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At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

The formation of sharp structures by ambipolar diffusion

The effect of ambipolar diffusion is investigated using simple numerical models. Examples are shown where sharp structures develop around magnetic nulls. In contrast to the case of ordinary diffusion, the magnetic field topology is conserved by ambipolar diffusion. This is demonstrated in an example where differential rotation winds up an initially uniform magnetic field and brings oppositely oriented field lines close together. It is argued that ambipolar diffusion produces structures of scales small enough for reconnection to occur.

Brandenburg, Axel↗
DOE OSTI2022

Design of a high energy density experiment to measure the suppression of hydrodynamic instability in an applied magnetic field

A transverse magnetic field can suppress the hydrodynamic instability growth of an unstable plasma interface. This effect, of interest to inertial confinement fusion and astrophysics, has mostly been studied via simulation. Here, we present the design of an experiment at the National Ignition Facility (NIF) to demonstrate this effect in a laboratory. Simulations indicate that the timescale for the diffusion of the magnetic field across the mixing region should be at least comparable to the timescale of the instability growth in order to have a measurable suppression effect. This motivates the use of lower density target materials than usual high energy density (HED) hydrodynamics experiments to permit faster hydrodynamics and higher plasma conductivities (through higher temperature), for a given laser drive and magnetic field. We discuss a target design for creating a Rayleigh–Taylor unstable HED plasma interface that uses 320 mg/cc iodine-doped carbon foam as the heavy material, 20 mg/cc carbon foam as the light material, and a 6 lm amplitude, a 120 lm wavelength ripple machined at the interface, which shows a measurable suppression effect with a nominal NIF drive and a 30 T magnetic field (the present facility limit). Here, models indicate lower density foams that may display even larger suppression effects, as the Hohlraum drive also radiatively preheats the foam to permit even higher temperatures and, hence, higher conductivities.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Physical properties of the quasi-two-dimensional square lattice antiferromagnet Ba 2 FeSi 2 O 7

In this work, we report magnetization (χ, M ), magnetic specific heat ( C M ), and neutron powder diffraction results on a quasi-two-dimensional (2D) S = 2 square lattice antiferromagnet Ba 2 FeSi 2 O 7 consisting of FeO 4 tetrahedrons with highly compressive tetragonal distortion (27%). Despite of the quasi-2D lattice structure, both χ and C M present three-dimensional magnetic long-range ordering below the Néel temperature T N = 5.2 K . Neutron diffraction data show a collinear Q m = (1,0,1/2) antiferromagnetic (AFM) structure below T N but the ordered moment aligned in the a b plane is suppressed by 26% from the ionic spin S = 2 value ( 4 μ B ). Both the AFM structure and the suppressed moments are well explained by using Monte Carlo simulations with a large single-ion in-plane anisotropy D = 1 .4 meV and a rather small Heisenberg exchange J intra = 0 .15 meV in the plane. The characteristic 2D spin fluctuations are recognized in the magnetic entropy release and diffuse scattering above T N . This new quasi-2D magnetic system also displays unusual nonmonotonic dependence of T N as a function of magnetic field H .

36 MATERIALS SCIENCE↗
DOE OSTI2025

Low-Field Regime of Magnon Transport in PLD-Grown YIG Films

The diffusive propagation of magnons in the archetypal magnetic insulator yttrium iron garnet (YIG) is being actively explored for low-power and low-loss data communication. However, operation under external magnetic fields reduces the magnon diffusion length and attenuates the voltage amplitude at measurement terminals of magnonic devices. Here, we explore the low-field and field-free regime of diffusive magnon transport in YIG films, demonstrating that the field-induced suppression of magnon diffusion length can be fully inhibited only at the zero-field limit. Even a modest field of 10 mT attenuates the nonlocal spin voltage by ~20% in an ~1 μm long transport channel. We further identify the often overlooked in-plane uniaxial magnetic anisotropy as the dominant factor governing magnon transport in the low-field regime. Using Stoner-Wohlfarth macrospin simulations, we quantify the anisotropy parameters and reveal a 10-fold enhancement at low temperatures, a key finding for field-free operation of magnonic devices under cryogenic conditions.

36 MATERIALS SCIENCE↗
NASA NTRS1974

Jupiter's radiation belts

Fluxes of electrons and protons in Jupiter's radiation belts are calculated with the source (radial diffusion inward from the solar wind) and the loss (synchroton radiation). The calculations are tested against the measured radio-wave wavelength distribution, the radio-wave distribution with distance from Jupiter, and the degree of polarization of the radio waves. The Fokker-Planck equation is solved by using the method of Farley and Walt (1971) with the fixed flux at the outer boundary and the zero flux at Jupiter's surface. It is found, in agreement with Brice and McDonough (1973), Jacques and Davis (1973), and Birmingham et al. (1974), that the usual magnetic and electric diffusions, which vary as L to the 10th power and L to the 6th power, respectively, are insufficient to furnish the required electrons and that the diffusion driven by ionospheric winds of Brice and McDonough is strong enough to furnish Jupiter's belts. An additional loss mechanism close to Jupiter is required to remove the electrons, particularly those at low energies.

