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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 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Localizing resonant magnetic perturbations for edge localized mode control in KSTAR

An external 3D magnetic perturbation typically drives a resonant response at the rational surfaces from the core to the edge of tokamak plasmas, due to strong mode coupling and amplification. In this paper, we present a method to isolate the edge from core resonant fields using the ideal perturbed equilibrium code and to design an edge-localized resonant magnetic perturbation (RMP) for effective edge localized mode (ELM) control. A robust feature of the edge-localized RMP is the curtailed response to the field at the low-field-side (LFS) midplane, as opposed to typical RMPs which strongly resonate with the LFS fields. This emphasizes the importance of off-midplane coils to improve ELM control without provoking a large core response that could lead to devastating instabilities. The conceptual design of new ELM control coils based on the edge-localized RMP in KSTAR shows how this new insight can be utilized to enhance the efficiency of our ELM suppression capabilities. Simple window-pane coils matching the edge-localized resonant mode structure substantially expand in the ELM suppression window beyond the existing coil. Further optimization using the flexible optimized coils using space-curves code leads to additional enhancement in the edge-localized control.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Determining the vortex tilt relative to a superconductor surface

Here, it is of interest to determine the exit angle of a vortex from a superconductor surface, since this affects the intervortex interactions and their consequences. Two ways to determine this angle are to image the vortex magnetic fields above the surface, or the vortex core shape at the surface. In this work we evaluate the field h(x,y,z) above a flat superconducting surface x,y and the currents J(x,y) at that surface for a straight vortex tilted relative to the normal to the surface, for both the isotropic and anisotropic cases. In principle, these results can be used to determine the vortex exit tilt angle from analyses of magnetic field imaging or density of states data.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Influence of the safety factor inhomogeneity on collisionless current generation in tokamaks

An improved scaling for the current density generated due to collisionless motion of {alpha}-particles in a tokamak is proposed. The dependence of the current density on the radial profile of the safety factor q is investigated. Monotonically increasing q profiles are considered, as well as q profiles with a minimum in the axial region of the plasma column. It is shown that the current density depends on the variation in q along the charged particle trajectories, rather than on the q value at the starting magnetic surface. The dependence of the current density on the gradient of q is strongest in the plasma core because of the large deviation of the drift surfaces from the magnetic ones in this region. At the plasma edge, the larger the second derivative of the plasma density, the greater the contribution of the gradient of q. For conventional plasma density profiles, the poloidal-angle-averaged current density calculated for a varying safety factor q is always lower than that calculated for a constant q. The effect of the nonuniformity of the safety factor on the current generation at the magnetic axis of a tokamak is investigated.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Subdominant modes and optimization trends of DIII-D reverse magnetic shear configurations

Alfvén Eigenmodes (AE) and magneto-hydrodynamic (MHD) modes are destabilized in DIII-D reverse magnetic shear configurations and may limit the performance of the device. We use the reduced MHD equations in a full 3D system, coupled with equations of density and parallel velocity moments for the energetic particles as well as the geodesic acoustic wave dynamics, to study the properties of instabilities observed in DIII-D reverse magnetic shear discharges, with the purpose of discovering ways to avoid or minimize MHD and AE activity for different magnetic field configurations and neutral beam injector (NBI) operation regimes. The simulations show at the beginning of the discharge, before the reverse shear region is formed, a plasma AE unstable and marginally MHD stable. As soon as the reverse shear region appears, ideal MHD modes are destabilized with larger growth rate than the AE. Both MHD modes and AE coexist along the discharge, although the MHD modes are more unstable as the reverse shear region deepens. AE stability can be improved if the NBI injection is off axis, because on-axis injection leads to AE with larger growth rate and frequency. In addition, decreasing the beam energy or increasing the NBI injection lead to an AE with larger growth rate and frequency, so a NBI operation in the weakly resonant regime requires higher beam energies than in the experiment (V th,ƒ /V A0 > 0.3). The MHD linear stability can be also improved if the reverse shear region and the q profile near the magnetic axis are in between the rational surfaces q = 2 and q = 1, specifically if there is a region in the core with negative shear, avoiding a flat q profile near the magnetic axis. The simulations also shows a smooth transition between MHD modes and low frequency AE, no critical β ƒ , pointing out an overlap between MHD and AE activity for modes with frequency lower than 30 kHz. This is in the range of Beta Acosutic Alfvén Eigenmodes (BAAE) and Beta induced Alfvén Eigenmodes (BAE).

