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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 1,531 records · Page 85
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Influence of Zr substitution on the stabilization of ThMn{sub 12}-type (Nd{sub 1−α}Zr{sub α})(Fe{sub 0.75}Co{sub 0.25}){sub 11.25}Ti{sub 0.75}N{sub 1.2−1.4} (α = 0–0.3) compounds

The influence of Zr substitution in ThMn{sub 12} compounds was investigated using strip casting alloys. It was found that Zr substitution stabilized (Nd{sub 1−α}Zr{sub α})(Fe{sub 0.75}Co{sub 0.25}){sub 11.25}Ti{sub 0.75}N{sub 1.2−1.4} (α = 0–0.3) compounds. Specifically, a reduction in the lattice constant along the a-axis was observed. Energy-dispersive X-ray spectroscopy mapping combined with Cs-corrected scanning transmission electron microscopy indicated that Zr atoms preferentially occupied Nd 2a sites. Both the magnetic anisotropy field and saturation polarization were maximum at Zr substitution ratio α = 0.1. The (Nd{sub 1−α}Zr{sub α})(Fe{sub 0.75}Co{sub 0.25}){sub 11.25}Ti{sub 0.75}N{sub 1.2−1.4} (α = 0–0.3) compounds displayed higher saturation polarization than Nd{sub 2}Fe{sub 14}B at high temperatures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

A curvature-based mechanism for the spontaneous emission of electromagnetic radiation at curved plasma–vacuum interfaces

In this work, we derive from first principles the dispersion relation for electromagnetic radiation that is spontaneously emitted at a 2D curved plasma–vacuum interface in the absence of both background and external electric and magnetic fields. This plasma geometry is motivated by plasmas generated through hypervelocity impact projectiles—an area that is of importance to satellite safety in the aerospace community, and may be of importance for the developing study of projectile impact-driven inertial confinement fusion. We find spontaneous radiation occurs when thermal surface waves propagate along the edge of a circular plasma, with a frequency lower than the plasma frequency and spatial decay proportional to 1/r. This is distinct from more well-known mechanisms of radiation, in which the radiation frequency is greater than or equal to the plasma frequency. This theory is validated against 2D, electromagnetic particle-in-cell simulations and shows reasonable agreement. Finally, we discuss the effects of plasma geometry and attribute the radiation generation mechanism to the acceleration of charged surface waves as they traverse azimuthally along a curved path.

Lau, Raymond (ORCID:0000000299318101)↗
DOE OSTI2012

KELVIN-HELMHOLTZ INSTABILITY IN PARTIALLY IONIZED COMPRESSIBLE PLASMAS

The Kelvin-Helmholtz instability (KHI) has been observed in the solar atmosphere. Ion-neutral collisions may play a relevant role for the growth rate and evolution of the KHI in solar partially ionized plasmas such as in, e.g., solar prominences. Here, we investigate the linear phase of the KHI at an interface between two partially ionized magnetized plasmas in the presence of a shear flow. The effects of ion-neutral collisions and compressibility are included in the analysis. We obtain the dispersion relation of the linear modes and perform parametric studies of the unstable solutions. We find that, in the incompressible case, the KHI is present for any velocity shear regardless of the value of the collision frequency. In the compressible case, the domain of instability depends strongly on the plasma parameters, especially the collision frequency and the density contrast. For high collision frequencies and low density contrasts the KHI is present for super-Alfvenic velocity shear only. For high density contrasts the threshold velocity shear can be reduced to sub-Alfvenic values. For the particular case of turbulent plumes in prominences, we conclude that sub-Alfvenic flow velocities can trigger the KHI thanks to the ion-neutral coupling.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Finite magnetic well effects on resistive and drift-resistive ballooning modes in a shaped tokamak

The impact of plasma shaping through magnetic well modifications on the stability of resistive ballooning modes (RBMs) in tokamaks is analysed, also including finite diamagnetic flows. Furthermore, various limiting cases of the dispersion relation, obtained by matching the ballooning equation across the ideal and resistive layers, are analysed. It is found that stability is generally improved by the combination of vertical elongation and positive triangularity, although, in some cases, the growth rate of the unstable mode can be enhanced by these effects. Usually, vertically elongated plasmas with no triangularity are prone to exhibit worse stability properties. A value for the critical β above which RBMs are driven unstable is identified, and a connection with type-III ELM activity is established.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Low Lying Spin Excitation in the Spin Ice Ho2Ti2O7