Stansberry, K. G.↗
NASA NTRS2021

Why Atmospheric Backscatter Is Important in the Formation of Electron Precipitation in the Diffuse Aurora

In addition to wave particle scattering in the magnetosphere, atmospheric backscatter of magnetospheric electrons is an important process that contributes to the formation of the precipitated electrons in the region of diffuse aurora. Two magnetically conjugate regions are involved in a complex magnetosphere-ionosphere (MI) particle and energy interplay. Based on synthesizing previous theoretical/modeling studies and experimental evidence, we demonstrate the need for improving the quantification of magnetospheric electrons backscatter processes that can affect inner magnetospheric electrodynamics, transport and loss in a way that is not easily predicted. We discuss how these complex and energy-dependent MI coupled processes can be treated in magnetospheric modeling.

diffuse aurora↗
DOE OSTI2024

Chain extension epoxide polymerization to well‐defined block polymers using a N‐Al Lewis pair catalyst

Block polyethers comprised of poly(propylene oxide) (PPO) and poly(ethylene oxide) (PEG or PEO) segments form the basis of ABA-type PEO-b-PPO-b-PEO poloxamer materials. The inverse architecture with an internal hydrophilic PEO segment flanked by hydrophobic blocks can be difficult to prepare with control of architecture by use of traditional anionic polymerization. These oxyanionic polymerizations are plagued by chain-transfer-to-monomer side reactions that occur with substituted epoxides such as propylene oxide (PO). Herein, we report a new method for the preparation of block polymers through a controlled polymerization involving a N-Al Lewis adduct catalyst and an aluminum alkoxide macroinitiator. The Lewis pair catalyst was able to chain-extend commercial PEO macroinitiators to prepare di-, tri-, and pentablock polyethers with low dispersity and reasonable monomer tolerance. Chain extension was confirmed using size exclusion chromatography and diffusion ordered nuclear magnetic resonance spectroscopy. In conclusion, the resulting block polymers were additionally analyzed with small-angle X-ray scattering to correlate the morphology to molecular architecture.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Cross-Code verification and sensitivity analysis to effectively model the electrothermal instability

This manuscript presents verification cases that are developed to study the electrothermal instability (ETI). Specific verification cases are included to ensure that the unit physics components necessary to model the ETI are accurate, providing a path for fluid-based codes to effectively simulate ETI in the linear and nonlinear growth regimes. Two software frameworks with different algorithmic approaches are compared for accuracy in their ability to simulate diffusion of a magnetic field, linear growth of the ETI, and a fully nonlinear ETI evolution. The nonlinear ETI simulations show early time agreement, with some differences emerging, as noted in the wavenumber spectrum, late into the nonlinear development of ETI. Here, a sensitivity study explores the role of equation-of-state (EOS), vacuum density, and vacuum resistivity. EOS and vacuum resistivity are found to be the most critical factors in the modeling of nonlinear ETI development.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Activation of Low-Valent, Multiply M–M Bonded Group VI Dimers toward Catalytic Olefin Metathesis via Surface Organometallic Chemistry

Olefin metathesis is a broadly employed reaction with applications that range from fine chemicals to materials and petrochemicals. The design and investigation of olefin metathesis catalysts have been ongoing for over half a century, with advancements made in terms of activity, stability, and selectivity. Immobilization of organometallic complexes onto solid supports such as silica or alumina is a promising strategy for catalyst heterogenization, often resulting in increased activity and stability. Consequently, a broad range of early transition metal catalysts bearing alkyl, oxide/alkoxide, and amide ligands have been grafted onto silica and their reactivities investigated. In this paper, we report a series of silica-supported tungsten and molybdenum dimers (X 3 M≡MX 3, where M = W and Mo; X = neopentyl, tert-butoxide, and dimethyl amide) and their reactivities toward catalytic olefin metathesis. Dynamic nuclear polarization (DNP)-enhanced solid-state nuclear magnetic resonance (SSNMR), diffuse reflectance infrared Fourier transform (DRIFT), UV resonance Raman, and X-ray absorption (XAS) spectroscopies suggest that upon heterogenization the dimers bind to the surface in a monopodal fashion, with the M≡M triple bond remaining intact. These structural assignments were further corroborated by density functional theory (DFT) calculations. While the homogeneous dimer counterparts are inert, the supported low-valent alkyl W and Mo dimers become active for the disproportionative self-metathesis of propylene to ethylene and butenes and 4-nonene to 4-octene and 5-decene under mild conditions. The lack of activity observed for the free and supported tert-butoxide and dimethyl amide dimers likely suggests that the neopentyl groups are necessary for the formation of a putative alkylidene active species. The difference in reactivity between the free and supported dimers could be explained either by the lowering of the activation barrier of the complex through the electronic effects of the surface or by site isolation of catalytically relevant reactive intermediates.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Incoherent Thomson scattering system for PHAse space MApping (PHASMA) experiment

