UMD Plasma Seminar

Plasma Physics Seminar

Spring 2021 Schedule

January 27th, Wed. 3:30PM Virtual Presentation	Kevin Genestreti, SwRI The onset of reconnection in Earth's magnetotail The elongated tail of our nightside magnetosphere stores energy from the solar wind. The oppositely-directed magnetic fields in the northern and southern tail are separated by a current sheet, which periodically becomes very thin then "short circuits". Understanding the causes of magnetic reconnection - the "short circuit" mechanism - is an important yet elusive problem in space physics. We have learned a tremendous amount about reconnection by observing it while it is occurring in space, yet it is still debated how reconnection starts in the first place. Ambiguity stems from the difficulty of determining the accurate time history of a reconnection event over the vast range of relevant spatial scales. The standard picture is that reconnection of the solar wind and Earth's dayside magnetic fields drives global (~1015 km3) magnetospheric convection, which can thin the current sheet by compressing it or depleting its internal pressure. However, tail reconnection does not necessarily follow dayside reconnection and some thin current sheets remain stable. Microscopic (~107 km3) magnetic reconnection sites are formed after one of several proposed instabilities is triggered. While several instabilities accompany reconnection, causal relationships have been difficult to verify. We start the seminar by summarizing the basic concepts of magnetospheric reconnection and the wide-ranging impacts of reconnection on near-Earth space. We then report a case study where many space and ground-based observatories witnessed magnetotail reconnection being initiated. We find that the tail current sheet became thin by evacuating its internal thermal pressure without significant dayside reconnection. The solar wind prompted the pressure evacuation and, eventually, initiated reconnection by momentarily compressing the tail. Reconnection was initiated in multiple locations once the current sheet surpassed the threshold for the electron-tearing instability, which requires a sufficiently thin current sheet with a weak magnetic field and a low ion-to-electron temperature ratio. One reconnection site quickly engulfed the others, becoming the dominant region that shredded the tail's magnetic field, fitting with simple models.
February 3rd, Wed. 3:30PM Virtual Presentation	Gareth Roberg-Clark, Max-Planck-Institut fur Plasmaphysik Calculating the linear critical gradient for the ion-temperature-gradient mode in magnetically confined plasmas A first-principles method to calculate the critical temperature gradient for the onsetof the ion-temperature-gradient mode (ITG) in linear gyrokinetics is presented. We find that conventional notions of the connection length previously invoked in tokamak research should be revised and replaced by a generalized correlation length to explain this onset in stellarators. Simple numerical experiments and gyrokinetic theory show that localized "spikes" in shear, a hallmark of stellarator geometry, are generally insufficient to constrain the parallel correlation length of the mode. ITG modes that localize within bad drift curvature wells that have a critical gradient set by peak drift curvature are also observed. A case study of nearly helical stellarators of increasing field period demonstrates that the critical gradient can indeed be controlled by manipulating magnetic geometry, but underscores the need for a general framework to evaluate the critical gradient. We conclude that average curvature and global shear set the correlation length of resonant ITG modes near the absolute critical gradient, the physics of which is included through direct solution of the gyrokinetic equation. Our method, which handles general geometry and is more efficient than conventional gyrokinetic solvers, could be applied to future studies of stellarator ITG turbulence optimization.
February 10th, Wed. 3:30PM Virtual Presentation	Open
February 17th, Wed. 3:30PM Virtual Presentation	Open
February 24th, Wed. 3:30PM Virtual Presentation	Open
March 3rd, Wed. 3:30PM Virtual Presentation	Open
March 10th, Wed. 3:30PM Virtual Presentation	Open
March 17th, Wed. 3:30PM Virtual Presentation	Spring Break
March 24th, Wed. 3:30PM Virtual Presentation	Marina Battaglia, FHNW, Switzerland A multi-wavelength view on electron acceleration in solar flares Solar flares are the most energetic phenomena in the solar system, releasing and converting energies up to 10^25 Joules. They allow us to study fundamental processes in magnetized plasmas such as energy release, particle acceleration, and particle transport. The dominant signatures of solar flare accelerated electrons are in the X-ray and radio-wavelength domains. I will introduce the main concepts of X-ray and radio diagnostics of flare accelerated electrons. How do we observe at these wavelengths? What do we see? What can we learn from such observations about electron acceleration and transport? I will present recent highlights from the analysis of data from past and present observatories, including first results from the Spectrometer/Telescope for Imaging X-rays (STIX) on Solar Orbiter.