The high flux and low background of the new backscattering spectrometer at the SNS combine to produce an excellent signal to noise ratio, allowing us to investigate a low lying weak excitation never seen before in the spin ice, Ho{sub 2}Ti{sub 2}O{sub 7}. This non-dispersive excitation has been observed at E = 26.3 {mu}eV below 100 K but is resolution limited only below {approx}65 K. It is indifferent to magnetic fields below {mu}{sub 0}H = 4.5 T, at 1.6 K. These characteristics help us to identify the excitation as due to the nuclear spin system.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Spin excitations used to probe the nature of exchange coupling in the magnetically ordered ground state of Pr 0.5 Ca 0.5 MnO 3

We have used time-of-flight inelastic neutron scattering to measure the spin wave spectrum of the canonical half-doped manganite Pr 0.5 Ca 0.5 MnO 3 in its magnetic and orbitally ordered phase. Comparison of the data, which cover multiple Brillouin zones and the entire energy range of the excitations, with several different models shows that only the CE-type ordered state provides an adequate description of the magnetic ground state, provided interactions beyond nearest neighbor are included. We are able to rule out a ground state in which there exist pairs of dimerized spins which interact only with their nearest neighbors. The Zener polaron ground state, which comprises strongly bound magnetic dimers, can be ruled out on the basis of gross features of the observed spin wave spectrum. A model with weaker dimerization reproduces the observed dispersion but can be ruled out on the basis of subtle discrepancies between the calculated and observed structure factors at certain positions in reciprocal space. Adding further neighbor interactions results in almost no dimerization, i.e. interpolating back to the CE model. These results are consistent with theoretical analysis of the degenerate double exchange model for half-doping.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Spin-dependent dark matter scattering in quasi-two-dimensional magnets

We study the prospects of detecting dark matter coupled to the spin of the electron, such that it may scatter and excite magnons—collective excitations of electronic spins. We show that materials exhibiting long-range magnetic order where the spins are coupled only along a plane may act as directional dark matter detectors. These quasi-two-dimensional materials possess anisotropic dispersion relations and structure functions which induce a sidereal modulation in the excitation rate. We calculate the expected signal rate for some candidate (anti)ferromagnets, demonstrating a possible route to the direct detection of spin-dependent dark matter in the keV to MeV mass range.

Dark matter direct detection↗
DOE OSTI2018

Magnetoelastic Waves in Submicron Yttrium–Iron Garnet Films Manufactured by Means of Ion-Beam Sputtering onto Gadolinium–Gallium Garnet Substrates

A series of equidistant oscillations have been revealed in the transmission spectrum and dispersion law of Damon–Eshbach surface magnetostatic waves (SMSWs) propagating in submicron (200-nm) yttrium–iron garnet (YIG) films manufactured by means of ion-beam sputtering onto gadolinium–gallium garnet (GGG) substrates. These oscillations correspond to the excitation of magnetoelastic waves in the YIG–GGG structure at frequencies of resonant interaction between the surface magnetostatic waves and the elastic shear modes of the wave-guiding YIG–GGG structure. The obtained results show that the studied YIG films are characterized by an efficient magnetoelastic coupling between their spin and elastic subsystems and the matching of acoustic impedances at the YIG–GGG interface, thus providing the possibility to consider the ion-beam sputtering of YIG films onto GGG substrates as a promising technology for the creation of magnonic and straintronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Development of the Electromagnetic Boom and MOP System (EMOP)

Several large-scale oil spills typically occur in the USA and other places around the world. Such events threaten the Great Lakes as well. The cleanup process is not necessarily efficient or environmentally safe. In addition, the remediation and recovery process is time consuming, expensive and can lead to harmful environmental effects. An innovative electromagnetic approach by Natural Science, LLC uses materials that are reusable, recoverable, and environmentally safe. When micron-sized magnetite (Fe3O4) particles are dispersed in oil on water, the particles form a unique and preferential bond with the oil due to a combination of forces, e.g. Van der Waals. Magnetic fields can then be used to manipulate, trap and remove the oil in an environmentally safe manner with high efficiency. In the case of oil spills on water, the water becomes the primary transport medium for manipulating the oil. Videos demonstrating the process will be shown. Both the particles and the oil are recaptured and separated for reuse. This process is being applied to electromagnetic systems that can replace and/or increase the efficiency of the passive boom and skimmer systems used today.

