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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS2011

Transition from the Sector Zone to the Unipolar Zone in the Heliosheath: Voyager 2 Magnetic Field Observations

The magnetic polarity pattern observed by Voyager 2 (V2) evolved with time from a nearly equal mixture of positive and negative polarity sectors in the sector zone from 2007.00 to 2007.67 to nearly uniform positive polarity (magnetic fields directed away from the Sun) in the unipolar zone from 2009.6 to 2010.3. This change was caused by the decreasing latitudinal extent of the sector zone, when the minimum extent of the heliospheric current sheet moved northward toward the solar equator as the solar activity associated with solar cycle 23 decreased a minimum in 2010. In the heliosheath, the distribution of daily averages of the magnetic field strength B was lognormal in the sector zone from 2008.83 to 2009.57 and Gaussian in the unipolar zone from 2009.57 to 2010.27. The distribution of daily increments of B was a Tsallis distribution (q-Gaussian distribution) with q = 1.66 +/- 0.010 in the sector zone and . Gaussian (q = 1.01+/-0.29) in the unipolar zone. The unipolar region appears to be in a relatively undisturbed equilibrium state.

Burlaga, L. F.↗
DOE OSTI2020

Crystal and magnetic structure of polar oxide HoCrWO 6

In this study, polar magnetic oxide HoCrWO 6 is synthesized and its crystal structure, magnetic structure, and thermodynamic properties are investigated. HoCrWO 6 forms the polar crystal structure (space group Pna 2 1 (#33)) due to the cation ordering of W 6+ and Cr 3+ . There is an antiferromagnetic transition at T N = 24.5 K along with the magnetic entropy change (~5 J.Kg. –1 K –1 at 70 kOe). Neutron diffraction measurement indicates that both Cr and Ho sublattices are ordered with the moment of 2.32(5)μ B and 8.7(4)μ B at 2 K, respectively. While Cr forms A-type collinear antiferromagnetic (AFM) structure with magnetic moment along the b axis, Ho sublattice orders in a non-coplanar AFM arrangement. A comparison with isostructural DyFeWO 6 and DyCrWO 6 indicates that the magnetic structure of this family of compounds is controlled by the presence or absence of e g electrons in the transition metal sublattice.

42 ENGINEERING↗
NASA NTRS1994

A test of the Hall-MHD model: Application to low-frequency upstream waves at Venus

Early studies suggested that in the range of parameter space where the wave angular frequency is less than the proton gyrofrequency and the plasma beta, the ratio of the thermal to magnetic pressure, is less than 1 magnetohydrodynamics provides an adequate description of the propagating modes in a plasma. However, recently, Lacombe et al. (1992) have reported significant differences between basic wave characteristics of the specific propagation modes derived from linear Vlasov and Hall-magnetohydrodynamic (MHD) theories even when the waves are only weakly damped. In this paper we compare the magnetic polarization and normalization magnetic compression ratio of ultra low frequency (ULF) upstream waves at Venus with magnetic polarization and normalized magnetic compression ratio derived from both theories. We find that while the 'kinetic' approach gives magnetic polarization and normalized magnetic compression ratio consistent with the data in the analyzed range of beta (0.5 less than beta less than 5) for the fast magnetosonic mode, the same wave characteristics derived from the Hall-MHD model strongly depend on beta and are consistent with the data only at low beta for the fast mode and at high beta for the intermediate mode.

Orlowski, D. S.↗
DOE OSTI2024

First Measurement of Drift-Alfvén Wave Polarization in Magnetically Confined Fusion Plasmas

Polarization of drift-Alfvén waves, defined as the ratio of electrostatic to electromagnetic fluctuations, has remained unmeasurable in fusion plasmas for decades, despite its pivotal role in understanding wave dynamics and their impact on plasmas. We report the first measurements of drift-Alfvén wave polarization in a hot, magnetically-confined plasma. Here, the breakthrough is enabled by a novel methodology developed from gyrokinetic theory, utilizing fluctuations of electron temperature and density. Analysis of data from the DIII-D tokamak reveals that the waves above the geodesic acoustic mode frequency exhibit dominant electromagnetic polarization, whereas lower frequency waves show a mix of electromagnetic and electrostatic polarization, indicating a strong coupling between shear Alfvén waves and drift-acoustic waves.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1972

Effects of interplanetary magnetic field azimuth on auroral zone and polar cap magnetic activity

During relatively quiet times in the period 1964-1968, AE is found to be greater when the interplanetary magnetic field (b sub IMF) is directed toward the sun in Jan., Feb., and Apr., and when B sub IMF is directed away from the sun in Oct. to Dec. Using Murmansk hourly H values and the AE components, AU and AL, it is shown that this sector dependence is present only in the negative H deviations. This observation supports the idea that negative bay magnitudes are determined chiefly by particle-produced ionization, while positive bay magnitudes are rather insensitive to increases in particle precipitation. The ratio of DP2-type magnetic activity in the southern polar cap to that in the northern polar cap is found to be greater by a factor of about 1.75 for B sub IMF toward the sun.