A new incoherent Thomson scattering system measures the evolution of electron velocity distribution functions perpendicular and parallel to the ambient magnetic field during kinking of a single flux rope and merging of two flux ropes through magnetic reconnection. The Thom-son scattering system provides sub-millimeter spatial resolution, sufficient to diagnose the several millimeters sized magnetic reconnection electron diffusion region in the PHAse Space MAppgin experiment. Due to the relatively modest plasma density ~ 10 19 m -3 and electron tem-perature ~1 eV, stray light suppression is critical for these measurements. Two volume Bragg gratings are used in series as a notch filter with a spectral bandwidth <0.1 nm in the collection branch. A CCD with a Gen III intensifier with peak quantum efficiency >47% is used as the detector in a 1.3 m spectrometer. Preliminary results of gun plasma electron temperature will be reported and compared with measurements obtained from a triple Langmuir probe.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Simulating a pulsed-power-driven plasma with ideal MHD

We describe a simple practical numerical method for simulating plasma driven within a vacuum chamber by a pulsed power generator. Typically, in this type of simulation, the vacuum region adjacent to the plasma is approximated as a highly resistive, light fluid; this involves computationally expensive solvers describing the diffusion of the magnetic field through this fluid. Instead, we provide a recipe for coupling pulsed power generators to the magnetohydrodynamics (MHD) domain by approximating the perfectly insulating vacuum as a light, perfectly conducting, inviscid MHD fluid and discuss the applicability of this counter-intuitive technique. This much more affordable ideal MHD representation is particularly useful in situations where a plasma exhibits interesting three-dimensional phenomena, either due to the design of the experiment or due to developing instabilities. We verified that this coupling recipe works by modeling an exactly solvable flux compression generator as well as a self-similar Noh-like solution and demonstrated convergence to the theoretical solution. We also showed examples of simulating complex three-dimensional pulsed power devices with this technique. We release our code implementation to the public.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Cross-machine comparison of runaway electron generation during tokamak start-up for extrapolation to ITER

A cross-machine comparison of global parameters that determine the runaway electron (RE) generation and loss process during tokamak start-up was carried out with the aim to extrapolate these to ITER. The study found that all considered discharges, also those that do not show signs of RE, are non-thermal at the start, i.e. have a streaming parameter larger than 0.1. During the current ramp-up the electric field, E, remains above the critical value, E c , that allows RE in the plasma. The distinction to be made is not if RE can form but, if sufficient RE can form fast enough such that they are detected or start to dominate the dynamics of the tokamak discharge. The dynamics of the value of E, density and temperature during tokamak are key to the formation of RE. It was found that larger devices operate with E closer to E c , due to their higher temperatures, hence the RE generation is relatively slower. The slower time scales for the formation of RE, estimated to be of the order of 100s of ms in ITER simplifies the development of avoidance schemes. The RE confinement time is also an important determinant of the entire process and is found to increase with the device size. The study also revealed that drift orbit losses, a mechanism often attributed as the main RE loss mechanism during the early tokamak discharge, are actually more difficult to achieve. RE losses might be more likely attributed to RE diffusion due to magnetic turbulence.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1971

Plasma physics abstracts, 1 January - 31 December 1971

Abstracts are presented on various aspects of plasma physics, including theoretical discussions and ionospheric plasmas. The topics considered cover Alfven waves, magnetized plasmas, plasma diffusion, Poynting flux measurements, electric fields, the magnetosphere, auroras, and plasma convection.

Montgomery, D. C.↗
NASA NTRS1974

Effect of small flares in the neutral component of secondary cosmic radiation

The results are presented of an investigation of the effect of small flares, scale divisions 1 and 1(+), in the neutron component of secondary cosmic radiation from the data of neutron supermonitors at the stations of Kiev, Bukhta Tiksi, and Deep River. It is shown that flares of scale divisions 1 and 1(+) are accompanied by an effect in the neutron component amounting to about 0.4%. A mechanism is presented for calculating the outflow of particles accelerated in small flares, owing to diffusion across the magnetic field of a trap.

Bondarenko, V. I.↗
NASA NTRS1979

Inert gas thrusters

Inert gas thrusters considered for space propulsion systems were investigated. Electron diffusion across a magnetic field was examined utilizing a basic model. The production of doubly charged ions was correlated using only overall performance parameters. The use of this correlation is therefore possible in the design stage of large gas thrusters, where detailed plasma properties are not available. Argon hollow cathode performance was investigated over a range of emission currents, with the positions of the inert, keeper, and anode varied. A general trend observed was that the maximum ratio of emission to flow rate increased at higher propellant flow rates. It was also found that an enclosed keeper enhances maximum cathode emission at high flow rates. The maximum cathode emission at a given flow rate was associated with a noisy high voltage mode. Although this mode has some similarities to the plume mode found at low flows and emissions, it is encountered by being initially in the spot mode and increasing emission. A detailed analysis of large, inert-gas thruster performance was carried out. For maximum thruster efficiency, the optimum beam diameter increases from less than a meter at under 2000 sec specific impulse to several meters at 10,000 sec. The corresponding range in input power ranges from several kilowatts to megawatts.

Kaufman, H. R.↗