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Emergence of Kondo Resonance in Graphene Intercalated with Cerium

The interaction between a magnetic impurity, such as cerium (Ce) atom, and surrounding electrons has been one of the core problems in understanding many-body interaction in solid and its relation to magnetism. Kondo effect, the formation of a new resonant ground state with quenched magnetic moment, provides a general framework to describe many-body interaction in the presence of magnetic impurity. In this Letter, a combined study of angle-resolved photoemission (ARPES) and dynamic mean-field theory (DMFT) on Ce-intercalated graphene shows that Ce-induced localized states near Fermi energy, E F , hybridized with the graphene π-band, exhibit gradual increase in spectral weight upon decreasing temperature. The observed temperature dependence follows the expectations from the Kondo picture in the weak coupling limit. These results provide a novel insight how Kondo physics emerges in the sea of two-dimensional Dirac electrons.

36 MATERIALS SCIENCE↗
DOE OSTI2012

THE FORMATION OF THE H{alpha} LINE IN THE SOLAR CHROMOSPHERE

We use state-of-the-art radiation-MHD simulations and three-dimensional (3D) non-LTE radiative transfer computations to investigate H{alpha} line formation in the solar chromosphere and apply the results of this investigation to develop the potential of H{alpha} as a diagnostic of the chromosphere. We show that one can accurately model H{alpha} line formation assuming statistical equilibrium and complete frequency redistribution provided the computation of the model atmosphere included non-equilibrium ionization of hydrogen and the Ly{alpha} and Ly{beta} line profiles are described by Doppler profiles. We find that 3D radiative transfer is essential in modeling hydrogen lines due to the low photon destruction probability in H{alpha}. The H{alpha} opacity in the upper chromosphere is mainly sensitive to the mass density and only weakly sensitive to the temperature. We find that the H{alpha} line-core intensity is correlated with the average formation height: The larger the average formation height is, the lower the intensity will be. The line-core width is a measure of the gas temperature in the line-forming region. The fibril-like dark structures seen in H{alpha} line-core images computed from our model atmosphere are tracing magnetic field lines. These structures are caused by field-aligned ridges of enhanced chromospheric mass density that raise their average formation height, and therefore make them appear dark against their deeper-formed surroundings. We compare with observations, and find that the simulated line-core widths are very similar to the observed ones, without the need for additional microturbulence.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Novel Transformer with Variable Leakage and Magnetizing Inductances

This paper introduces the concept of a novel variable inductance transformer (VIT) whose leakage and magnetizing inductances can be varied dynamically and independently to aid the advancement of research in isolated power electronic converters. In this transformer, the leakage inductance can be varied by varying the length of overlap between the primary and secondary windings, while the magnetizing inductance can be varied by varying the air gap between the core legs. Individual controls for the two inductances ensure that one can be varied independent of the other, as needed to satisfy different power conversion objectives. In this paper, the analytical models of the variable leakage and magnetizing inductances are presented. Analytical results are compared with those obtained from Finite Element Method (FEM) and an experimental prototype of a VIT. Analytical solutions of the variable leakage inductance present an error of less than 2.35 % when compared to FEM solutions, while the analytical solutions of the variable magnetizing inductance present a maximum error of 1.82 % when compared to experimental measurements.

