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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.

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At least 811 records · Page 45
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Collisional broadening of nonlinear resonant wave–particle interactions

A general procedure for understanding plasma behaviour when resonant wave–particle interactions are the sole destabilizing and transport mechanism or only heating and/or current drive source is highlighted without recourse to involved numerical or analytical treatments. These phenomena are characterized by transport that appears to be collisionless even though collisions play a central role in narrow collisional boundary layers. The order of magnitude estimates, which include nonlinear effects, are shown to provide expressions in agreement with the principal results of recent toroidal Alfvén eigenmode (TAE), toroidal magnetic field ripple, and heating and current drive treatments. More importantly, the retention of nonlinearities leads to new estimates of the alpha particle energy diffusivity at saturation for TAE modes, and the ripple threshold at which superbanana plateau evaluations of alpha particle transport are modified by nonlinear radial drift effects. In addition, the estimates indicate when quasilinear descriptions for heating and current drive will begin to fail. The phenomenological procedure demonstrates that in magnetic fusion relevant plasmas, narrow collisional boundary layers must be retained for resonant wave–particle interactions as they enhance the role of collisions, and make stochastic particle motion unlikely to be more important than other nonlinear processes.

Physics↗
DOE OSTI2013

Magnetic reconnection process in transient coaxial helicity injection

The physics of magnetic reconnection and fast flux closure in transient coaxial helicity injection experiments in NSTX is examined using resistive MHD simulations. These simulations have been performed using the NIMROD code with fixed boundary flux (including NSTX poloidal coil currents) in the NSTX experimental geometry. Simulations show that an X point is formed in the injector region, followed by formation of closed flux surfaces within 0.5 ms after the driven injector voltage and injector current begin to rapidly decrease. As the injector voltage is turned off, the field lines tend to untwist in the toroidal direction and magnetic field compression exerts a radial J × B force and generates a bi-directional radial E{sub toroidal}×B{sub poloidal} pinch flow to bring oppositely directed field lines closer together to reconnect. At sufficiently low magnetic diffusivity (high Lundquist number), and with a sufficiently narrow injector flux footprint width, the oppositely directed field lines have sufficient time to reconnect (before dissipating), leading to the formation of closed flux surfaces. The reconnection process is shown to have transient Sweet-Parker characteristics.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Structure and dynamics of water confined in nanoporous carbon

We study the structure and dynamics of water confined in a nanoporous graphitic carbon structure using molecular-dynamics (MD) simulations. The carbon structure is generated by a reactive MD simulation of oxidation of a silicon carbide nanoparticle. We embed water molecules in the nanopores and study structural and dynamical properties of nanoconfined water as a function of temperature. MD simulation results indicate the presence of high-density water (HDW) and low-density water. Radial distribution functions and spatial density functions indicate that the second solvation shell of the HDW is broken. We calculate the self-diffusion coefficient of confined water molecules as a function of temperature and find a significant decrease in the diffusion of water molecules around T = 190K. The cage correlation function c(t) of confined water molecules at T = 200K exhibits stretched exponential decay, c(t) = exp[–(t/τ) β ], with β = 0.43, which matches exactly with the theoretical prediction β = 3/7. Finally, the self-intermediate scattering function at T = 200K indicates differences in small-scale and large-scale dynamics of water molecules.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Collisional broadening of nonlinear resonant wave-particle interactions

A general procedure for understanding plasma behavior when resonant wave-particle interactions are the sole destabilizing and transport mechanism or only heating and/or current drive source is highlighted without recourse to involved numerical or analytical treatments. These phenomena are characterized by transport that appears to be collisionless even though collisions play a central role in narrow collisional boundary layers. The order of magnitude estimates, which include nonlinear effects, are shown to provide expressions in agreement with the principle results of recent toroidal Alfvén eigenmode (TAE), toroidal magnetic field ripple, and heating and current drive treatments. More importantly, the retention of nonlinearities leads to new estimates of the alpha particle energy diffusivity at saturation for TAE modes, and the ripple threshold at which superbanana plateau evaluations of alpha particle transport are modified by nonlinear radial drift effects. In addition, the estimates indicate when quasilinear descriptions for heating and current drive will begin to fail. The phenomenological procedure demonstrates that in magnetic fusion relevant plasmas narrow collisional boundary layers must be retained for resonant wave-particle interactions as they enhance the role of collisions, and make stochastic particle motion unlikely to be more important than other nonlinear process.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1973

Jupiter's radiation belts and the sweeping effect of its satellites.

