Monday February 28, 2022 at 2 p.m., 4-330 PAB: Laszlo Bardoczi, General Atomics: New Insights and Solutions to Long Standing Problems in Nuclear Fusion: Magnetic Island Flux Tunneling, Heteroclinic Bifurcation and Seeding by Non-Linear Three-Wave Coupling.
Abstract: Tearing modes are magnetohydrodynamic instabilities that form magnetic island structures in tokamaks whose topology supports fast transport and rapid growth. These characteristics enable the islands to degrade the magnetic confinement or even rapidly terminate tokamak plasmas, hindering the development of operational scenarios in the path to fusion energy production. Four new important aspects of tearing mode physics have been confirmed through recent experiments: seeding by nonlinear three-wave coupling, heteroclinic bifurcation, interaction with micro-turbulence, and flux tunneling. The physics insights from these experiments, coupled with modeling and theory support, offer new solutions to avoiding and mitigating tearing modes.
Monday March 7, 2022 at 2 p.m., 4-330 PAB: Chaojie Zhang, University of California, Los Angeles: Optical-field ionized gases: a new route for studying kinetic plasma instabilities
More than 99% of the observable matter in our universe is in the plasma state. When the velocity distribution functions of the plasma electrons, ions, or both are nonthermal, plasmas are susceptible to kinetic instabilities. Kinetic instabilities are ubiquitous in both laboratory and astrophysical plasmas and their theory has been one of the foundational pillars of plasma physics. Yet the experimental studies of them have been relatively limited due to the lack of suitable experimental platforms. In this talk, I will introduce a new platform that is suitable for investigating kinetic instabilities such as streaming, filamentation, and Weibel instabilities. The platform allows one to initialize nonthermal and anisotropic electron velocity distributions in a controllable manner and then to follow the onset, growth, and saturation of these instabilities. Experimental results will be presented.
Monday March 14, 2022 at 2 p.m., 4-330 PAB: Derek Schaeffer, Princeton University: Collisionless Shockwaves in Magnetized High-Energy-Density Laboratory Plasmas
As a fundamental process for converting kinetic to thermal energy, collisionless shocks are ubiquitous throughout the heliosphere and astrophysical systems, from Earth's magnetosphere to supernova remnants. While these shocks have been studied for decades by spacecraft, telescopes, and numerical simulations, there remain key open questions in the fundamental physics of collisionless shocks, such as: How do shocks accelerate particles to extremely high energies? or How is energy partitioned between particles across a shock?
In this talk, I will discuss results from high-energy-density experiments and simulations on the formation of supercritical collisionless shocks created through the interaction of a supersonic laser-driven magnetic piston and magnetized ambient plasma. Through proton and refractive imaging, we observe for the first time a fast, high-Mach-number shock, comparable to some of the strongest shocks in the heliosphere. By probing particle velocity distributions with Thomson scattering, we directly measure the coupling interactions between the piston and ambient plasmas that are critical steps in the formation of magnetized collisionless shocks. Particle-in-cell simulations constrained by experimental data further detail the shock formation process and predict key signatures that are observed in experiments. I will also discuss how the development of this experimental platform can complement, and in some cases overcome, the limitations of similar measurements undertaken by spacecraft missions and can allow novel investigations of energy partitioning and particle acceleration in shocks.