Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “electron density profile”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 775 records · Page 43
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Anode heat transfer for a flowing argon plasma at elevated electron temperature.

Theoretical investigation of heat transfer from a preionized gaseous plasma flowing over an anode surface at an elevated electron temperature in the presence of an electric field normal to the surface. A laminar boundary layer is considered in which only the velocity profile is locally similar and fluid properties are assumed to change uniformly in the gas flow direction. Results obtained by an approximation method show that for moderate current densities, the velocity and temperature distributions are insensitive to current. In addition, the effect of elevated electron temperature is negligible on convective heat transfer, but is significant for the overall heat transfer due to the enthalpy transport by current. Total heat flux to the anode is obtained by evaluating the Nusselt number and adding terms due to the potential drop in the sheath and the surface work function.

Bose, T. K.↗
NASA NTRS2016

Observations and Simulations of the Na I D-1 Line profiles in an M-Class Solar Flare

We study the temporal evolution of the Na I D1 line profiles in the M3.9 flare SOL2014-06-11T21:03UT, using observations at high spectral resolution obtained with the Interferometric Bidimensional Spectrometer instrumentation the Dunn Solar Telescope combined with radiative hydrodynamic simulations. Our results show a significant increase in the intensities of the line core and wings during the flare. The analysis of the line profiles from the flare ribbons reveals that the Na I D1 line has a central reversal with excess emission in the blue wing (blue asymmetry).We combine RADYN and RH simulations to synthesize Na I D1 line profiles of the flaring atmosphere and find good agreement with the observations. Heating with a beam of electrons modifies the radiation field in the flaring atmosphere and excites electrons from the ground state 3s 2S to the first excited state 3p 2P, which in turn modifies the relative population of the two states. The change in temperature and the population density of the energy states make the sodium line profile revert from absorption into emission. Furthermore, the rapid changes in temperature break the pressure balance between the different layers of the lower atmosphere, generating upflow/downflow patterns. Analysis of the simulated spectra reveals that the asymmetries of the Na I D1 flare profile are produced by the velocity gradients in the lower solar atmosphere.

methods: numerical↗
DOE OSTI2023

Transition from ITG to MTM linear instabilities near pedestals of high density plasmas

Here, investigation of linear gyrokinetic ion-scale modes ( k θ ρ s = 0.3) finds that a transition from ion temperature gradient to microtearing mode (MTM) dominance occurs as the density is increased near the pedestal region of a parameterized DIII-D sized tokamak. H-modes profile densities, temperatures, and equilibria are parameterized utilizing the OMFIT PRO_create module. With these profiles, linear gyrokinetic ion-scale instabilities are predicted with CGYRO. This transition (nMTM) has a weak dependence on radial location in the region near the top of the pedestal ( ρ = 0.7 – 0.9), which allows simulating single radii to examine the approximate scaling of nMTM with global parameters. The critical nMTM is found to scale with plasma current. Additionally, increasing the minor radius by decreasing the aspect ratio and increasing the major radius are found to reduce nMTM. However, any relationship between nMTM and density limit physics remains unclear as nMTM increases relative to the Greenwald density with larger minor radius and with larger magnetic field, suggesting that the transport due to MTM may be less important for a reactor. Additionally, nMTM is sensitive to the pedestal temperature, the local electron and ion gradients, the ratio of ion to electron temperature T i / T e, and the current profile. MTMs are predicted to be the dominant instability in the core at similar Greenwald fractions for DIII-D, NSTX, and NSTX-U H-mode experiments, supporting the results of the parameterized study. Additionally, MTMs continue to be the dominant linear instability in a DIII-D L-mode after an H–L transition as the plasma approaches a density limit disruption despite the large change in plasma profiles.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Efficient Modeling of Structural, Electronic, and Optical Properties of Silver and Gold Metal Nanoclusters and Alloys Using Optimized SCC-DFTB Parameters