image method, leakage inductance, magnetizing indu↗
DOE OSTI2023

A Flexible Field Mapping System for Accelerator Magnets

Magnetic field mapping is a fundamental magnetic measurement method that typically uses Hall and NMR sensors. In magnet measurement facilities, such systems are likely used in various configurations suitable for a specific task at hand. To address this diversity, the authors developed a flexible field mapping system capable of being configured and tailored to each particular measurement case. Further, the system needs to address the variability introduced by differences in sensors and their readout systems, probe positioning systems, power supply systems, and required mapping geometry (mapping space and grid, measurement steps and sequences). Although the discussed field mapping systems range from a self-propelled multi-sensor mapper of a large detector magnet to a single 3D Hall sensor system to scan a small permanent magnet, they were all built with the same core mapping system. The variability present in field mapping systems, the measurement system architecture addressing this variability, as well as examples of several field mapping systems built in this architecture are presented.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Correlation of electrostatic fluctuation and reversal of toroidal field in the reversed-field pinch plasma

The magnetic fluctuations and electrostatic probe potential have been measured in the Toroidal Pinch Experiment - RX (TPE-RX) reversed-field pinch plasma [Y. Yagi et al., Fusion Eng. Des. 45, 421 (1999)] (at the plasma surface r/a = 1.00). Fast electrons with energy comparable to or slightly higher than the core electron temperature are observed as many spikes in the electrostatic probe signal. These electrons are diffused by a fluctuating magnetic field from the core region. During the period of mild deepening of the reversal of the edge toroidal field, a significant reduction in the spike signal, increases in electron density and soft x-ray radiation, and a decrease in the D{alpha} line radiation are observed, even though the reduction in magnetic fluctuations is not significant during the same period, which indicates that the mild deepening of the reversal of the toroidal field can improve the confinement of fast electrons.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

ACCELERATION OF COMPACT RADIO JETS ON SUB-PARSEC SCALES

Jets of compact radio sources are highly relativistic and Doppler boosted, making studies of their intrinsic properties difficult. Observed brightness temperatures can be used to study the intrinsic physical properties of relativistic jets, and constrain models of jet formation in the inner jet region. We aim to observationally test such inner jet models. The very long baseline interferometry (VLBI) cores of compact radio sources are optically thick at a given frequency. The distance of the core from the central engine is inversely proportional to the frequency. Under the equipartition condition between the magnetic field energy and particle energy densities, the absolute distance of the VLBI core can be predicted. We compiled the brightness temperatures of VLBI cores at various radio frequencies of 2, 8, 15, and 86 GHz. We derive the brightness temperature on sub-parsec scales in the rest frame of the compact radio sources. We find that the brightness temperature increases with increasing distance from the central engine, indicating that the intrinsic jet speed (the Lorentz factor) increases along the jet. This implies that the jets are accelerated in the (sub-)parsec regions from the central engine.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Reconstructing paleo-SMT positions on the Casadia margin using magnetic susceptibility

Magnetic susceptibility (κ) is a mixed signal in marine sediments, representing primary depositional and secondary diagenetic processes. Production of hydrogen sulfide via anaerobic oxidation of methane (AOM) at the sulfate-methane transition (SMT) and organoclastic sulfate reduction above the SMT can result in the dissolution of iron oxides, altering κ in sediments in methane gas and gas hydrate bearing regions. We investigated records of κ on the Cascadia margin (ODP Sites 1249 and 1252; IODP Site 1325) using a Zr/Rb heavy mineral proxy from XRF core scanning to identify intervals of primary detrital magnetic susceptibility and intervals and predict intervals affected by magnetite dissolutions. We also measured total sulfur content, grain size distributions, total organic carbon (TOC) content, and magnetic mineral assemblage. The upper 100 m of Site 1252 contains a short interval of κ driven by primary magnetite, with multiple intervals (> 90 m total) of decreased κ correlated with elevated sulfur content, consistent with dissolution of magnetite and re-precipitation of pyrite. In the upper 90 m of Site 1249, κ is almost entirely altered by diagenetic processes, with much of the low κ explained by a high degree of pyritization, and some intervals affected by the precipitation of magnetic iron sulfides. At Site 1325, κ between 0-20 and 51-73 mbsf represents primary mineralogy, and in the interval 24-51 mbsf, κ may be reduced due to pyritization. This integrated approach allows for a prediction of primary κ and the amount of κ loss at each site when compared to actual κ measurements. In the case of magnetite dissolution and full pyritization, these drawdowns in κ are supported by sulfur measurements, and the exposure times of magnetite to hydrogen sulfide can be modeled. The presence of methane and methane hydrates at these sites, as well as large variations in TOC content, suggest that the past migration rates of the SMT and variation in sulfate reduction rates may influence κ alteration along the Cascadia margin.