March 31st, Wed. 3:30PM Virtual Presentation	Dan Gordon, NRL Overview of Strong Field QED in Laser Plasmas The world's highest intensity lasers produce a combination of irradiance and electron energy such that quantum electrodynamics (QED) effects are important. We give an overview of the theoretical methodology that underlies the QED cross sections used in particle-in-cell (PIC) codes such as EPOCH and OSIRIS. We highlight generation of copious gamma rays and electron-positron pairs as a compelling physical outcome of the QED laser-plasma interactions, and show that EPOCH and OSIRIS are in quantitative agreement. We comment on physics that is left out of the standard treatment, and propose a novel cross section that goes beyond the current state of the art.
April 7th, Wed. 3:30PM Virtual Presentation	Richard Buttery, General Atomics The Advanced Tokamak Path to a Compact Fusion Pilot Plant The Advanced Tokamak concept provides an attractive path to develop a compact and inexpensive "pilot plant" to demonstrate net electricity and resolve nuclear technology and breeding. It works through a fortuitous alignment of high pressure operation with strong self-driven "bootstrap" current and low turbulent transport. Here, great research progress in transport, pedestal, stability and energetic particle physics has identified the key principles behind a solution, which will be explained in this talk. Furthermore, new "full physics" simulations show the trade-offs and path to optimize the approach: raising pressure increases fusion performance, but increasing the density has greater leverage, raising bootstrap current and decreasing auxiliary current drive demands from expensive RF systems. The efficient solutions found have high energy confinement, reducing the necessary fusion performance, heat and neutron loads for a net electric goal. Viable devices are predicted with a compact major radius of ~4m radius giving 200MW electricity at ~6T using conventional superconductors, or better still using high Tc superconductors at 7T, which provides greater performance margins and permits easier maintenance for the nuclear research mission. The plasma exhaust is managed by a combination of core radiation, flux expansion and radiative divertor, although the challenges continuous operation require further configuration research to reduce erosion. Overall this Compact Advanced Tokamak (CAT) approach provides an attractive and robust path to fusion energy, with high performance and stability. Further exciting research is underway at the DIII-D National Fusion Facility and elsewhere to validate the approach and test key technologies to make such a device a reality.
April 14th, Wed. 3:30PM Virtual Presentation	Katherine Goodrich, UC-Berkeley Shocks Under the Microscope: Examining the Microphysics of Collisionless Shocks with High Resolution, Multi-point Space Plasma Measurements Collisionless shocks are an important and universal phenomenon in astrophysical plasmas. Shocks form when a supersonic plasma flow interacts with an impermeable obstacle. Examples of such interactions include galactic jets or supernova remnants interacting with the interstellar medium, or plasma wind from stars encountering stellar system bodies, such as planets, comets and moons. The shock performs the necessary function of converting kinetic energy to thermal energy, heating the originally supersonic plasma flow until its speed is reduced and it can flow around the obstacle. The energy conversion processes that take place inside collisionless shocks, however, are not well established and have thus been a subject of interest for several decades. The closest and perhaps the most relevant collisionless shock to us humans, is the Earth's bow shock. This shock forms 10-12 Earth radii upstream of our planet when supersonic plasma ejected from the sun, called the solar wind, meets the Earth's intrinsic magnetosphere. The bow shock has been observed by multiple spacecraft such as ISEE, Cluster, WIND, and THEMIS over three decades. These missions have provided us with a wealth of information on plasma conditions both upstream and downstream of the shock, however, limited instrument capabilities have prevented us from resolving the physical processes active at the shock itself. The energy conversion processes active within shocks are expected to occur at primarily electron spatial and time scales (milliseconds to seconds), which up until recently were beyond the reach of our particle observations. While we can rely on highly time resolved magnetic and electric field measurements from these missions, they provide an incomplete view of the inner workings of the shock. Observations from the Magnetospheric Multiscale (MMS) mission (launched in 2014) provide particle observations on the order of 10s of milliseconds, providing an unprecedented opportunity to directly observe microscale processes inside collisionless shocks. In this talk, we'll take a first look at the direct connection between electric and magnetic field signals with electron and ion dynamics in the Earth's bow shock. We find that energy conversion can occur from multiple processes, some unexpected, within varied areas of the shock. The new dataset provided by MMS enables a new age of collisionless shock analysis as well as motivation for future space missions dedicated to the shock, signaling an exciting time for space physicists!
April 21st, Wed. 3:30PM Virtual Presentation	Open
April 28th, Wed. 3:30PM Virtual Presentation	Open
May 5th, Wed. 3:30PM Virtual Presentation	Open

Contact Information: Marc Swisdak or Ian Abel