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At least 1,405 records · Page 78
All sourcesNASA NTRSDOE OSTI
NASA NTRS2008

The Dissipation Mechanism of Magnetic Reconnection

Magnetic reconnection is arguably the most efficient transport and energy conversion mechanism in almost ideal plasmas. Reconnection controls the overall dynamics in space and astrophysics plasmas, as well as in many laboratory plasma systems. Reconnection operates by means of a localized diffusion region, where deviations from the plasma idealness condition generate electric fields and permit plasma transport even far away from the diffusion region itself. Recent advances in analytic theory and computer modeling have begun to shed light on the internal dynamics of the diffusion region. In particular, we begin to understand the delicate nature of the force balance in the inner diffusion region, where particles can become unmagnetized and where electric field forces are important. This presentation will provide a brief introduction of the reconnection process and its applications. This introduction will be followed by a detailed analysis of the current understanding of dissipation region physics, and by an outlook toward future research.

Hesse, Michael↗
NASA NTRS2007

The Inner Workings of Magnetic Reconnection

Magnetic reconnection is arguably the most efficient transport and energy conversion mechanism in almost ideal plasmas. Reconnection controls the overall dynamics in space and astrophysics plasmas, as well as in many laboratory plasma systems. Reconnection operates by means of a localized diffusion region, where deviations from the plasma idealness condition generate electric fields and permit plasma transport even far away from the diffusion region itself. Recent advances in analytic theory and computer modeling have begun to shed light on the internal dynamics of the diffusion region. In particular, we begin to understand the delicate nature of the force balance in the inner diffusion region, where particles can become unmagnetized and where electric field forces are important. This presentation will provide a brief introduction of the reconnection process and its applications. This introduction will be followed by a detailed analysis of the current understanding of dissipation region physics, and by an outlook toward future research.

Hesse, Michael↗
DOE OSTI2025

Stability Frontiers in the AM 6 X 6 Kagome Metals: The Ln Nb 6 Sn 6 ( Ln :Ce–Lu,Y) Family and Density-Wave Transition in LuNb 6 Sn 6

The kagome motif is a versatile platform for condensed matter physics, hosting rich interactions between magnetic, electronic, and structural degrees of freedom. In recent years, the discovery of a charge density wave (CDW) in the AV 3 Sb 5 superconductors and structurally-derived bond density waves (BDW) in FeGe and ScV 6 Sn 6 have stoked the search for new kagome platforms broadly exhibiting density wave (DW) transitions. Here, in this work, we evaluate the known AM 6 X 6 chemistries and construct a stability diagram that summarizes the structural relationships among the >125 member family. Subsequently, we introduce our discovery of the broader LnNb 6 Sn 6 (Ln:Ce–Nd,Sm,Gd–Tm,Lu,Y) family of kagome metals and an analogous DW transition in LuNb 6 Sn 6 . Our X-ray scattering measurements clearly indicate a (1/3, 1/3, 1/3) ordering wave vector (√$\bar{3}$ x √$\bar{3}$ x $3$ superlattice) and diffuse scattering on half-integer L-planes. Our analysis of the structural data supports the “rattling mode” DW model proposed for ScV 6 Sn 6 and paints a detailed picture of the steric interactions between the rare-earth filler element and the host Nb–Sn kagome scaffolding. We also provide a broad survey of the magnetic properties within the HfFe 6 Ge 6 -type LnNb 6 Sn 6 members, revealing a number of complex antiferromagnetic and metamagnetic transitions throughout the family. This work integrates our new LnNb 6 Sn 6 series of compounds into the broader AM 6 X 6 family, providing new material platforms and forging a new route forward at the frontier of kagome metal research.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1999

Solidification of II-VI Compounds in a Rotating Magnetic Field

This project is aimed at using a rotating magnetic field (RMF) to control fluid flow and transport during directional solidification of elemental and compound melts. Microgravity experiments have demonstrated that small amounts of residual acceleration of less than a micro-g can initiate and prolong fluid flow, particularly when there is a static component of the field perpendicular to the liquid solid interface. Thus a true diffusion boundary layer is not formed, and it becomes difficult to verify theories of solidification or to achieve diffusion controlled solidification. The RMF superimposes a stirring effect on an electrically conducting liquid, and with appropriate field strengths and frequencies, controlled transport of material through a liquid column can be obtained. As diffusion conditions are precluded and complete mixing conditions prevail, the technique is appropriate for traveling solvent zone or float zone growth methods in which the overall composition of the liquid can be maintained throughout the growth experiment. Crystals grown by RMF techniques in microgravity in previous, unrelated missions have shown exceptional properties. The objective of the project is two-fold, namely (1) using numerical modeling to simulate the behavior of a solvent zone with applied thermal boundary conditions and demonstrate the effects of decreasing gravity levels, or an increasing applied RMF, or both, and (2) to grow elements and II-VI compounds from traveling solvent zones both with and without applied RMFs, and to determine objectively how well the modeling predicts solidification parameters. Numerical modeling has demonstrated that, in the growth of CdTe from a tellurium solution, a rotating magnetic field can advantageously modify the shape of the liquid solid interface such that the interface is convex as seen from the liquid. Under such circumstances, the defect structure is reduced as any defects which are formed tend to grow out and not propagate. The flow of liquid, however, is complex due to the competing flow induced by the rotating magnetic field and the buoyancy driven convection. When the acceleration forces are reduced to one thousandth of gravity, the flow pattern is much simplified and well controlled material transport through the solvent zone can be readily achieved. Triple axis diffractometry and x-ray synchrotron topography have demonstrated that there is no significant improvement in crystal quality for HgCdTe grown on earth from a tellurium solution when a rotating magnetic field is applied. However, modeling shows that the flow in microgravity with a rotating magnetic field would produce a superior product.

