond any purely linear estimates. The triplet correlation time is further used to improve a highly reduced model for fast heat-flux prediction in gyrokinetics, which shows significant improvement in several cases that demonstrate heat-flux onset upshift from the linear critical gradient for gradient scans. The role of the coupling coefficient in gyrokinetics is still under investigation. Join Zoom Meeting Meeting ID: 325 961 7260 Passcode: 61T1p8
Host: Paul Terry, Faculty Advisor
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Graduate Introductory Seminar (Physics 701)
Intersection of Neutrino/Dark Matter Physics and Astrophysics with Quantum Information Science
Time: 12:05 pm - 12:55 pm
Place: 2241 Chamberlin
Speaker: A. Balantekin, UW Madison Department of Physics
Host: Sridhara Dasu
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Physics Department Colloquium
Supermassive Black Holes and Merging Galaxies: Low-Frequency Gravitational Wave Detection with Pulsar Timing Arrays
Time: 3:30 pm
Place: 2103 Chamberlin Hall
Speaker: Sarah Vigeland, UW-Milwaukee
Abstract: Observations have shown that nearly all galaxies harbor massive or supermassive black holes at their centers. Gravitational wave (GW) observations of these black holes will shed light on their growth and evolution, and the merger histories of galaxies. Pulsar timing arrays (PTAs) use observations of millisecond pulsars to detect low-frequency GWs with frequencies ~1-100 nHz, and can detect GWs emitted by supermassive black hole binaries, which form when two galaxies merge. In this talk, I will discuss the current status of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) PTA, with an emphasis on results from our most recent search for the stochastic GW background. I will also discuss future prospects for detecting and characterizing GWs from individual supermassive binary black holes with PTAs.
Host: Ellen Zweibel
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