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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 1,801 records · Page 100
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Ultra-fast magnetic vortex core reversal by a local field pulse

Magnetic vortex core reversal of a 20-nm-thick permalloy disk with a diameter of 100 nm was studied by micromagnetic simulations. By applying a global out-of-plane magnetic field pulse, it turned out that the final core polarity is very sensitive to pulse width and amplitude, which makes it hard to control. The reason for this phenomenon is the excitation of radial spin waves, which dominate the reversal process. The excitation of spin waves can be strongly suppressed by applying a local field pulse within a small area at the core center. With this approach, ultra-short reversal times of about 15 ps were achieved, which are ten times faster compared to a global pulse.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Magnetic vortex crystal formation in the antidot complement of square artificial spin ice

We have studied ferromagnetic nickel thin films patterned with square lattices of elongated antidots that are negative analogues of square artificial spin ice. Micromagnetic simulations and direct current magnetic moment measurements reveal in-plane anisotropy of the magnetic hysteresis loops, and the formation of a dense array of magnetic vortices with random polarization and chirality. These multiply-connected antidot arrays could be superior to lattices of disconnected nanodisks for investigations of vortex switching by applied electric current.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

On the use of the double floating probe method to infer the difference between the electron and the heavy particles temperatures in an atmospheric pressure, vortex-stabilized nitrogen plasma jet

Sweeping double probe measurements in an atmospheric pressure direct current vortex-stabilized plasma jet are reported (plasma conditions: 100 A discharge current, N{sub 2} gas flow rate of 25 Nl/min, thoriated tungsten rod-type cathode, copper anode with 5 mm inner diameter). The interpretation of the double probe characteristic was based on a generalization of the standard double floating probe formulae for non-uniform plasmas coupled to a non-equilibrium plasma composition model. Perturbations caused by the current to the probe together with collisional and thermal processes inside the probe perturbed region were taken into account. Radial values of the average electron and heavy particle temperatures as well as the electron density were obtained. The calculation of the temperature values did not require any specific assumption about a temperature relationship between different particle species. An electron temperature of 10 900 ± 900 K, a heavy particle temperature of 9300 ± 900 K, and an electron density of about 3.5 × 10{sup 22} m{sup −3} were found at the jet centre at 3.5 mm downstream from the torch exit. Large deviations from kinetic equilibrium were found toward the outer border of the plasma jet. These results showed good agreement with those previously reported by the authors by using a single probe technique. The calculations have shown that this method is particularly useful for studying spraying-type plasma torches operated at power levels of about 15 kW.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Symmetrical Josephson vortex interferometer as an advanced ballistic single-shot detector

We consider a ballistic detector formed in an interferometer manner which operational principle relies on Josephson vortex scattering at a measurement potential. We propose an approach to symmetrize the detector scheme and explore arising advantages in the signal-to-noise ratio and in the back-action on a measured object by means of recently presented numerical and analytical methods for modeling of a soliton scattering dynamics in the presence of thermal fluctuations. The obtained characteristics for experimentally relevant parameters reveal practical applicability of the considered schemes including possibility of coupling with standard digital rapid single flux quantum circuits.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Electron vortex magnetic holes: A nonlinear coherent plasma structure

We report the properties of a novel type of sub-proton scale magnetic hole found in two dimensional particle-in-cell simulations of decaying turbulence with a guide field. The simulations were performed with a realistic value for ion to electron mass ratio. These structures, electron vortex magnetic holes (EVMHs), have circular cross-section. The magnetic field depression is associated with a diamagnetic azimuthal current provided by a population of trapped electrons in petal-like orbits. The trapped electron population provides a mean azimuthal velocity and since trapping preferentially selects high pitch angles, a perpendicular temperature anisotropy. The structures arise out of initial perturbations in the course of the turbulent evolution of the plasma, and are stable over at least 100 electron gyroperiods. We have verified the model for the EVMH by carrying out test particle and PIC simulations of isolated structures in a uniform plasma. It is found that (quasi-)stable structures can be formed provided that there is some initial perpendicular temperature anisotropy at the structure location. The properties of these structures (scale size, trapped population, etc.) are able to explain the observed properties of magnetic holes in the terrestrial plasma sheet. EVMHs may also contribute to turbulence properties, such as intermittency, at short scale lengths in other astrophysical plasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Current-driven vortex domain wall motion in wire-tube nanostructures