43 PARTICLE ACCELERATORS↗
NASA NTRS1978

Fluctuation spectra in the NASA Lewis bumpy-torus plasma

The electrostatic potential fluctuation spectrum in the NASA Lewis bumpy-torus plasma was studied with capacitive probes in the low pressure (high impedance) mode and in the high pressure (low impedance) mode. Under different operating conditions, the plasma exhibited electrostatic potential fluctuations (1) at a set of discrete frequencies, (2) at a continuum of frequencies, and (3) as incoherent high-frequency turbulence. The frequencies and azimuthal wave numbers were determined from digitally implemented autopower and cross-power spectra. The azimuthal dispersion characteristics of the unstable waves were examined by varying the electrode voltage, the polarity of the voltage, and the neutral background density at a constant magnetic field strength.

Singh, C. M.↗
NASA NTRS1992

Alfvenic drift Kelvin-Helmholtz instability in the presence of an equilibrium electric field

The Alfvenic drift Kelvin-Helmholtz instability of a high-beta plasma in the presence of equilibrium magnetic and electric fields perpendicular to each other are studied. The plasma components are assumed to have 2D sheared velocity in y and z directions. The dispersion relation is derived, and the instability criterion is determined. It is shown that the equilibrium electric field has either stabilizing or destabilizing effect depending on certain conditions discussed in the paper.

Sharma, Avadhesh C.↗
DOE OSTI2018

Excess Conductivity Investigation of Y{sub 3}Ba{sub 5}Cu{sub 8}O{sub 18±δ} Superconductors Prepared by Various Parameters of Planetary Ball Milling Technique

Superconducting samples of type Y{sub 3}Ba{sub 5}Cu{sub 8}O{sub 18±δ} were elaborated by using a planetary high-energy ball milling (HEBM) technique with various parameters. Phases, microstructure, and superconductivity have been examined using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and electrical resistivity measurements. SEM investigation shows the occurrence of nanoentities embedded into the superconducting matrix for samples prepared by a ball milling technique. The main objective of this study is to investigate the influence of ball milling parameters on the fluctuation-induced conductivity (FIC). The electrical resistivity versus temperature, ρ(T), above T{sub c} was analyzed using Lawrence–Doniach (LD) and Aslamazov–Larkin (AL) models. Different fluctuation regimes indicated by short-wave fluctuation (SWF) and three-dimensional (3D), two-dimensional (2D), one-dimensional (1D), and critical region (CR) fluctuations were identified. The coherence length at zero temperature, the effective layer thickness, the critical magnetic fields, and the critical current densities are determined. The superconducting parameters strongly depend on the planetary ball milling technique, and the results are explained in relevancy to the microstructure. It was found that the achievement of a microstructure with well-dispersed nanoentities is an effective way for introducing pinning centers for enhancing critical current density and flux pinning capability of bulk Y-358.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Bell-Plesset effects in Rayleigh-Taylor instability of finite-thickness spherical and cylindrical shells

Bell-Plesset (BP) effects account for the influence of global convergence or divergence of the fluid flow on the evolution of the interfacial perturbations embedded in the flow. The development of the Rayleigh-Taylor instability in radiation-driven spherical capsules and magnetically-driven cylindrical liners necessarily includes a significant contribution from BP effects due to the time dependence of the radius, velocity, and acceleration of the unstable surfaces or interfaces. An analytical model is presented that, for an ideal incompressible fluid and small perturbation amplitudes, exactly evaluates the BP effects in finite-thickness shells through acceleration and deceleration phases. The time-dependent dispersion equations determining the “instantaneous growth rate” are derived. It is demonstrated that by integrating this approximate growth rate over time, one can accurately evaluate the number of perturbation e-foldings during the inward acceleration phase of the implosion. As a result, in the limit of small shell thickness, exact thin-shell perturbationequations and approximate thin-shell dispersion equations are obtained, generalizing the earlier results [E. G. Harris, Phys. Fluids 5, 1057 (1962); E. Ott, Phys. Rev. Lett. 29, 1429 (1972); A. B. Bud'ko et al., Phys. Fluids B 2, 1159 (1990)].

