Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Plasma Physics
Volume 65, Number 11
Monday–Friday, November 9–13, 2020; Remote; Time Zone: Central Standard Time, USA
Session GI01: Invited: Low Temperature PlasmaLive
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Chair: Michael Campanell, LLNL |
Tuesday, November 10, 2020 9:30AM - 10:00AM Live |
GI01.00001: Modeling and simulation of Hall thruster plasma discharges at JPL Invited Speaker: Alejandro Lopez Ortega Numerical simulations of Hall-effect thrusters (HET) are of paramount importance in their development and flight qualification cycle as they predict operation in space, support to laboratory tests, guide thruster design, and provide insight into the inherently complex plasma physics of these devices. Hall2De, a hydrodynamics code for the simulation of the plasma discharge in Hall thrusters, has been in development for the last ten years and is continuously improved and validated at the Jet Propulsion Laboratory. The code was instrumental in the discovery of magnetic shielding, a design technique that tailors the applied magnetic field in a way that minimizes erosion of the thruster walls by ion sputtering. More recently, Hall2De simulations also explained the physics behind the much reduced but finite erosion of magnetic poles observed in a laboratory Hall thruster and how to minimize it. It has also been employed to predict thruster operation in space for Psyche, the first mission to use Hall thrusters beyond lunar orbit. There are still some physics in these devices however that remain elusive, most notably, the existence of anomalous electron transport across magnetic field lines. In this presentation we will provide an overview of the impact these physics-based simulations have had on our understanding of these low-temperature discharges and some of the challenges we are faced with in the near term. [Preview Abstract] |
Tuesday, November 10, 2020 10:00AM - 10:30AM Live |
GI01.00002: Low-Temperature Plasmas Generated by Intense Electron Beams Invited Speaker: A. S. Richardson A renewed effort is underway at the Naval Research Laboratory (NRL) to characterize and understand plasmas formed by injecting intense, pulsed electron beams into low-pressure (0.1--10 Torr) gases. The low-temperature plasmas formed in this way are primarily generated by direct beam-impact ionization and by the electric field induced by the rapidly changing beam current. This research effort is a combination of laboratory experiments and numerical modeling. In the experiment, an intense ($\sim$4 kA, $\sim$100 keV), pulsed ($\sim$40 ns) electron beam is injected into a low-pressure gas cell. Electrical, optical, and spectral diagnostics are used to measure the properties of the resulting plasma. By varying parameters such as gas pressure, beam current density, and gas constituents, the plasmas can be tuned from weakly to strongly ionized, from fluid-like to highly kinetic, and we can explore the impact of varying plasma chemistry. The goal of these experiments is to map out this parameter space, and gather data to use for validating numerical and plasma chemistry models. A complementary modeling and simulation effort is also underway, with the goal of testing various plasma models by comparing them to experimental results and benchmarking them against each other. New tools are in development to allow for the rapid prototyping and comparison of plasma models. The main tool for this comparison work is a new, lightweight computational physics framework for Python called turboPy [https://arxiv.org/abs/2002.08842, https://github.com/arichar6/turbopy]. TurboPy modules were created for several weakly ionized fluid models, as well as a temporally and spatially dependent two-term Boltzmann solver. By comparing turboPy results to experiment, regions of validity in pressure–current-density parameter space were mapped out for each model. [Preview Abstract] |
Tuesday, November 10, 2020 10:30AM - 11:00AM Live |
GI01.00003: Studies of plasma sheaths using novel numerical schemes with self-consistent emitting walls and full Fokker-Planck collisions Invited Speaker: Petr Cagas Continuum-kinetic plasma models are successfully used to study plasma sheaths by directly evolving ion and electron distribution functions using the Vlasov equation coupled with Maxwell's equations. This way, the particle distributions are available for analysis and are unaffected by particle noise. Furthermore, the shape of a distribution at the edge of a bounded plasma is the key input parameter for particle emission from the boundary. Since the emission can significantly alter the plasma, the governing boundary conditions need to be both physics-based and self-consistent. While plasma sheaths are in theory often considered collisionless, the source of particles for a sheath is in a collisional presheath and details of the collisions can have a significant impact. Further, in many machines the loss cone needs detailed understanding of the collisional processes and often can be electrically connected to the sheath. In this talk, these two key processes, Fokker-Planck collisions and self-consistent first-principles emitting boundary conditions, are discussed along with a description of their incorporation into a high-order accurate continuum-kinetic scheme. The presented scheme is carefully crafted using a novel version of the discontinuous Galerkin (DG) method based on the concepts of weak equality and recovery to obtain gradients at discontinuous interfaces. An integral part of this super-convergent scheme is a unique multi-dimensional recovery approach which utilizes computer algebra tools for efficiency. The important role of accurate collisions in the presheath and wall emissions on sheath characteristics will be presented. [Preview Abstract] |
Tuesday, November 10, 2020 11:00AM - 11:30AM Live |