Burch, J. L.↗
NASA NTRS1982

Structure of the heliospheric current sheet in the early portion of sunspot cycle 21

The structure of the heliospheric current sheet on a spherical source surface of radius 2.35 solar radii has been computed via the use of a potential field model during the first year and a half after the last sunspot minimum. The solar polar magnetic field that is not fully observed in conventional magnetograph scans was included in the computation. The computed heliospheric current sheet had a quasi-stationary structure consisting of two northward and two southward maxima in latitude per solar rotation. The extent in latitude slowly increased from about 15 deg near the start of the interval to about 45 deg near the end. The magnetic field polarity (away from the sun or toward the sun) at the subterrestrial latitude on the source surface agreed with the interplanetary magnetic field polarity observed or inferred at the earth on 82 percent of the days. The interplanetary field structure observed at the earth at this time is finely tuned to the structure of low-latitude fields on the source surface.

Hoeksema, J. T.↗
DOE OSTI2021

Ferroelectric Self-Polarization Controlled Magnetic Stratification and Magnetic Coupling in Ultrathin La 0.67 Sr 0.33 MnO 3 Films

Multiferroic oxide heterostructures consisting of ferromagnetic and ferroelectric components hold the promise for nonvolatile magnetic control via ferroelectric polarization, advantageous for the low-dissipation spintronics. Modern understanding of the magnetoelectric coupling in these systems involves structural, orbital, and magnetic reconstructions at interfaces. Previous works have long proposed polarization-dependent interfacial magnetic structures; however, direct evidence is still missing, which requires advanced characterization tools with near-atomic-scale spatial resolutions. Here, extensive polarized neutron reflectometry (PNR) studies have determined the magnetic depth profiles of PbZr 0.2 Ti 0.8 O 3 /La 0.67 Sr 0.33 MnO 3 (PZT/LSMO) bilayers with opposite self-polarizations. When the LSMO is 2–3 nm thick, the bilayers show two magnetic transitions on cooling. However, temperature-dependent magnetization is different below the lower-temperature transition for opposite polarizations. PNR finds that the LSMO splits into two magnetic sublayers, but the inter-sublayer magnetic couplings are of opposite signs for the two polarizations. Near-edge X-ray absorption spectroscopy further shows contrasts in both the Mn valences and the Mn–O bond anisotropy between the two polarizations. Finally, this work completes the puzzle for the magnetoelectric coupling model at the PZT/LSMO interface, showing a synergic interplay among multiple degrees of freedom toward emergent functionalities at complex oxide interfaces.

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

Geometric concepts for stellarator permanent magnet arrays

The development of stellarators that use permanent magnet arrays to shape their confining magnetic fields has been a topic of recent interest, but the requirements for how such magnets must be shaped, manufactured, and assembled remain to be determined. To address these open questions, we have performed a study of geometric concepts for magnet arrays with the aid of the newly developed MAGPIE code. A proposed experiment similar to the National Compact Stellarator Experiment (NCSX) is used as a test case. Two classes of magnet geometry are explored: curved bricks that conform to a regular grid in cylindrical coordinates, and hexahedra that conform to the toroidal plasma geometry. In addition, we test constraints on the magnet polarization. While magnet configurations constrained to be polarized normally to a toroidal surface around the plasma are unable to meet the required magnetic field parameters when subject to physical limitations on the strength of present-day magnets, configurations with unconstrained polarizations are shown to satisfy the physics requirements for a targeted plasma.

stellarator, permanent magnet, optimization↗
DOE OSTI2020

Geometric concepts for stellarator permanent magnet arrays

The development of stellarators that use permanent magnet arrays to shape their confining magnetic fields has been a topic of recent interest, but the requirements for how such magnets must be shaped, manufactured, and assembled remain to be determined. To address these open questions, we have performed a study of geometric concepts for magnet arrays with the aid of the newly developed MAGPIE code. A proposed experiment similar to the National Compact Stellarator Experiment (NCSX) is used as a test case. Here, two classes of magnet geometry are explored: curved bricks that conform to a regular grid in cylindrical coordinates, and hexahedra that conform to the toroidal plasma geometry. In addition, we test constraints on the magnet polarization. While magnet configurations constrained to be polarized normally to a toroidal surface around the plasma are unable to meet the required magnetic field parameters when subject to physical limitations on the strength of present-day magnets, configurations with unconstrained polarizations are shown to satisfy the physics requirements for a targeted plasma.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1998