Gillies, D. C.↗
DOE OSTI2023

NMR relaxometry characterization of water adsorption in corn stover anatomical fractions

Nuclear magnetic resonance (NMR) relaxometry is applied to provide direct measurement of water adsorption in anatomical fractions of corn stover. NMR transverse T 2 relaxation time distribution measurements indicate multiple water populations, which vary with anatomical fraction and water adsorption. Measured T 2 data are used to calculate thermodynamic properties of Brunauer-Emmet-Teller adsorption theory using a model to estimate mono and bilayer relaxation. T 2 data are used directly to determine rotational diffusion correlation times indicating adsorption interaction strength. T 1 -T 2 longitudinal-transverse relaxation time correlation measurements quantify differences in the molecular level structural order of the adsorbate surface water as a function of water activity, i.e. relative humidity or water vapor partial pressure. The T 1 /T 2 ratio provides a measure of the surface energy related to the adsorption strength and surface diffusive mobility of the water adsorbate and differentiates the anatomical fractions. Furthermore, the results indicate that direct measurement of NMR relaxation times can be used to characterize corn stover biomass water adsorption, data relevant to processing and handling considerations.

09 BIOMASS FUELS↗
NASA NTRS2003

Neuroprotection trek--the next generation: neuromodulation I. Techniques--deep brain stimulation, vagus nerve stimulation, and transcranial magnetic stimulation

Neuromodulation denotes controlled electrical stimulation of the central or peripheral nervous system. The three forms of neuromodulation described in this paper-deep brain stimulation, vagus nerve stimulation, and transcranial magnetic stimulation-were chosen primarily for their demonstrated or potential clinical usefulness. Deep brain stimulation is a completely implanted technique for improving movement disorders, such as Parkinson's disease, by very focal electrical stimulation of the brain-a technique that employs well-established hardware (electrode and pulse generator/battery). Vagus nerve stimulation is similar to deep brain stimulation in being well-established (for the treatment of refractory epilepsy), completely implanted, and having hardware that can be considered standard at the present time. Vagus nerve stimulation differs from deep brain stimulation, however, in that afferent stimulation of the vagus nerve results in diffuse effects on many regions throughout the brain. Although use of deep brain stimulation for applications beyond movement disorders will no doubt involve placing the stimulating electrode(s) in regions other than the thalamus, subthalamus, or globus pallidus, the use of vagus nerve stimulation for applications beyond epilepsy-for example, depression and eating disorders-is unlikely to require altering the hardware significantly (although stimulation protocols may differ). Transcranial magnetic stimulation is an example of an external or non-implanted, intermittent (at least given the current state of the hardware) stimulation technique, the clinical value of which for neuromodulation and neuroprotection remains to be determined.

Review, Tutorial↗
DOE OSTI2020

Superconducting magnesium diboride films for levitation of laser targets

Inertial confinement fusion targets would benefit from being levitated inside hohlraums to avoid capsule support-related implosion perturbations. Levitation inside a magnetic trap requires coating the capsule with a thin film that is superconducting at 20 K. Such non-epitaxial film growth on non-planar substrates is challenging. Here, we study Mg vapor annealing and solid-phase reactive inter-diffusion methods to form superconducting magnesium diboride (MgB 2 ) films on different planar and spherical carbon substrates, evaluating glassy carbon, polycrystalline diamond made by chemical vapor deposition, and carbon deposited by magnetron sputtering of graphite targets. Thin films of B and Mg are produced by magnetron sputtering onto stationary planar or rolling spherical substrates and annealed at either 600 or 850 °C in Mg vapor. The films are characterized by a combination of high-energy ion scattering, electron microscopy, and magnetometry. Results show that the critical superconducting temperature of resultant films depends on film microstructure and oxygen impurity content. The formation of MgB 2 films is also strongly substrate dependent, even in this case of non-epitaxial growth. Important factors to consider are oxygen outgassing of the substrate during thermal processing, substrate surface roughness, and the matching of thermal expansion coefficients of different layers in the multilayer structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1972