We have investigated the current-driven domain wall motion in nanostructures comprised of a pair of nanotube and nanowire segments. Under certain values of external magnetic fields, it is possible to pin a vortex domain wall in the transition zone between the wire and tube segments. We explored the behavior of this domain wall under the action of an electron flow applied in the opposite direction to the magnetic field. Thus, for a fixed magnetic field, it is possible to release a domain wall pinned simply by increasing the intensity of the current density, or conversely, for a fixed current density, it is possible to release the domain wall simply decreasing the magnetic external field. When the domain wall remains pinned due to the competition between the current density and the magnetic external field, it exhibits a oscillation frequency close to 8 GHz. The amplitude of the oscillations increases with the current density and decreases over time. On the other hand, when the domain wall is released and propagated through the tube segment, this shows the standard separation between a steady and a precessional regime. The ability to pin and release a domain wall by varying the geometric parameters, the current density, or the magnetic field transforms these wire-tube nanostructures in an interesting alternative as an on/off switch nano-transistor.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

Annular vortex merging processes in non-neutral electron plasmas

Non-neutral electron plasmas in a uniform magnetic field are investigated experimentally as a two dimensional (2D) fluid. Previously, it was reported that 2D phase space volume increases during a vortex merging process with viscosity. However, the measurement was restricted to a plasma with a high density. Here, an alternative method is introduced to evaluate a similar process for a plasma with a low density.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Coupling of magnetic electron drift vortex mode with longitudinal perturbations in collision-less and dissipative electron and electron-ion plasmas

The plasma density non-uniformity gives rise to the coupling of transverse magnetic electron drift vortex (MEDV) mode with the longitudinal perturbations in dissipative and non-dissipative electron plasmas. This coupling produces partially transverse and partially longitudinal low frequency instabilities in classical un-magnetized laser plasmas. The MEDV mode couples with the ion acoustic wave, when the ion dynamics is also included. Both the modes have frequencies of the same order of magnitude and couple to give rise to electromagnetic instabilities in un-magnetized plasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Constructive spin-orbital angular momentum coupling can twist materials to create spiral structures in optical vortex illumination

It was discovered that optical vortices twist isotropic and homogenous materials, e.g., azo-polymer films to form spiral structures on a nano- or micro-scale. However, the formation mechanism has not yet been established theoretically. To understand the mechanism of the spiral surface relief formation in the azo-polymer film, we theoretically investigate the optical radiation force induced in an isotropic and homogeneous material under irradiation using a continuous-wave optical vortex with arbitrary topological charge and polarization. It is revealed that the spiral surface relief formation in azo-polymer films requires the irradiation of optical vortices with a positive (negative) spin angular momentum and a positive (negative) orbital angular momentum (constructive spin-orbital angular momentum coupling), i.e., the degeneracy among the optical vortices with the same total angular momentum is resolved.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Orientation of bluff body for designing efficient energy harvesters from vortex-induced vibrations

The characteristics and performances of four distinct vortex-induced vibrations (VIVs) piezoelectric energy harvesters are experimentally investigated and compared. The difference between these VIV energy harvesters is the installation of the cylindrical bluff body at the tip of cantilever beam with different orientations (bottom, top, horizontal, and vertical). Experiments show that the synchronization regions of the bottom, top, and horizontal configurations are almost the same at low wind speeds (around 1.5 m/s). The vertical configuration has the highest wind speed for synchronization (around 3.5 m/s) with the largest harvested power, which is explained by its highest natural frequency and the smallest coupled damping. The results lead to the conclusion that to design efficient VIV energy harvesters, the bluff body should be aligned with the beam for low wind speeds (<2 m/s) and perpendicular to the beam at high wind speeds (>2 m/s)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Vortex motion of dust particles due to non-conservative ion drag force in a plasma

Vortex motion of the dust in a dusty plasma is shown to result because non-parallelism of the ion density gradient and the gradient of the magnitude of the ion ambipolar velocity cause the ion drag force on dust grains to be non-conservative. Furthermore, dust grain poloidal vortices consistent with the model predictions are experimentally observed, and the vortices change character with imposed changes in the ion temperature profile as predicted. For a certain ion temperature profile, two adjacent co-rotating poloidal vortices have a well-defined X-point analogous to the X-point in magnetic reconnection.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Toroidally symmetric plasma vortex at tokamak divertor null point

Reduced MHD equations are used for studying toroidally symmetric plasma dynamics near the divertor null point. Numerical solution of these equations exhibits a plasma vortex localized at the null point with the time-evolution defined by interplay of the curvature drive, magnetic restoring force, and dissipation. Convective motion is easier to achieve for a second-order null (snowflake) divertor than for a regular x-point configuration, and the size of the convection zone in a snowflake configuration grows with plasma pressure at the null point. In conclusion, the trends in simulations are consistent with tokamak experiments which indicate the presence of enhanced transport at the null point.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

On the acoustic signature of tandem airfoils: The sound of an elastic airfoil in the wake of a vortex generator