Jupiter's electron and proton radiation belts are analyzed, with particular reference to the sweeping effect of its five inner satellites, located deep within its magnetosphere. The characteristics of trapped electrons and protons with a magnetic moment of 50 MeV/G, considered typical at Jupiter, are calculated. The assumption is then made that a particle would be removed from the radiation belt if, in its normal motion, it would happen to impact a satellite. The mean absorption time before impact is calculated for particles located at the radial distance of each of the satellites. This average lifetime is found to be of the order of a few days. A characteristic diffusion time near each satellite was calculated, assuming violation of the third invariant due to magnetic fluctuations associated with fluctuations in the solar wind.

Mead, G. D.↗
NASA NTRS1993

Time-dependent modulation of galactic cosmic rays by merged interaction regions

Models that solve the one-dimensional, solar modulation equation have reproduced the 11-year galactic cosmic ray using functional representations of global merged interaction regions (MIRs). This study extends those results to the solution of the modulation equation with explicit time dependence. The magnetometers on Voyagers 1 and 2 provide local magnetic field intensities at regular intervals, from which one calculates the ratio of the field intensity to the average local field. These ratios in turn are inverted to form diffusion coefficients. Strung together in radius and time, these coefficents then fall and rise with the strength of the interplanetary magnetic field, becoming representations of MIRs. These diffusion coefficients, calculated locally, propagate unchanged from approx. 10 AU to the outer boundary (120 AU). Inside 10 AU, all parameters, including the diffusion coefficient are assumed constant in time and space. The model reproduces the time-intensity profiles of Voyager 2 and Pioneer 10. Radial gradient data from 1982-1990 between Pioneer 10 and Voyager 2 are about the same magnitude as those calculated in the model. It is also shows agreement in rough magnitude with the radial gradient between Pioneer 10 and 1 AU. When coupled with enhanced, time-dependent solar wind speed at the probe's high latitude, as measured by independent observers, the model also follows Voyager 1's time-intensity profile reasonably well, providing a natural source the model also follows Voyager 1's time-intensity profile reasonably well, providing a natural source for the observed negative latitudinal gradients. The model exhibits the 11-year cyclical cosmic ray intensity behavior at all radii, including 1 AU, not just at the location of the spacecraft where the magnetic fields are measured. In addition, the model's point of cosmic ray maximum correctly travels at the solar wind speed, illustrating the well-known propagation of modulation. Finally, at least in the inner heliosphere this model accounts for the delay experienced by lower-rigidity protons in reaching their time-intensity peak. The actual delays in this model, however, are somewhat smaller than the data. In the outer heliosphere the models sees no delays, and the data are ambiguous as to their existence. It appears that strong magnetic field compression regions (merged interaction regions) that are 3-4 times the average field strength can, at least in a helioequatorial band, disrupt effects, such as drifts, that could dominate in quieter magnetic fields. The question remains: Is the heliosphere ever quiet enough to allow such effects to be unambiguously measured, at least in the midlatitudes?

Perko, J. S.↗
NASA NTRS1996

The 5-6 December 1991 FIRE IFO 2 Jet Stream Cirrus Case Study: Possible Influences of Volcanic Aerosols

In presenting an overview of the cirrus clouds comprehensively studied by ground based and airborne sensors from Coffeyville, Kansas, during the 5-6 December 1992 First ISCCP Regional Experiment (FIRE) intensive field observation (IFO) case study period, evidence is provided that volcanic aerosols from the June 1991 Pinatubo eruptions may have significantly influenced the formation and maintenance of the cirrus. Following the local appearance of a spur of stratospheric volcanic debris from the subtropics, a series of jet streaks subsequently conditioned the troposphere through tropopause foldings with sulfur based particles that became effective cloud forming nuclei in cirrus clouds. Aerosol and ozone measurements suggest a complicated history of stratospheric-tropospheric exchanges embedded with the upper level flow, and cirrus cloud formation was noted to occur locally at the boundaries of stratospheric aerosol enriched layers that became humidified through diffusion, precipitation, or advective processes. Apparent cirrus cloud alterations include abnormally high ice crystal concentrations (up to approximately 600 L(exp. 1)), complex radial ice crystal types, and relatively large haze particles in cirrus uncinus cell heads at temperatures between -40 and -50 degrees C. Implications for volcanic-cirrus cloud climate effects and unusual (nonvolcanic) aerosol jet stream cirrus cloud formation are discussed.

