27 research outputs found
Energy-dependent implementation of secondary electron emission models in continuum kinetic sheath simulations
The plasma-material interactions present in multiple fusion and propulsion
concepts between the flow of plasma through a channel and a material wall drive
the emission of secondary electrons. This emission is capable of altering the
fundamental structure of the sheath region, significantly changing the expected
particle fluxes to the wall. The emission spectrum is separated into two major
energy regimes, a peak of elastically backscattered primary electrons at the
incoming energy, and cold secondary electrons inelastically emitted directly
from the material. The ability of continuum kinetic simulations to accurately
represent the secondary electron emission is limited by relevant models being
formulated in terms of monoenergetic particle interactions which cannot be
applied directly to the discrete distribution function. As a result, rigorous
implementation of energy-dependent physics is often neglected in favor of
simplified, constant models. We present here a novel implementation of
semi-empirical models in the boundary of continuum kinetic simulations which
allows the full range of this emission to be accurately captured in
physically-relevant regimes
The effect of spatially-varying collision frequency on the development of the Rayleigh-Taylor instability
The Rayleigh-Taylor (RT) instability is ubiquitously observed, yet has
traditionally been studied using ideal fluid models. Collisionality can vary
strongly across the fluid interface, and previous work demonstrates the
necessity of kinetic models to completely capture dynamics in certain
collisional regimes. Where previous kinetic simulations used spatially- and
temporally-constant collision frequency, this work presents 5-dimensional (two
spatial, three velocity dimensions) continuum-kinetic simulations of the RT
instability using a more realistic spatially-varying collision frequency. Three
cases of collisional variation are explored for two Atwood numbers: low to
intermediate, intermediate to high, and low to high. The low to intermediate
case exhibits no RT instability growth, while the intermediate to high case is
similar to a fluid limit kinetic case with interface widening biased towards
the lower collisionality region. A novel contribution of this work is the low
to high collisionality case that shows significantly altered instability growth
through upward movement of the interface and damped spike growth due to
increased free-streaming particle diffusion in the lower region. Contributions
to the energy-flux from the non-Maxwellian portions of the distribution
function are not accessible to fluid models and are greatest in magnitude in
the spike and regions of low collisionality. Increasing the Atwood number
results in greater RT instability growth and reduced upward interface movement.
Deviation of the distribution function from Maxwellian is inversely
proportional to collision frequency and concentrated around the fluid
interface. The linear phase of RT instability growth is well-described by
theoretical linear growth rates accounting for viscosity and diffusion
Resolving the mystery of electron perpendicular temperature spike in the plasma sheath
A large family of plasmas has collisional mean-free-path much longer than the
non-neutral sheath width, which scales with the plasma Debye length. The
plasmas, particularly the electrons, assume strong temperature anisotropy in
the sheath. The temperature in the sheath flow direction () is
lower and drops towards the wall as a result of the decompressional cooling by
the accelerating sheath flow. The electron temperature in the transverse
direction of the flow field () not only is higher but also spikes
up in the sheath. This abnormal behavior of spike is found to be
the result of a negative gradient of the parallel heat flux of transverse
degrees of freedom () in the sheath. The non-zero heat flux is
induced by pitch-angle scattering of electrons via either their interaction
with self-excited electromagnetic waves in a nearly collisionless plasma or
Coulomb collision in a collisional plasma, or both in the intermediate regime
of plasma collisionality
Robust estimation of bacterial cell count from optical density
Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data
Numerical Methods for 3-dimensional Magnetic Confinement Configurations using Two-Fluid Plasma Equations
The 5-moment two-fluid plasma model uses Euler equations to describe the ion and electron fluids, and Maxwell's equations to describe the electric and magnetic fields. Two-fluid physics becomes significant when the characteristic spatial scales are on the order of the ion skin depth and characteristic time scales are on the order of the inverse ion cyclotron frequency. The two-fluid plasma model has disparate characteristic speeds ranging from the ion and electron speeds of sound to the speed of light. In addition, the characteristic frequencies in the system are the ion and electron plasma frequency, and the ion and electron cyclotron frequency. Explicit and implicit time-stepping schemes are explored for the two-fluid plasma model to study the accuracy and computational effectiveness with which they could capture two-fluid physics. The explicit schemes explored include the high resolution wave propagation method (a finite volume method) and the Runge-Kutta discontinuous Galerkin (RKDG) method (a finite element method). The ideal two-fluid model is a purely dispersive equation system with no physical or artificial dissipation. The dispersions are physical effects responsible for the wide variety of plasma waves; they are not numerical artifacts. This sets the two-fluid plasma model apart from other equation systems. The finite volume and finite element methods are compared for accuracy and computational expense for applications of the two-fluid plasma model. For realistic regimes, the explicit time-step for the two-fluid plasma model can be very restrictive making it computationally expensive. This motivates the implicit time-stepping scheme. A semi-implicit two-fluid plasma model is developed using the discontinuous Galerkin method where the electron fluid equations and Maxwell's equations are evolved implicitly eliminating the restrictions set by the speed of light, and the electron plasma and cyclotron frequencies. Resolving all ion time-scales is a minimum to capture two-fluid physics, so the ion fluid equations are solved explicitly. This allows for accuracy and physics considerations alone to determine the time-step. Non-ideal terms are added to the two-fluid plasma model in the form of resistivity, viscosity, and heat flux to provide a self-consistent and physically relevant two-fluid plasma model and these are compared to solutions of the ideal two-fluid plasma model. The two-fluid plasma model is compared to the more commonly used Hall-MHD model for accuracy and computational effort using an explicit time-stepping scheme. Simulations of two-fluid instabilities in the Z-pinch and the field-reversed configuration are presented in 3-dimensions.Air Force Office of Scientific Researc
A Comparison between the Discontinuous Galerkin Method and the High Resolution Wave Propagation Algorithm for the Full Two-Fluid Plasma Model
and have found that it is complete and satisfactory in all respects, and that any and all revisions required by the final examining committee have been made