A self-consistent model for Jupiter's radiation belts

The solar wind is assumed to be a sufficient source for the radiation belts of Jupiter, with radial diffusion as the process for getting the solar wind fluxes into the inner regions of the radiation belts. The process of a radial diffusion source is combined with pitch-angle diffusion losses to estimate an upper limit to the proton and electron fluxes. The upper limit to the integral proton and electron fluxes in the outer zone scale as L to the -6th power throughout the outer zone, assuming that the magnetic moment is conserved. It is important that these fluxes are above the stably trapped flux levels in the outer regions of the belts; the instability will work because the energies of the particles are above the critical value required.

Thorne, R. M.↗
NASA NTRS1972

Transport of cosmic rays in the solar corona

A method by which energetic cosmic ray particles in the interplanetary medium can diffuse in solar longitude. The method consists of particle motion occurring along current sheets separating discontinuous field structures in the corona. These sheets can serve as pathways along which energetic particles drift at nearly their propagation speed. A model of the coronal magnetic field and the large scale field behavior are analyzed. A mechanism by which particles can travel across the coronal magnetic field and arrive at the earth is suggested.

Schatten, K. H.↗
NASA NTRS1972

Theory on the precipitation of magnetospheric electrons at the time of a sudden commencement.

At the time of a sudden commencement or even of a sudden impulse, very-low-frequency waves can be excited. In turn, the wave turbulence drives the pitch-angle diffusion of energetic electrons trapped in the magnetosphere, causing them to be precipitated into the lower ionosphere. The distribution of this precipitation as a function of geomagnetic latitude and local time is shown to be directly related to the characteristics of the magnetic disturbances.

Perona, G. E.↗
NASA NTRS1974

Two views of cosmic ray propagation in the solar system

Diffusion and scatter free cosmic ray propagation theories are discussed in terms of the quality of their alternate viewpoints. A hypothetical model of conditions in interplantary space is described which aids in the comparison. A plot of the intensity of the interplanetary magnetic field over a long period is presented, and the association of these regions with high velocity streams is shown to support the scatter free viewpoint.

Barouch, E.↗
NASA NTRS1981

The dissipation of inhomogeneous magnetic fields and the problem of coronae. I - Dislocation and flattening of flux tubes. II - The dynamics of dislocated flux

Attention is given to the dynamical dissipation arising in a magnetic field extending up through a tenuous atmosphere when an elemental flux tube in the field (1) is displaced from its equilibrium position and/or (2) is inflated by an internal fluid pressure different from the external fluid pressure. It is pointed out that as a consequence the tension in the lines of force of the ambient field flattens the dislocated tube so that the thickness of the tube decreases without limit and that the local field gradients increase rapidly with the passage of time until destroyed by one or more dissipative effects. The magnetic energy of a dislocated flux tube is therefore soon converted into thermal energy no matter how low the molecule resistivity of the fluid. Some formal illustrations of local conditions along a misaligned flux tube are presented, showing the simultaneous onset of diffusion, fluid motion, and hydromagnetic wave propagation. The examples demonstrate that the total effect is complicated and subject only to estimation, rather than formal calculation, at the present time.

Parker, E. N.↗
NASA NTRS1982

Evidence for a poleward meridional flow on the sun

Two subsets of all polar zone filaments, designated polemost filament and polar filament bands, are defined for observational study of their behavior, which is compared with the evolution of the polar magnetic field over Howard and LaBonte's (1981) activity cycle. The magnetic data show that the polar magnetic fields are built up and maintained by the episodic arrival of discrete f-polarity regions originating in active region latitudes and subsequently drifting to the poles. F-polarity regions are carried poleward by a meridional flow, rather than by diffusion. It is noted that the mean latitude of the polemost filaments tracks the boundary of the polar field cap, and undergoes an equatorward dip during each arrival of additional polar field, and that the polar filament bands track the boundary latitudes of the unipolar regions, drifting poleward with the regions at about 10 m/sec.

Topka, K.↗
NASA NTRS1986

Effects of three-dimensional heliospheric structures on cosmic-ray modulation

The theory of cosmic-ray transport in the heliosphere contains four distinct physical processes - diffusion, convection, adiabatic cooling, and drifts. The last of these has only recently been evaluated. Extrapolation of present understanding of the regions near the heliospheric equator to high heliographic latitudes leads to the conclusion that particle drift in the large-scale magnetic field plays an important role in cosmic-ray modulation. The large-scale, three-dimensional structure of the interplanetary magnetic field is therefore very important in understanding cosmic rays. Several key observed modulation effects are summarized, each of which is a natural consequence of drift, but which requires special assumptions if drift plays no role. It is concluded that particle drifts play an important and possibly dominant role in transport in the heliosphere.