Computation of optical properties using conventional time-dependent density functional theory (TD-DFT) is time-consuming and memory-intensive. In this study, we investigate the accuracy and efficiency of the density functional tight binding (DFTB) framework with newly optimized Slater–Koster (SK) parameters for modeling the structural, electronic properties, and absorption spectra of silver and gold nanoclusters and their alloys. Our investigation of the ground state (GS) properties demonstrates that the newly developed GS-SK parameters enable DFTB to closely approximate DFT-calculated bond lengths for octahedron, tetrahedron, icosahedra, and truncated octahedron with sizes Ag n /Au n (n = 19, 20, 38, 55), nanoclusters and Ag 20 /Au 20 nanoalloys, with a maximum deviation of approximately 0.15 Å. Formation energy results indicate that the GS-SK parameters can closely estimate changes in formation energies with alloy composition, and the comparison of electronic structures for Ag 20 , Au 20 , and AgAu alloy nanoclusters using the DFTB approximation reveals good agreement in the projected density of states (DOS) profiles and energy levels. A second set of SK parameters, ES-SK, has been developed to describe excited state (ES) properties, including the absorption spectra of silver octahedron Ag 19 , tetrahedral Ag n (n = 20, 56, 84), truncated octahedron Ag 38 , and icosahedra Ag 55 closed-shell clusters and their gold and alloy counterparts over a broad range of alloy compositions. This parametrization uses TD-DFTB calculations and fine-tunes the d and p eigenvalues by comparing them to reference absorption spectra from first-principles TD-DFT. This enables the generation of absorption spectra that closely match the reference spectra when plasmon excitation is dominant, as demonstrated by studying the plasmonic properties of icosahedral Ag n and Au n (n = 309 and 561) nanoparticles. This includes the rapid loss in plasmon quality when Au partially replaces Ag in alloy clusters. Furthermore, these results provide a foundation for addressing computational bottlenecks in plasmonics and with new prospects for applications in the quantum plasmonics for bimetallic alloys.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Why Are There so Few Reports of High-Energy Electron Drift Resonances? Role of Radial Phase Space Density Gradients

Models of monochromatic Pc5 (2–7 mHz) ultralow frequency (ULF) wave interactions with high energy (greater than ~1 MeV) electrons predict drift resonant interactions that can cause rapid radial transport and acceleration. There are few reports of electron drift resonance at energies greater than ~1 MeV, in contrast to lower energies; moreover, all previous reports occur in the aftermath of interplanetary shocks. These two facts are difficult to reconcile with theory and numerical simulations predicting that greater than ~1 MeV drift resonances should occur more often and in a wider variety of driving conditions. In this study, we show that a combination of observational sampling biases and nominal radial phase space density gradients is one explanation for this discrepancy between theory and observations. In particular, we examine electron dynamics in two case studies with very similar satellite coverage, solar wind conditions, and Pc5 wave properties, yet with different radial phase space density profiles. Using global wave and particle observations, we show that the events have vastly different particle responses despite having similar wave properties. Placing these results in context with past studies, we further show here that nominal radial PSD gradients near geostationary orbit can mask the expected drift resonance particle response and explain (1) the small number of past greater than ~1 MeV drift resonance reports and (2) the restriction of these reports to interplanetary shock events. We argue that future observational studies characterizing radial transport via drift resonance should examine global particle dynamics, including observations of the radial phase space density profile.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Gyrokinetic and extended-MHD simulations of a flow shear stabilized Z-pinch experiment

Axisymmetric (m = 0) gyrokinetic and extended-MHD simulations of a sheared-flow Z-pinch plasma are performed with the high-order finite volume code COGENT. The present gyrokinetic model solves the long-wavelength limit of the gyrokinetic equation for both ion and electron species coupled to the electrostatic gyro-Poisson equation for the electrostatic potential. The extended-MHD model is electromagnetic and includes the effects of the gyro-viscous pressure tensor, diamagnetic electron and ion heat fluxes, and generalized Ohm's law. Additionally, a prominent feature of this work is that the radial profiles for the plasma density and temperature are taken from the fusion Z-pinch experiment (FuZE), and the magnetic field profile is obtained as a solution of the MHD force balance equation. Such an approach allows to address realistic plasma parameters and provide insights into the current and planned experiments. In particular, it is demonstrated that the radial profiles play an important role in stabilization, as the embedded guiding center (E×B ) drift has a strong radial shear, which can contribute to the Z-pinch stabilization even in the absence of the fluid flow shear. The results of simulations for the FuZE plasma parameters show a decrease in the linear growth rate with an increase in the flow shear; however, full stabilization in the linear regime is not observed even for large (comparable to the Alfvén velocity) radial variations of the axial flow. Nonlinear stability properties of the FuZE plasmas are also studied, and it is found that profile broadening can have a pronounced stabilizing effect in the nonlinear regime.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2026