Sassen, Kenneth↗
NASA NTRS2020

A Disc-Dominated and Clumpy Circumgalactic Medium of the Milky Way Seen in X-Ray Emission

The Milky Way galaxy is surrounded by a circumgalactic medium (CGM) that may play a key role in galaxy evolution as the source of gas for star formation and a repository of metals and energy produced by star formation and nuclear activity. The CGM may also be a repository for baryons seen in the early universe, but undetected locally. The CGM has an ionized component at temperatures near 2 x 10^6 K studied primarily in the soft X-ray band. Here we report a survey of the southern Galactic sky with a soft X-ray spectrometer optimized to study diffuse soft X-ray emission. The X-ray emission is best fit with a disc-like model based on the radial profile of the surface density of molecular hydrogen, a tracer of star formation, suggesting that the X-ray emission is predominantly from hot plasma produced via stellar feedback. Strong variations in the X-ray emission on angular scales of ~10 degrees indicate that the CGM is clumpy. Addition of an extended, and possibly massive, halo component is needed to match the halo density inferred from other observations.

P Kaaret↗
NASA NTRS1989

Ballistic transport in planetary ring systems due to particle erosion mechanisms. I - Theory, numerical methods, and illustrative examples

Ballistic transport, defined as the net radial transport of mass and angular momentum due to exchanges of meteoroid hypersonic-impact ejecta by neighboring planetary ring regions on time-scales orders-of-magnitude shorter than the age of the solar system, is presently considered as a problem in mathematical physics. The preliminary results of a numerical scheme for following the combined effects of ballistic transport and viscous diffusion demonstrate that ballistic transport generates structure near sharp edges already present in the ring-mass distribution; the entire ring system ultimately develops an undulatory structure whose length scale is typically of the order of the radial excursion of the impact ejecta.

Durisen, Richard H.↗
DOE OSTI2015

Pulsed laser generation of ultrasound in a metal plate between the melting and ablation thresholds

The generation of ultrasound in a metal plate exposed to nanosecond pulsed laser heating, sufficient to cause melting but not ablation, is treated. Consideration is given to the spatial and temporal profiles of the laser pulse, penetration of the laser beam into the sample, the evolution of the melt pool, and thermal conduction in the melt and surrounding solid. The excitation of the ultrasound takes place over a few nanoseconds, and occurs predominantly within the thermal diffusion length of a micron or so beneath the surface. Because of this, the output of the thermal simulations can be represented as axially symmetric transient radial and normal surface force distributions. The epicentral displacement response at the opposite surface to these forces is obtained by two methods, the one based on the elastodynamic Green’s functions for plate geometry determined by the Cagniard generalized ray method, and the other using a finite element numerical method. The two approaches are in very close agreement. Numerical simulations are reported of the epicentral displacement response of a 3.12mm thick tungsten plate irradiated with a 4 ns pulsed laser beam with Gaussian spatial profile, at intensities below and above the melt threshold. Comparison is made between results obtained using available temperature dependent thermophysical data, and room temperature materials constants except near the melting point.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Room temperature thermal conductivity measurements of neat MOF-5 compacts with high pressure hydrogen and helium