Jokipii, J. R.↗
NASA NTRS1988

A mathematical model of the middle and high latitude ionosphere

A time-dependent three-dimensional model of the middle and high latitude ionosphere is described. The density distributions of six ion species NO(+), O(2+), N(2+), O(+), N(+), He(+), and the electron and ion temperatures are obtained from a numerical solution of the appropriate continuity, momentum, and energy equations. The equations are solved as a function of height for an inclined magnetic field at E and F region altitudes. The three-dimensional nature of the model is obtained by following flux tubes of plasma as they convect or corotate through a moving neutral atmosphere. The model takes account of field-aligned diffusion, cross-field electrodynamic drifts, thermospheric winds, polar wind escape, energy-dependent chemical reactions, neutral composition changes, ion production due to solar EUV radiation and auroral precipitation, thermal conduction, diffusion-thermal heat flow and local heating and cooling processes. The model also takes account of the offset between the geomagnetic and geographic poles.

Schunk, R. W.↗
NASA NTRS1994

Diffusive shock acceleration in the presence of current sheets

We study the nature of diffusive shock acceleration in the presence of current sheets which are oriented normal to the shock, so that the drift along the sheets carries particles either away from or toward the shock plane. We consider a system with periodic regions of alternating magnetic field direction. This can be taken to be a very crude representation of the termination of the solar wind, with the equator being one sheet and the pole, with its rapid opposite drift, being another. We find very important effects of the current sheets. For example, particle intensities at the shock will be enhanced near one sheet and depressed at the other. Furthermore, the intensity at high energies (averaged over the shock plane) may in fact be considerably larger than in the standard one-dimensional infinite shock case. Implications of these results for the possible acceleration of cosmic electrons up to high energies at the solar-wind termination shock will briefly be discussed.

Kota, J.↗
NASA NTRS2007

Chaotic Motion of Relativistic Electrons Driven by Whistler Waves

Canonical equations governing an electron motion in electromagnetic field of the whistler mode waves propagating along the direction of an ambient magnetic field are derived. The physical processes on which the equations of motion are based .are identified. It is shown that relativistic electrons interacting with these fields demonstrate chaotic motion, which is accompanied by the particle stochastic heating and significant pitch angle diffusion. Evolution of distribution functions is described by the Fokker-Planck-Kolmogorov equations. It is shown that the whistler mode waves could provide a viable mechanism for stochastic energization of electrons with energies up to 50 MeV in the Jovian magnetosphere.

Khazanov, G. V.↗
NASA NTRS2014

Behavior of Particle Depots in Molten Silicon During Float-Zone Growth in Strong Static Magnetic Fields

In the photovoltaics industry, the largest market share is represented by solar cells made from multicrystalline silicon, which is grown by directional solidification. During the growth process, the silicon melt is in contact with the silicon nitride coated crucible walls and the furnace atmosphere which contains carbon monoxide. The dissolution of the crucible coating, the carbon bearing gas, and the carbon already present in the feedstock, lead to the precipitation of silicon carbide, and silicon nitride, at later stages of the growth process. The precipitation of Si3N4 and SiC particles of up to several hundred micrometers in diameter leads to severe problems during the wire sawing process for wafering the ingots. Furthermore the growth of the silicon grains can be negatively influenced by the presence of particles, which act as nucleation sources and lead to a grit structure of small grains and are sources for dislocations. If doped with Nitrogen from the dissolved crucible coating, SiC is a semi conductive material, and can act as a shunt, short circuiting parts of the solar cell. For these reasons, the incorporation of such particles needs to be avoided. In this contribution we performed model experiments in which the transport of intentionally added SiC particles and their interaction with the solid-liquid interface during float zone growth of silicon in strong steady magnetic fields was investigated. SiC particles of 7μm and 60μm size are placed in single crystal silicon [100] and [111] rods of 8mm diameter. This is achieved by drilling a hole of 2mm diameter, filling in the particles and closing the hole by melting the surface of the rod until a film of silicon covers the hole. The samples are processed under a vacuum of 1x10(exp -5) mbar or better, to prevent gas inclusions. An oxide layer to suppress Marangoni convection is applied by wet oxidation. Experiments without and with static magnetic field are carried out to investigate the influence of melt convection on the distribution of particles and their incorporation into the crystal. The field strengths applied by a superconducting magnet are 1T, 3T, 4.5T, and 5T. The increase in field strength dampens the melt flow, and so this study provides comparative data to the crystal growth experiment to be carried out onboard the sounding rocket mission TEXUS 51, where purely diffusive growth condition will be achieved under microgravity conditions.

Jauss, T.↗