Interpreting AI for fusion: An application to plasma profile analysis for tearing mode stability

Artificial intelligence models have demonstrated strong predictive capabilities for various instabilities in fusion devices such as Tokamaks, including tearing modes (TM), edge localized modes, and disruptive events, but their opaque nature raises concerns about safety and trustworthiness when applied to fusion power plants. Here, we present a physics-based interpretation framework using a TM prediction model as a demonstration that is validated through a dedicated DIII-D TM avoidance experiment. By applying Shapley analysis, we identify how profiles such as rotation, temperature, and density contribute to the model's prediction of TM stability. Our analysis shows that in our experimental scenario, core electron temperature and rotation peaking play the primary role in TM stability, while density changes have smaller effects on stability. We show that off-axis ion temperature stabilizes TMs, suggesting that off-axis neutral beam heating can further stabilize this scenario. This work presents a generalizable ML-based event prediction methodology, from training to physics-driven interpretation, bridging the gap between physics understanding and opaque ML models.

Farre-Kaga, Hiro J. [Princeton Univ., NJ (United S↗
DOE OSTI2022

Boron nitride on SiC(0001)

Here, in the field of van der Waals heterostructures, the twist angle between stacked two-dimensional layers has been identified to be of utmost importance for the properties of the heterostructures. In this context, we previously reported the growth of a single layer of unconventionally oriented epitaxial graphene that forms in a surfactant atmosphere [F. C. Bocquet et al., Phys. Rev. Lett. 125, 106102 (2020)]. The resulting G-R0° layer is aligned with the SiC lattice, and hence represents an important milestone towards high-quality twisted bilayer graphene, a frequently investigated model system in this field. Here, we focus on the surface structures obtained in the same surfactant atmosphere, but at lower preparation temperatures at which a boron nitride template layer forms on SiC(0001). In a comprehensive study based on complementary experimental and theoretical techniques, we find—in contrast to the literature—that this template layer is a hexagonal B x N y layer, but not high-quality hBN. It is aligned with the SiC lattice and gradually replaced by low-quality graphene in the 0° orientation of the B x N y template layer upon annealing.

36 MATERIALS SCIENCE↗
NASA NTRS1973

Initial results from the NASA Lewis Bumpy Torus experiment

Initial results are given for low power operation of the NASA Lewis Bumpy Torus experiment in which a steady-state ion heating method based on a modified Penning discharge is applied to a bumpy torus confinement geometry. The magnet facility consists of 12 superconducting coils capable of 3.0 T each and equally spaced in a toroidal array 1.52 m in major diameter. A 18 cm i.d. anode ring is located at each of the 12 midplanes and is maintained at high positive potentials by a dc power supply. Initial observations indicate electron temperatures from 10 to 150 eV, and ion kinetic temperatures from 200 eV to 1200 eV. Two modes of operation with different radial density profiles can be achieved, depending on the background pressure applied.

Roth, J. R.↗
NASA NTRS1994

An investigation of magnetic field effects on plume density and temperature profiles of an applied-field MPD thruster

Applied-field magnetoplasmadynamic (MPD) thruster performance is below levels required for primary propulsion missions. While MPD thruster performance has been found to increase with the magnitude of the applied-field strength, there is currently little understanding of the impact of applied-field shape on thruster performance. The results of a study in which a single applied-field thruster was operated using three solenoidal magnets with diameters of 12.7, 15.2, and 30.4-cm are presented. Thruster voltage and anode power deposition were measured for each applied field shape over a range of field strengths. Plume electron number density and temperature distributions were measured using a Langmuir probe in an effort to determine the effect of field shape on plume confinement by the diverging magnetic-field for each of the three magnetic field shapes. Results show that the dependence of the measured thruster characteristics on field shape were non-monotonic and that the field shape had a significant effect on the plume density and temperature profiles.