Investigation of laser polarized xenon magnetic resonance

Ground-based investigations of a new biomedical diagnostic technology: nuclear magnetic resonance of laser polarized noble gas are addressed. The specific research tasks discussed are: (1) Development of a large-scale noble gas polarization system; (2) biomedical investigations using laser polarized noble gas in conventional (high magnetic field) NMR systems; and (3) the development and application of a low magnetic field system for laser polarized noble gas NMR.

Walsworth, Ronald L.↗
NASA NTRS1994

The determination of interplanetary magnetic field polarities around sector boundaries using E greater than 2 keV electrons

The determination of the polarities of interplanetary magnetic fields (whether the field direction is outward from or inward toward the sun) has been based on a comparison of observed field directions with the nominal Parker spiral angle. These polarities can be mapped back to the solar source field polarities. This technique fails when field directions deviate substantially from the Parker angle or when fields are substantially kinked. We introduce a simple new technique to determine the polarities of interplanetary fields using E greater than 2 keV interplanetary electrons which stream along field lines away from the sun. Those electrons usually show distinct unidirectional pitch-angle anisotropies either parallel or anti-parallel to the field. Since the electron flow direction is known to be outward from the sun, the anisotropies parallel to the field indicate outward-pointing, positive-polarity fields, and those anti-parallel indicate inward-pointing, negative-polarity fields. We use data from the UC Berkeley electron experiment on the International Sun Earth Explorer 3 (ISSE-3) spacecraft to compare the field polarities deduced from the electron data, Pe (outward or inward), with the polarities inferred from field directions, Pd, around two sector boundaries in 1979. We show examples of large (greater than 100 deg) changes in azimuthal field direction Phi over short (less than 1 hr) time scales, some with and some without reversals in Pe. The latter cases indicate that such large directional changes can occur in unipolar structures. On the other hand, we found an example of a change in Pe during which the rotation in Phi was less than 30 deg, indicating polarity changes in nearly unidirectional structures. The field directions are poor guides to the polarities in these cases.

Kahler, S.↗
DOE OSTI2020

High-Pressure, High-Temperature Synthesis and Characterization of Polar and Magnetic LuCrWO 6

We report a new polar and magnetic oxide, LuCrWO 6 , was synthesized under high pressure (6 GPa) and high temperature (1673 K). LuCrWO 6 is isostructural with the previously reported polar YCrWO 6 (SG: Pna21, no. 33). The ordering of CrO 6 and WO 6 octahedra in the edge-shared dimers induce the polar structure. The effective size of rare earth, Ln cation does not seem to affect the symmetry of LnCrWO 6 . Second harmonic generation measurements of LuCrWO 6 confirmed the noncentrosymmetric character and strong piezoelectric domains are observed from piezoresponse force microscopy at room temperature. LuCrWO 6 exhibits antiferromagnetic behavior, T N , of ~18 K with a Weiss temperature of -30.7 K.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

The role of microstructural evolution during spark plasma sintering on the soft magnetic and electronic properties of a CoFe–Al2O3 soft magnetic composite

Abstract For transformers and inductors to meet the world’s growing demand for electrical power, more efficient soft magnetic materials with high saturation magnetic polarization and high electrical resistivity are needed. This work aimed at the development of a soft magnetic composite synthesized via spark plasma sintering with both high saturation magnetic polarization and high electrical resistivity for efficient soft magnetic cores. CoFe powder particles coated with an insulating layer of Al 2 O 3 were used as feedstock material to improve the electrical resistivity while retaining high saturation magnetic polarization. By maintaining a continuous non-magnetic Al 2 O 3 phase throughout the material, both a high saturation magnetic polarization, above 1.5 T, and high electrical resistivity, above 100 μΩ·m, were achieved. Through microstructural characterization of samples consolidated at various temperatures, the role of microstructural evolution on the magnetic and electronic properties of the composite was elucidated. Upon consolidation at relatively high temperature, the CoFe was to found plastically deform and flow into the Al 2 O 3 phase at the particle boundaries and this phenomenon was attributed to low resistivity in the composite. In contrast, at lower consolidation temperatures, perforation of the Al 2 O 3 phase was not observed and a high electrical resistivity was achieved, while maintaining a high magnetic polarization, ideal for more efficient soft magnetic materials for transformers and inductors.

36 MATERIALS SCIENCE↗