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Empirical model of whistler anisotropy instability

Empirical formulae for the real frequency and growth rate associated with the whistler anisotropy instability are obtained. The electrons are assumed to have an anisotropic distribution function, with Maxwellian parallel distribution. Under such an assumption complex roots of the dispersion relation depend only on two dimensionless parameters, namely, the temperature anisotropy factor A = T{sub perpendiculare}/T{sub ||e} - 1, where T{sub perpendiculare} and T{sub ||e} are the perpendicular and parallel electron temperatures, respectively, and parallel electron beta, {beta}{sub ||} = (8{pi}nT{sub ||e}/B{sup 2}){sup 1/2}, where n and B are the plasma density and magnetic field intensity, respectively. Comparison against exact numerical roots show that analytical formulae describe the whistler instability over a wide range of parallel electron beta and temperature anisotropy factor. The present result may be useful for circumstances in which the use of exact numerical roots becomes impractical, such as in the radiation belt quasi-linear transport coefficient calculation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Calibration of imaging plates to electrons between 40 and 180 MeV

This paper presents the response calibration of Imaging Plates (IPs) for electrons in the 40-180 MeV range using laser-accelerated electrons at Laboratoire d’Optique Appliquée (LOA), Palaiseau, France. In the calibration process, the energy spectrum and charge of electron beams are measured by an independent system composed of a magnetic spectrometer and a Lanex scintillator screen used as a calibrated reference detector. It is possible to insert IPs of different types or stacks of IPs in this spectrometer in order to detect dispersed electrons simultaneously. The response values are inferred from the signal on the IPs, due to an appropriate charge calibration of the reference detector. The effect of thin layers of tungsten in front and/or behind IPs is studied in detail. GEANT4 simulations are used in order to analyze our measurements.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2024

Electronic rotons and Wigner crystallites in a two-dimensional dipole liquid

A key concept proposed by Landau to explain superfluid liquid helium is the elementary excitation of quantum particles called rotons1–8. The irregular arrangement of atoms in a liquid leads to the aperiodic dispersion of rotons, which played a pivotal role in understanding fractional quantum Hall liquids (magneto-rotons)9,10 and the supersolidity of Bose–Einstein condensates11–13. Even for a two-dimensional electron or dipole liquid, in the absence of a magnetic field, the repulsive interactions have been predicted to form a roton minimum14–19, which can be used to trace the transition to Wigner crystals20–24 and superconductivity25–27, although this has not yet been observed. Here, we report the observation of such electronic rotons in a two-dimensional dipole liquid of alkali-metal ions donating electrons to surface layers of black phosphorus. Our data reveal the striking aperiodic dispersion of rotons, which is characterized by a local minimum of energy at finite momentum. As the density of dipoles decreases so that interactions dominate over the kinetic energy, the roton gap reduces to 0, as in a crystal, signalling Wigner crystallization. Our model shows the importance of short-range order arising from repulsion between dipoles, which can be viewed as the formation of Wigner crystallites (bubbles or stripes) floating in the sea of a Fermi liquid. Our results reveal that the primary origin of electronic rotons (and the pseudogap) is strong correlations.

Park, Soobin↗
DOE OSTI2022

Resistive wall tearing mode disruptions in DIII-D and ITER tokamaks

Disruptions are a serious problem in tokamaks, in which thermal and magnetic energy confinement is lost. This paper uses data from the DIII-D experiment, theory, and simulations to demonstrate that resistive wall tearing modes (RWTMs) produce the thermal quench (TQ) in a typical locked mode shot. Analysis of the linear RWTM dispersion relation shows the parameter dependence of the growth rate, particularly on the resistive wall time. Linear simulations of the locked mode equilibrium show that it is unstable with a resistive wall and stable with an ideally conducting wall. Nonlinear simulations demonstrate that the RWTM grows to sufficient amplitude to cause a complete thermal quench. The RWTM growth time is proportional to the thermal quench time. Here, the nonlinearly saturated RWTM magnetic perturbation amplitude agrees with experimental measurements. The onset condition is that the q = 2 rational surface is sufficiently close to the resistive wall. Collectively, this identifies the RWTM as the cause of the TQ. In ITER, RWTMs will produce long TQ times compared to present-day experiments. ITER disruptions may be significantly more benign than previously predicted.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Exchange enhancement of the electron g factor in strained InGaAs/InP heterostructures

The exchange enhancement of the electron g factor in strained InGaAs/InP heterostructures with a two-dimensional electron gas is studied. Analysis of the temperature dependence of the resistance in the minima of the Shubnikov-de Haas oscillations in perpendicular magnetic fields up to 12 T in the vicinity of the odd filling factors of the Landau levels yields the values of the effective electron Lande factor g* from −8.6 to −10.1. The experimental values are compared with the results of theoretical calculations of the g factor of quasiparticles. The calculations are performed using an eight-band k · p Hamiltonian and take into account exchange interaction in the two-dimensional electron gas. It is shown that, under the conditions of a large overlap between the spin-split Landau levels, the maximum value of the quasiparticle g factor can be attained in the vicinity of even filling factors. This is caused by the nonparabolicity of the electron dispersion relation.

36 MATERIALS SCIENCE↗