Bullock, S. Ray↗
DOE OSTI2022

Kinetic modeling of neutral transport for a continuum gyrokinetic code

In this work we present the first-of-its-kind coupling of a continuum full- f gyrokinetic turbulence model with a 6D continuum model for kinetic neutrals, carried out using the Gkeyll code. Our objective is to improve the first-principle understanding of the role of neutrals in plasma fueling, detachment, and their interaction with edge plasma profiles and turbulence statistics. Our model includes only atomic hydrogen and incorporates electron-impact ionization, charge exchange, and wall recycling. These features have been successfully verified with analytical predictions and benchmarked with the DEGAS2 Monte Carlo neutral code. We carry out simulations for a scrape-off layer (SOL) with simplified geometry and National Spherical Torus Experiment parameters. We compare these results to a baseline simulation without neutrals and find that neutral interactions reduce the normalized density fluctuation levels and associated skewness and kurtosis, while increasing auto-correlation times. A flatter density profile is also observed, similar to the SOL density shoulder formation in experimental scenarios with high fueling.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1995

An estimate of solar wind velocity profiles in an coronal hole and a coronal streamer area (6-40 solar radius)

Using the total electron content data obtained by the Ulysses Solar Corona Experiment during the superior solar conjunction in summer 1991, we selected two data sets, one associated with a coronal hole and the other one with coronal streamer crossings. By doing this data splitting, we find two entirely different density profiles varying as r(exp -2.7) and r(exp -2.3) for the coronal hole and coronal streamers, respectively. Assuming mass flux conservation from the inner corona to one AU, an estimate for the velocity profiles or acceleration in these two different regions can be determined. The more negative exponent of the coronal hole density profile indicates a more extended heating and acceleration region or more flaring, or both. Various possible explanations will be discussed.

Paetzold, M.↗
NASA NTRS1986

Transition to unstable ion flow in parallel electric fields

The stability of ionospheric O(+)-H(+) outflows accelerated by a nonambipolar parallel electric field is considered under conditions where the ion motion initially develops adiabatically and the ambient plasma is vertically stratified with an effective temperature that increases with altitude. Such conditions are expected near the bottom of the auroral acceleration region where ion and electron streaming instabilities first develop. It is shown for a particular equilibrium profile that the differentially accelerated ion flows become unstable within about 100 km from their entry point in the acceleration region. At O(+)/H(+) density ratios less than about 9, the instability is dominated by a violent H(+)-O(+) two-stream interaction which couples the O(+) and H(+) acoustic modes, and which mediates a transition to nonadiabatic acceleration. At higher altitudes and/or larger O(+)/H(+) density ratios, a much weaker resonant instability exists, which is driven by the relative drift between electrons and O(+) or H(+) ions. The results suggest that the H(+)-O(+) two-stream instability may be a viable mechanism for heating upflowing auroral ions.

Bergmann, R.↗
NASA NTRS1976

Energetic electrons in the Jovian magnetosphere

Results are reported for a detailed analysis of Pioneer 10 data on energetic particle species in the magnetodisk region of Jupiter's magnetosphere. It is shown that the observed counting rates in the magnetodisk (beyond 20 Jupiter radii) were caused primarily by electrons with energies exceeding 0.06 MeV. Absolute omnidirectional electron intensities in the magnetodisk are presented for five integral energy ranges, and a model electron differential energy spectrum is found to fit the intensities throughout most of the encounter trajectory. It is suggested that the observed spectral shape results from losses of high-energy electrons by pitch-angle scattering. Observed equatorial energy spectra are used to compute distribution functions for several values of the first adiabatic invariant, mu. The radial profiles of the functions are found to have maxima at about 50 Jupiter radii inbound as well as at about 90 radii outbound and to diminish strongly for lesser radii. The large decreases in density are shown to require strong losses, and resonant electron whistler-mode pitch-angle scattering is suggested as a loss mechanism.