03 NATURAL GAS↗
DOE OSTI2013

Magnetic properties of two-dimensional nanodots: Ground state and phase transition

We study the effect of perpendicular single-ion anisotropy, −As{sub z}{sup 2}, on the ground-state structure and finite-temperature properties of a two-dimensional magnetic nanodot in presence of a dipolar interaction of strength D. By a simulated annealing Monte Carlo method, we show that in the ground state a vortex core perpendicular to the nanodot plane emerges already in the range of moderate anisotropy values above a certain threshold level. In the giant-anisotropy regime the vortex structure is superseded by a stripe domain structure with stripes of alternate domains perpendicular to the surface of the sample. We have also observed an intermediate stage between the vortex and stripe structures, with satellite regions of tilted nonzero perpendicular magnetization around the core. At finite temperatures, at small A, we show by Monte Carlo simulations that there is a transition from the the in-plane vortex phase to the disordered phase characterized by a peak in the specific heat and the vanishing vortex order parameter. At stronger A, we observe a discontinuous transition with a large latent heat from the in-plane vortex phase to perpendicular stripe ordering phase before a total disordering at higher temperatures. In the regime of perpendicular stripe domains, namely with giant A, there is no phase transition at finite T: the stripe domains are progressively disordered with increasing T. Finite-size effects are shown and discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Nanolaminated Permalloy Core for High-Flux, High-Frequency Ultracompact Power Conversion

Metallic magnetic materials have desirable magnetic properties, including high permeability, and high saturation flux density, when compared with their ferrite counterparts. However, eddy-current losses preclude their use in many switching converter applications, due to the challenge of simultaneously achieving sufficiently thin laminations such that eddy currents are suppressed (e.g., 500 nm-1 mu m for megahertz frequencies), while simultaneously achieving overall core thicknesses such that substantial power can be handled. A CMOS-compatible fabrication process based on robot-assisted sequential electrodeposition followed by selective chemical etching has been developed for the realization of a core of substantial overall thickness (tens to hundreds of micrometers) comprised of multiple, stacked permalloy (Ni80Fe20) nanolaminations. Tests of toroidal inductors with nanolaminated cores showed negligible eddy-current loss relative to total core loss even at a peak flux density of 0.5 T in the megahertz frequency range. To illustrate the use of these cores, a buck power converter topology is implemented with switching frequencies of 1-2 MHz. Power conversion efficiency greater than 85% with peak operating flux density of 0.3-0.5 T in the core and converter output power level exceeding 5 W was achieved.

Kim, J↗
DOE OSTI2015

The r-process nucleosynthesis in the various jet-like explosions of magnetorotational core-collapse supernovae

The r-process nucleosynthesis in core-collapse supernovae (CC-SNe) is studied, with a focus on the explosion scenario induced by rotation and strong magnetic fields. Nucleosynthesis calculations are conducted based on magneto-hydrodynamical explosion models with a wide range of parameters for initial rotation and magnetic fields. The explosion models are classified in two different types: prompt-magnetic-jet and delayed-magnetic-jet, for which the magnetic fields of proto-neutron stars (PNSs) during collapse and the core-bounce are strong and comparatively moderate, respectively. Following the hydrodynamical trajectories of each explosion model, we confirmed that r-processes successfully occur in the prompt-magnetic-jets, which produce heavy nuclei including actinides. On the other hand, the r-process in the delayed-magnetic-jet is suppressed, which synthesizes only nuclei up to the second peak (A∼130). Thus, the r-process in the delayed-magnetic-jets could explain only “weak r-process” patterns observed in metal-poor stars rather than the “main r-process,” represented by the solar abundances. Our results imply that CC-SNe are possible astronomical sources of heavy r-process elements if their magnetic fields are strong enough, while weaker magnetic explosions may produce “weak r-process” patterns (A≲130). We show the potential importance and necessity of magneto-rotational SNe for explaining the galactic chemical evolution, as well as abundances of r-process enhanced metal-poor stars. We also examine the effects of the remaining uncertainties in the nature of PNSs due to weak interactions that determine the final neutron-richness of ejecta. Additionally, we briefly discuss radioactive isotope yields in primary jets (e.g., {sup 56}Ni), with relation to several optical observation of SNe and relevant high-energy astronomical phenomena.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Manifested luminescence and magnetic responses of stoichiometry dependent Cd{sub 1− x}Mn{sub x}Se quantum dots

