47 research outputs found
Effect of a weak ion collisionality on the dynamics of kinetic electrostatic shocks
In strictly collisionless electrostatic shocks, the ion distribution function
can develop discontinuities along phase-space separatrices, due to partial
reflection of the ion population. In this paper, we depart from the strictly
collisionless regime and present a semi-analytical model for weakly collisional
kinetic shocks. The model is used to study the effect of small but finite
collisionalities on electrostatic shocks, and they are found to smooth out
these discontinuities into growing boundary layers. More importantly, ions
diffuse into and accumulate in the previously empty regions of phase space,
and, by upsetting the charge balance, lead to growing downstream oscillations
of the electrostatic potential. We find that the collisional age of the shock
is the more relevant measure of the collisional effects than the
collisionality, where the former can become significant during the lifetime of
the shock, even for weak collisionalities.Comment: Published in J. Plasma Phy
A Deep Dive into the Distribution Function: Understanding Phase Space Dynamics with Continuum Vlasov-Maxwell Simulations
In collisionless and weakly collisional plasmas, the particle distribution function is a rich tapestry of the underlying physics. However, actually leveraging the particle distribution function to understand the dynamics of a weakly collisional plasma is challenging. The equation system of relevance, the Vlasov--Maxwell--Fokker--Planck (VM-FP) system of equations, is difficult to numerically integrate, and traditional methods such as the particle-in-cell method introduce counting noise into the distribution function.
In this thesis, we present a new algorithm for the discretization of VM-FP system of equations for the study of plasmas in the kinetic regime. Using the discontinuous Galerkin (DG) finite element method for the spatial discretization and a third order strong-stability preserving Runge--Kutta for the time discretization, we obtain an accurate solution for the plasma's distribution function in space and time.
We both prove the numerical method retains key physical properties of the VM-FP system, such as the conservation of energy and the second law of thermodynamics, and demonstrate these properties numerically. These results are contextualized in the history of the DG method. We discuss the importance of the algorithm being alias-free, a necessary condition for deriving stable DG schemes of kinetic equations so as to retain the implicit conservation relations embedded in the particle distribution function, and the computational favorable implementation using a modal, orthonormal basis in comparison to traditional DG methods applied in computational fluid dynamics.
A diverse array of simulations are performed which exploit the advantages of our approach over competing numerical methods. We demonstrate how the high fidelity representation of the distribution function, combined with novel diagnostics, permits detailed analysis of the energization mechanisms in fundamental plasma processes such as collisionless shocks. Likewise, we show the undesirable effect particle noise can have on both solution quality, and ease of analysis, with a study of kinetic instabilities with both our continuum VM-FP method and a particle-in-cell method.
Our VM-FP solver is implemented in the Gkyell framework, a modular framework for the solution to a variety of equation systems in plasma physics and fluid dynamics
Low Mach-number collisionless electrostatic shocks and associated ion acceleration
The existence and properties of low Mach-number () electrostatic
collisionless shocks are investigated with a semi-analytical solution for the
shock structure. We show that the properties of the shock obtained in the
semi-analytical model can be well reproduced in fully kinetic Eulerian
Vlasov-Poisson simulations, where the shock is generated by the decay of an
initial density discontinuity. Using this semi-analytical model, we study the
effect of electron-to-ion temperature ratio and presence of impurities on both
the maximum shock potential and Mach number. We find that even a small amount
of impurities can influence the shock properties significantly, including the
reflected light ion fraction, which can change several orders of magnitude.
Electrostatic shocks in heavy ion plasmas reflect most of the hydrogen impurity
ions.Comment: In Plasma Physics and Controlled Fusio
Electron Energization in Reconnection: Eulerian versus Lagrangian Perspectives
Particle energization due to magnetic reconnection is an important unsolved
problem for myriad space and astrophysical plasmas. Electron energization in
magnetic reconnection has traditionally been examined from a particle, or
Lagrangian, perspective using particle-in-cell (PIC) simulations.