Metal-organic frameworks (MOFs) are a highly porous crystalline material with potential in various applications including on-board vehicle hydrogen storage for fuel cell vehicles. The thermal conductivity of MOFs is an important parameter in the design and ultimate performance of an on-board hydrogen storage system. However, in-situ thermal conductivity measurements have not been previously reported. The present study reports room temperature thermal conductivity and thermal diffusivity measurements performed on neat MOF-5 cylindrical compacts (ρ = 0.4 g/mL) as a function of pressure (0.27–90 bar) and gas type (hydrogen and helium). The transient plane source technique was used to measure both the non-directional thermal properties (isotropic method) and the directional thermal properties (anisotropic method). High pressure measurements were made using our in-house built low-temperature, high pressure thermal conductivity sample cell. The intrinsic thermal properties of neat MOF-5 measured under vacuum were—Isotropic: k isotropic = 0.1319 W/m K, α isotropic = 0.4165 mm 2 /s; Anisotropic: k axial = 0.1477 W/m K, k radial = 0.1218 W/m K, α axial = 0.5096 mm 2 /s, and α radial = 0.4232 mm 2 /s. The apparent thermal properties of neat MOF-5 increased with increasing hydrogen and helium pressure, with the largest increase occurring in the narrow pressure range of 0–10 bar and then monotonically asymptoting with increasing pressures up to around 90 bar. On average, a greater than two-fold enhancement in the apparent thermal properties was observed with neat MOF-5 in the presence of helium and hydrogen compared to the intrinsic values of neat MOF-5 measured under vacuum. The apparent thermal properties of neat MOF-5 measured with hydrogen were higher than those measured with helium, which were directly related to the gas-specific thermal properties of helium and hydrogen. Neat MOF-5 exhibited a small degree of anisotropy under all conditions measured with thermal conductivities and diffusivities in the axial direction being higher than those in the radial direction. As a result, the low temperature specific heat capacities of neat MOF-5 were also measured and reported for the temperature range of 93–313 K (–180–40 °C).

36 MATERIALS SCIENCE↗
DOE OSTI2017

Convection Effects During Bulk Transparent Alloy Solidification in DECLIC-DSI and Phase-Field Simulations in Diffusive Conditions

In order to study the dynamical formation and evolution of cellular and dendritic arrays under diffusive growth conditions, three-dimensional (3D) directional solidification experiments were conducted in microgravity on a model transparent alloy onboard the International Space Station using the Directional Solidification Insert in the DEvice for the study of Critical LIquids and Crystallization. Selected experiments were repeated on Earth under gravity-driven fluid flow to evidence convection effects. Both radial and axial macrosegregation resulting from convection are observed in ground experiments, and primary spacings measured on Earth and microgravity experiments are noticeably different. The microgravity experiments provide unique benchmark data for numerical simulations of spatially extended pattern formation under diffusive growth conditions. The results of 3D phase-field simulations highlight the importance of accurately modeling thermal conditions that strongly influence the front recoil of the interface and the selection of the primary spacing. The modeling predictions are in good quantitative agreements with the microgravity experiments.

36 MATERIALS SCIENCE↗
DOE OSTI2022

The effect of saturated thermal conduction on clouds in a hot plasma

We numerically investigate the internal evolution of multiphase clouds, which are at rest with respect to an ambient, highly ionized medium (HIM) representing the hot component of the circumgalactic medium. Time-dependent saturated thermal conduction and its implications like condensation rates and mixing efficiency are assessed in multiphase clouds. Our simulations are carried out by using the adaptive mesh refinement code FLASH. The model clouds are initially in both hydrostatic and thermal equilibrium and are in pressure balance with the HIM. Thus, they have steep gradients in both temperature and density at the interface to HIM leading to non-negligible thermal conduction. Several physical processes are considered numerically or semi-analytically: thermal conduction, radiative cooling and external heating of gas, self-gravity, mass diffusion, and dissociation of molecules and ionization of atoms. It turns out that saturated thermal conduction triggers a continuous condensation irrespective of cloud mass. Dynamical interactions with ambient HIM all relate to the radial density gradient in the clouds: (1) mass flux due to condensation is the higher the more homogeneous the clouds are; (2) mixing of condensed gas with cloud gas is easier in low-mass clouds, because of their shallower radial density gradient; and thus (3) accreted gas is distributed more efficiently. A distinct and sub-structured transition zone forms at the interface between cloud and HIM, which starts at smaller radii and is much narrower as deduced from analytical theory.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Radial Deconsolidation and Leach-Burn-Leach of Eight As-Irradiated AGR-3/4 TRISO Fuel Compacts