GI01.00004: Laser Diagnostics for Nanosecond Pulse and Hybrid Plasmas: Electrical and Chemical Properties Invited Speaker: Igor Adamovich Applications of nonequilibrium plasmas to chemical syntheses, such as plasma-assisted combustion, fuel reforming, and catalysis require efficient generation of excited species and radicals. Self-sustained electric discharges allow only a limited degree of control over the reduced electric field (E/N) and therefore the range of species generated in the plasma. The use of ``hybrid'' electric discharges, sustained by two independent voltage waveforms, helps circumvent this constraint. Hybrid plasmas are sustained by a combination of two overlapping discharges, (i) high peak voltage, high repetition rate, ns pulse discharge producing electron impact ionization, and (ii) sub-breakdown quasi-steady-state (DC or RF) discharge, which does not generate ionization by itself but couples additional energy to the pre-ionized flow. This approach improves the discharge stability, while generation of desired excited species and radicals is optimized by combining a high peak E/N ns pulse waveform with a ``tailored'' E/N value in the quasi-steady-state discharge. In the present work, the plasma is sustained by a ns pulse discharge train combined with a capacitively coupled RF voltage waveform, which enables selective generation of vibrationally and electronically excited molecules. The advantages of this method include the use of a single pair of electrodes external to the plasma chemical reactor, which provides better stability at high pressures and input powers, since the RF discharge remains non-self-sustained in the entire gap. Electrical and chemical properties of the plasma, such as the electric field distribution, gas temperature, vibrational level populations of diatomic molecules, and number densities of excited metastable electronic states are measured using laser diagnostic techniques such as Electric Field Induced Second Harmonic (EFISH) generation, Coherent Anti-Stokes Raman Scattering (CARS), Cavity Ring Down Spectroscopy (CRDS), and Tunable Diode Laser Absorption Spectroscopy (TDLAS). These data provide detailed insight into kinetics of ionization, vibrational relaxation, quenching of excited electronic states, molecular dissociation, and plasma chemical reactions. [Preview Abstract] |
Tuesday, November 10, 2020 11:30AM - 12:00PM Live |
GI01.00005: Effective Field Theories Motivated by Applications in Low-Temperature Plasmas Invited Speaker: Scott D. Baalrud (Stix winner) Effective field theories are systematic derivations of a description of matter from first principles based upon an expansion parameter and a renormalization strategy. In principle, they allow quantification of error associated with successive orders of approximation. Lennard-Balescu theory is a common example in plasma physics, where the expansion parameter is associated with the strength of interactions and the renormalization is associated with the dynamics of binary collisions. This talk will overview three recent examples of effective field theories in plasmas along with the applications that motivated them. The first is a quasilinear extension of Lenard-Balescu theory that treats convectively unstable plasmas. This instability-enhanced kinetic theory has found application in understanding ion flow near sheaths, and in explaining anomalous electron transport in ExB discharges. The second is an extension of linear response kinetic theory to treat strongly magnetized plasmas in which the gyrofrequency exceeds the plasma frequency. This strongly magnetized kinetic theory led to the prediction of a novel friction force and associated transport relevant to non-neutral plasmas and ultracold neutral plasmas. The third example is a kinetic theory for strongly coupled plasmas based on an expansion in terms of the deviation of correlations from their equilibrium values, rather than the strength of correlations. This mean force kinetic theory has found application in the description of transport properties of high energy density plasmas and ultracold plasmas. This work was supported by the U.S. Department of Energy, Office of Fusion Energy Sciences under Award Number DE-SC0016159, by the National Science Foundation under Award No. PHY-1453736, and by the Air Force Office of Scientific Research under Award No. FA9550-16-1-0221. [Preview Abstract] |
Tuesday, November 10, 2020 12:00PM - 12:30PM Live |
GI01.00006: Friction Force in Strongly Magnetized Plasmas Invited Speaker: David Bernstein The friction force on a single particle moving through a plasma is conventionally thought to act anti-parallel to its velocity. However, recent work predicts that it has an additional component perpendicular to the velocity vector and the Lorentz force vector when the plasma is strongly magnetized, i.e. when the gyro-frequency exceeds the plasma frequency [1]. This transverse force is predicted to significantly alter the trajectory of the projectile particle [1]. This presentation shows results of first-principles molecular dynamics simulations that confirm the existence of the predicted transverse force. A transverse force is observed when the projectile's velocity is at an oblique angle with respect to the magnetic field. A predicted asymmetry in the electrostatic potential wake about the projectile that causes the transverse force is also observed. This effect is predicted to influence transport properties in many applications in which strongly magnetized plasmas are found, including non-neutral plasmas, ultra-cold plasmas, antimatter traps, and fusion experiments. For example, in antimatter traps the transverse force may influence the dynamics of injected anti-protons as they cool by slowing on strongly magnetized electrons. In this application, plasmas can be both strongly magnetized and strongly coupled, i.e. when the average inter-particle potential energy exceeds the average thermal energy. The combined influence of strong coupling and strong magnetization on the transverse force is also quantified, and the results show that the transverse component becomes a larger fraction of the total friction force when the plasma is strongly coupled. [1] - T. Lafleur and S. D. Baalrud, Plasma Phys. Control. Fusion 61 125004 (2019). [Preview Abstract] |
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