Guiding-center analyses of ensembles of PIC particles have suggested that Fermi
(curvature drift) acceleration and direct acceleration via the reconnection
electric field are the primary electron energization mechanisms. However, both
PIC guiding-center ensemble analyses and spacecraft observations are performed
in an Eulerian frame. For this work, we employ the continuum Vlasov-Maxwell
solver within the Gkeyll simulation framework to re-examine electron
energization from a kinetic continuum, Eulerian, perspective. We separately
examine the contribution of each drift energization component to determine the
dominant electron energization mechanisms in a moderate guide-field Gkeyll
reconnection simulation. In the Eulerian perspective, we find that the
diamagnetic and agyrotropic drifts are the primary electron energization
mechanisms away from the reconnection x-point, where direct acceleration
dominates. We compare the Eulerian (Vlasov Gkeyll) results with the wisdom
gained from Lagrangian (PIC) analyses.Comment: 10 pages, 7 figure
Isolation and Phase-Space Energization Analysis of the Instabilities in Collisionless Shocks
We analyze the generation of kinetic instabilities and their effect on the
energization of ions in non-relativistic, oblique collisionless shocks using a
3D-3V simulation by , a hybrid particle-in-cell code. At
sufficiently high Mach number, quasi-perpendicular and oblique shocks can
experience rippling of the shock surface caused by kinetic instabilities
arising from free energy in the ion velocity distribution due to the
combination of the incoming ion beam and the population of ions reflected at
the shock front. To understand the role of the ripple on particle energization,
we devise the new instability isolation method to identify the unstable modes
underlying the ripple and interpret the results in terms of the governing
kinetic instability. We generate velocity-space signatures using the
field-particle correlation technique to look at energy transfer in phase space
from the isolated instability driving the shock ripple, providing a viewpoint
on the different dynamics of distinct populations of ions in phase space. We
generate velocity-space signatures of the energy transfer in phase space of the
isolated instability driving the shock ripple using the field-particle
correlation technique. Together, the field-particle correlation technique and
our new instability isolation method provide a unique viewpoint on the
different dynamics of distinct populations of ions in phase space and allow us
to completely characterize the energetics of the collisionless shock under
investigation.Comment: 32 pages, 14 figures, accepted by the Journal of Plasma Physic
Revolutionizing our Understanding of Particle Energization in Space Plasmas Using On-Board Wave-Particle Correlator Instrumentation
The development of on-board wave-particle correlator instrumentation using existing instrumental capabilities makes it possible to probe the collisionless wave-particle interactions governing plasma heating and particle acceleration in the heliosphere at full measurement cadence while circumventing telemetry constraints, enabling us to maximize the scientific return from future missions
Phase Space Energization of Ions in Oblique Shocks
Examining energization of kinetic plasmas in phase space is a growing topic
of interest, owing to the wealth of data in phase space compared to traditional
bulk energization diagnostics. Via the field-particle correlation (FPC)
technique and using multiple means of numerically integrating the plasma
kinetic equation, we have studied the energization of ions in phase space
within oblique collisionless shocks. The perspective afforded to us with this
analysis in phase space allows us to characterize distinct populations of
energized ions. In particular, we focus on ions which reflect multiple times
off the shock front through shock-drift acceleration, and how to distinguish
these different reflected populations in phase space using the FPC technique.
We further extend our analysis to simulations of three-dimensional shocks
undergoing more complicated dynamics, such as shock ripple, to demonstrate the
ability to recover the phase space signatures of this energization process in a
more general system. This work thus extends previous applications of the FPC
technique to more realistic collisionless shock environments, providing
stronger evidence of the technique's utility for simulation, laboratory, and
spacecraft analysis.Comment: 9 pages, 5 figure
CD8+ T cells specific for an immunodominant SARS-CoV-2 nucleocapsid epitope display high naive precursor frequency and TCR promiscuity
To better understand primary and recall T cell responses during coronavirus disease 2019 (COVID-19), it is important to examine unmanipulated severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-specific T cells. By using peptide-human leukocyte antigen (HLA) tetramers for direct ex vivo analysis, we characterized CD8+ T cells specific for SARS-CoV-2 epitopes in COVID-19 patients and unexposed individuals. Unlike CD8+ T cells directed toward subdominant epitopes (B7/N257, A2/S269, and A24/S1,208) CD8+ T cells specific for the immunodominant B7/N105 epitope were detected at high frequencies in pre-pandemic samples and at increased frequencies during acute COVID-19 and convalescence. SARS-CoV-2-specific CD8+ T cells in pre-pandemic samples from children, adults, and elderly individuals predominantly displayed a naive phenotype, indicating a lack of previous cross-reactive exposures. T cell receptor (TCR) analyses revealed diverse TCRαβ repertoires and promiscuous αβ-TCR pairing within B7/N105+CD8+ T cells. Our study demonstrates high naive precursor frequency and TCRαβ diversity within immunodominant B7/N105-specific CD8+ T cells and provides insight into SARS-CoV-2-specific T cell origins and subsequent responses