Eight as-irradiated AGR-3/4 fuel compacts were subjected to destructive post-irradiation examination via radial-deconsolidation-leach-burn-leach (RDLBL) at INL. The RDLBL process deconsolidated the compacts in multiple, radial steps, followed by a final, single-step axial deconsolidation. The samples generated at each step were analyzed for isotopes of key fission products and actinides. After each deconsolidation step, the compact volume was assessed, and this was used to normalize the measured quantity of nuclides of interest to give a volumetric concentration as a function of radial position within the compact. The total inventories of measured fission products and actinides and the radial concentration profiles were compared among the eight compacts deconsolidated at INL and four other as-irradiated compacts examined at ORNL. The results were analyzed for the effects of irradiation temperature. These results will be used for comparisons with fission product transport models and as input from which fission product diffusivities can be calculated.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2011

CHEMICAL EVOLUTION OF PROTOPLANETARY DISKS-THE EFFECTS OF VISCOUS ACCRETION, TURBULENT MIXING, AND DISK WINDS

We calculate the chemical evolution of protoplanetary disks considering radial viscous accretion, vertical turbulent mixing, and vertical disk winds. We study the effects on the disk chemical structure when different models for the formation of molecular hydrogen on dust grains are adopted. Our gas-phase chemistry is extracted from the UMIST Database for Astrochemistry (Rate06) to which we have added detailed gas-grain interactions. We use our chemical model results to generate synthetic near- and mid-infrared local thermodynamic equilibrium line emission spectra and compare these with recent Spitzer observations. Our results show that if H{sub 2} formation on warm grains is taken into consideration, the H{sub 2}O and OH abundances in the disk surface increase significantly. We find that the radial accretion flow strongly influences the molecular abundances, with those in the cold midplane layers particularly affected. On the other hand, we show that diffusive turbulent mixing affects the disk chemistry in the warm molecular layers, influencing the line emission from the disk and subsequently improving agreement with observations. We find that NH{sub 3}, CH{sub 3}OH, C{sub 2}H{sub 2}, and sulfur-containing species are greatly enhanced by the inclusion of turbulent mixing. We demonstrate that disk winds potentially affect the disk chemistry and the resulting molecular line emission in a manner similar to that found when mixing is included.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Anchoring Polar Magnetic Field in a Stationary Thick Accretion Disk

We investigate the properties of a hot accretion flow bathed in a poloidal magnetic field. We consider an axisymmetric viscous-resistive flow in the steady-state configuration. We assume that the dominant mechanism of energy dissipation is due to turbulence viscosity and magnetic diffusivity. A certain fraction of that energy can be advected toward the central compact object. We employ the self-similar method in the radial direction to find a system of ODEs with just one varible, θ in the spherical coordinates. For the existence and maintenance of a purely poloidal magnetic field in a rotating thick disk, we find that the necessary condition is a constant value of angular velocity along a magnetic field line. We obtain an analytical solution for the poloidal magnetic flux. We explore possible changes in the vertical structure of the disk under the influences of symmetric and asymmetric magnetic fields. Our results reveal that a polar magnetic field with even symmetry about the equatorial plane makes the disk vertically thin. Moreover, the accretion rate decreases when we consider a strong magnetic field. Finally, we notice that hot magnetized accretion flows can be fully advected even in a slim shape.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1979

Accretion by rotating magnetic neutron stars. II - Radial and vertical structure of the transition zone in disk accretion

The radial and vertical structure of the transition zone at the magnetospheric boundary of an aligned rotating neutron star accreting matter from a Keplerian disk are calculated. The results obtained indicate that: (1) the inner edge of the disk is located where the integrated magnetic stress acting on the disk plasma becomes comparable to the integrated material stress associated with its inward radial drift and orbital motion; (2) the stellar magnetic field threads the disk near its inner edge via the Kelvin-Helmholtz instability, turbulent diffusion, and reconnection, producing a broad transition zone between the unperturbed disk flow and corotating magnetosphere; (3) the transition zone consists of two qualitatively different regions, viz., a broad outer transition zone where the motion is Keplerian and a narrow inner zone, or boundary layer, where the departure from Keplerian motion is substantial; (4) the stellar magnetic field is largely but not entirely screened by currents flowing in the boundary layer; and (5) there are no steady-flow solutions for sufficiently fast stellar rotation.

Ghosh, P.↗