January 30, 2023 (Monday) 4:00-5:00p.m.
Speaker: Yan Yang, University of Delaware
Hosts: S. Mordijck and M. Sher
Title: Multiscale Nature of Turbulence in Space Plasmas
Abstract: Turbulence enters into space plasmas in many guises. The complexity and variability of the behavior of plasma turbulence is in large part due to the involvement of dynamics at many scales, ranging from macroscopic fluid to sub-electron scales. Based on what plasma properties we are interested in studying, be they dominant at small or large scales, a plasma can be treated as tractable models in various limits, such as the kinetic theory and magnetohydrodynamic (MHD) theory. Turbulence flows are characterized by the nonlinear transfer of energy and other quantities across a huge range of scales. Observed turbulence in space is expected to involve cross-scale energy transfer and subsequent dissipation and heating. Space plasmas are frequently taken to be weakly collisional or collisionless. Therefore, an explicit form of viscous dissipation as in collisional (e.g., MHD) cases cannot be easily defined. A variety of approaches have attempted to characterize specific mechanisms (e.g., magnetic reconnection, wave-particle interaction and turbulent-driven intermittency) and to quantify the dissipation. However, the community has not come to a consensus solution applicable to all systems. In this talk I will first give an overview of some basic properties for turbulence. Then I will briefly review turbulence theory application in space plasmas. I will discuss in detail how to disentangle multiscale properties, how plasma dynamics bridges multiple scales, what new ingredients are introduced in cross-scale transfer as models progress from fluid to kinetic, and how to identify key steps in energy transfer and estimate energy dissipation rate in weakly collisional plasmas. These also motivate several unresolved issues that may be addressed by future studies. Where feasible, examples are given from MHD, Particle in Cell, and hybrid Vlasov-Maxwell simulations, and from Magnetospheric Multiscale (MMS) observations.
February 13, 2023 (Monday) 4:00-5:00p.m.
Speaker: Rogerio Jorge, IST, University of Lisbon
Hosts: S. Mordijck and M. Sher
Title: Recent nuclear fusion breakthroughs using stellarator optimization
Abstract: The success of magnetic confinement nuclear fusion requires magnetic fields with optimal properties. Namely, these should provide the necessary plasma performance to sustain high-temperature plasmas for a long enough time. However, there are still important outstanding issues in magnetic fusion, such as the understanding and control of plasma density, fast particles, and turbulence. To achieve good confinement, recent experiments such as HSX and W7-X have been designed using optimization based on the calculation of MHD equilibria at each evaluation of the objective function. Here, we show how new design methods have been able to reduce the computational cost of such optimization efforts by orders of magnitude, while also providing new insights into the space of solutions. These methods enable a direct geometrical construction of magnetic fields with extremely good confining properties such as quasisymmetry and quasi-isodynamic fields. Thanks to the reduced computational cost, we are able to achieve good particle confinement with much higher accuracy than reported before and perform wide surveys over parameter space. Furthermore, many figures of merit can be now calculated directly, including MHD stability and heat flux, making the design of a fusion machine optimized for turbulent transport and fast particles a possibility in the near future.