Baker, D. N.↗
DOE OSTI2021

Quantum Spin Torque Driven Transmutation of an Antiferromagnetic Mott Insulator

The basic model of spin-transfer torque (STT) in antiferromagnetic spintronics considers the exchange of angular momentum between quantum spins of flowing electrons and noncollinear-to-them localized spins treated as classical vectors. These vectors are assumed to realize Néel order in equilibrium, ↑ ↓ ∙∙∙ ↑ ↓, and their STT-driven dynamics is described by the Landau-Lifshitz-Gilbert (LLG) equation. However, many experimentally employed materials (such as archetypal NiO) are strongly electron-correlated antiferromagnetic Mott insulators (AFMIs) whose localized spins form a ground state quite different from the unentangled Néel state | ↑ ↓ ∙∙∙ ↑ ↓$\rangle$. The true ground state is entangled by quantum spin fluctuations, leading to the expectation value of all localized spins being zero, so that LLG dynamics of classical vectors of fixed length rotating due to STT cannot even be initiated. Instead, a fully quantum treatment of both conduction electrons and localized spins is necessary to capture the exchange of spin angular momentum between them, denoted as quantum STT. We use a recently developed time-dependent density matrix renormalization group approach to quantum STT to predict how injection of a spin-polarized current pulse into a normal metal layer coupled to an AFMI overlayer via exchange interaction and possibly small interlayer hopping—mimicking, e.g., topological-insulator/NiO bilayer employed experimentally—will induce a nonzero expectation value of AFMI localized spins. This new nonequilibrium phase is a spatially inhomogeneous ferromagnet with a zigzag profile of localized spins. The total spin absorbed by AFMI increases with electron-electron repulsion in AFMIs, as well as when the two layers do not exchange any charge.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1974

Coronal density structures in regions of type III activity

The density of the corona over the active regions generating the type III exciters are examined, taking into account radio data together with daily white light coronal observations obtained with a K-coronameter. The intensity profiles and the synoptic maps derived from the K-corona data make it possible to distinguish between dense structures and low intensity regions. A deconvolution technique developed by Leblanc et al. (1970) is applied to calculate the electron density of these structures. For the low intensity regions van de Hulst's (1950) method has been used to compute the electron density of models.

Leblanc, Y.↗
NASA NTRS1983

Role of thermal conduction in the acceleration of the solar wind

The thermal and suprathermal processes involving solar wind electrons are discussed from a theoretical point of view. A model for the electron distribution function, f(e), based on the solutions of the Boltzmann equation in Krook's approximation is outlined: the angular and energy dependences of for various distances from the Sun between the coronal base and the Earth are presented with the express purpose of obtaining the radial profile of the heat flux generated by the suprathermals. The basic fluid-dynamical equations for the solar wind plasma as a whole along any given magnetic field line are solved. Results are in good agreement with observations. The predicted density and temperature profiles for positive ions exhibit the characteristics of recent measurements, both in the corona (above the coronal holes) and between 0.3 and 1 A.U. (the Helios region). It is concluded that the suprathermal electrons play an important role in the formation and the dynamics of the solar wind.

Olbert, S.↗
DOE OSTI2022

Characterization of an aerosolized nanoparticle beam beyond the diffraction limit through strong field ionization

Abstract The study of nanomaterials is an active area of research for technological applications as well as fundamental science. A common method for studying properties of isolated nanoparticles is by an in-vacuum particle beam produced via an aerodynamic lens. Despite being common practice, characterization of such beams has proven difficult as light scattering detection techniques fail for particles with sizes beyond the diffraction limit. Here we present a new technique for characterizing such nanoparticle beams using strong field ionization. By focusing an ultrafast, mJ-level laser into the particle beam, a nanoparticle within the laser focus is ionized and easily detected by its ejected electrons. This method grants direct access to the nanoparticle density at the location of the focus, and by scanning the focus through the transverse and longitudinal profiles of the particle beam we attain the 3-dimensional particle density distribution for a cylindrically symmetric beam. Further, we show that strong field ionization is effective in detecting spherical nanoparticles as small as 10 nm in diameter. Additionally, this technique is an effective tool in optimizing the particle beam for specific applications. As an example we show that the particle beam density and width can be manipulated by restricting the gas flow into the aerodynamic lens.

77 NANOSCIENCE AND NANOTECHNOLOGY↗