Highlights: • Thio-glycolic acid (TGA) coated Cd{sub 1−x}Mn{sub x}Se quantum dots have been prepared with varying x • Formation of QD heterostructure can be expected at a nonzero, but higher value of x. • The deep defect related emission can be attributed to V{sub Cd}–V{sub Se} di-vacancies, which is dominant for smaller values of x. • An appreciable charge transfer between Mn{sup 2+} ions and capping agent TGA is anticipated. - Abstract: We report on stoichiometry dependent manifested physical properties of thioglycolic acid (TGA) coated Cd{sub 1− x}Mn{sub x}Se QDs. While possessing a wurtzite phase, with increasing x, the QDs exhibited a notable blue-shifting of the onset of absorption. Attributed to V{sub Cd}–V{sub Se} di-vacancies, the QDs describe an intense deep-defect related emission response at smaller values of x (=0 to 0.3). Due to the facilitation of magnetic Mn{sup 2+} ion migration from the core to the QD surfaces, {sup 4}T{sub 1}–{sup 6}A{sub 1} transition based Mn{sup 2+} orange emission get suppressed at a higher x (=0.6 to 1). While the FT-IR spectra of the alloyed QDs display characteristic Mn–OH stretching mode at ∼644 cm{sup −1}, the peak located at ∼703 cm{sup −1} is assigned to Cd-Se bending. Furthermore, the QDs with a low x (=0.3), exhibit paramagnetic characteristics owing to the presence of uncorrelated, isolated Mn{sup 2+} spins. The collective luminescence and magnetic features would find immense scope in bio-labeling and imaging applications, apart from solid state luminescent components.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Design Considerations of a Transverse Flux Machine for Direct-Drive Wind Turbine Applications

This paper presents the design considerations of a double-sided transverse flux machine (TFM) for direct-drive wind turbine applications. The TFM has a modular structure with quasi-U stator cores and ring windings. The rotor is constructed with ferrite magnets in a flux-concentrating arrangement to achieve high air gap flux density. The design considerations for this TFM with respect to initial sizing, pole number selection, key design ratios, and pole shaping are presented in this paper. Pole number selection is critical in the design process of a TFM because it affects both the torque density and power factor under fixed magnetic and changing electrical loading. Several key design ratios are introduced to facilitate the design procedure. The effect of pole shaping on back-emf and inductance is also analyzed. These investigations provide guidance toward the required design of a TFM for direct-drive applications. The analyses are carried out using analytical and three-dimensional finite element analysis. A prototype is under construction for experimental verification.

17 WIND ENERGY↗
DOE OSTI2014

Development of a new virtual diagnostic for V3FIT

The determination of plasma equilibria from diagnostic information is a fundamental issue. V3FIT is a fully three-dimensional reconstruction code capable of solving the inverse problem using both magnetic and kinetic measurements. It uses VMEC as core equilibrium solver and supports both free- and fixed-boundary reconstruction approaches. In fixed-boundary mode VMEC does not use explicit information about currents in external coils, even though it has important effects on the shape of the safety factor profile. Indeed, the edge safety factor influences the reversal position in RFP plasmas, which then determines the position of the m = 0 island chain and the edge transport properties. In order to exploit such information a new virtual diagnostic has been developed, that thanks to Ampère's law relates the external current through the center of the torus to the circulation of the toroidal magnetic field on the outermost flux surface. The reconstructions that exploit the new diagnostic are indeed found to better interpret the experimental data with respect to edge physics.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