The acoustic signature of an acoustically compact tandem airfoil setup in uniform high-Reynolds number flow is investigated. The upstream airfoil is considered rigid and is actuated at its leading edge with small-amplitude harmonic pitching motion. The downstream airfoil is taken passive and elastic, with its motion forced by the vortex-street excitation of the upstream airfoil. The non-linear near-field description is obtained via potential thin-airfoil theory. It is then applied as a source term into the Powell-Howe acoustic analogy to yield the far-field dipole radiation of the system. To assess the effect of downstream-airfoil elasticity, results are compared with counterpart calculations for a non-elastic setup, where the downstream airfoil is rigid and stationary. Depending on the separation distance between airfoils, airfoil-motion and airfoil-wake dynamics shift between in-phase (synchronized) and counter-phase behaviors. Consequently, downstream airfoil elasticity may act to amplify or suppress sound through the direct contribution of elastic-airfoil motion to the total signal. Resonance-type motion of the elastic airfoil is found when the upstream airfoil is actuated at the least stable eigenfrequency of the downstream structure. This, again, results in system sound amplification or suppression, depending on the separation distance between airfoils. With increasing actuation frequency, the acoustic signal becomes dominated by the direct contribution of the upstream airfoil motion, whereas the relative contribution of the elastic airfoil to the total signature turns negligible.

42 ENGINEERING↗
DOE OSTI2016

Vortex dynamics in a thin superconducting film with a non-uniform magnetic field applied at its center with a small coil

This article models the dynamics of vortices that are generated in the middle of a thin, large-area, superconducting film by a low-frequency magnetic field from a small coil, motivated by a desire to better understand measurements of the superconducting coherence length made with a two-coil apparatus. When the applied field exceeds a critical value, vortices and antivortices originate near the middle of the film at the radius where the Lorentz force of the screening supercurrent is largest. The Lorentz force from the screening supercurrent pushes vortices toward the center of the film and antivortices outward. In an experiment, vortices are detected as an increase in mutual inductance between drive coil and a coaxial “pickup” coil on the opposite side of the film. Lastly, the model shows that the essential features of measurements are well described when vortex pinning and the attendant hysteresis are included.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Molecular hydrodynamics: Vortex formation and sound wave propagation

In the present study, quantitative feasibility tests of the hydrodynamic description of a two-dimensional fluid at the molecular level are performed, both with respect to length and time scales. Using high-resolution fluid velocity data obtained from extensive molecular dynamics simulations, we computed the transverse and longitudinal components of the velocity field by the Helmholtz decomposition and compared them with those obtained from the linearized Navier-Stokes (LNS) equations with time-dependent transport coefficients. By investigating the vortex dynamics and the sound wave propagation in terms of these field components, we confirm the validity of the LNS description for times comparable to or larger than several mean collision times. The LNS description still reproduces the transverse velocity field accurately at smaller times, but it fails to predict characteristic patterns of molecular origin visible in the longitudinal velocity field. Based on these observations, we validate the main assumptions of the mode-coupling approach. The assumption that the velocity autocorrelation function can be expressed in terms of the fluid velocity field and the tagged particle distribution is found to be remarkably accurate even for times comparable to or smaller than the mean collision time. This suggests that the hydrodynamic-mode description remains valid down to the molecular scale.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Shock-driven discrete vortex evolution on a high-Atwood number oblique interface

Here, we derive a model describing vorticity deposition on a high-Atwood number interface with a sinusoidal perturbation by an oblique shock propagating from a heavy into a light material. Limiting cases of the model result in vorticity distributions that lead to Richtmyer-Meshkov and Kelvin-Helmholtz instability growth. For certain combinations of perturbation amplitude, wavelength, and tilt of the shock, a regime is found in which discrete, co-aligned, vortices are deposited on the interface. The subsequent interface evolution is described by a discrete vortex model, which is found to agree well with both RAGE simulations and experiments at early times.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Electromechanical control of polarization vortex ordering in an interacting ferroelectric-dielectric composite dimer

Using a free-energy based computational model, we have investigated the response of a system comprising two interacting ferroelectric nanospheres, embedded in a dielectric medium, to a static external electric field. The system response is hysteretic and tunable by changing the inter-particle distance and the orientation of the applied field, which strongly modulates the field-driven long-range elastic interactions between the particles that propagate through the dielectric matrix. At small separations, the sensitivity of the system behavior with respect to the electric field direction originates from drastically different configurations of the local vortex-like polarization states in ferroelectric particles. In conclusion, this suggests new routes for the design of composite metamaterials whose dielectric properties can be controlled and tuned by selecting the mutual arrangement of their ferroelectric components.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Ion acceleration in laser generated megatesla magnetic vortex

We report that Magnetic Vortex Acceleration (MVA) from near critical density targets is one of the promising schemes of laser-driven ion acceleration. 3D particle-in-cell simulations are used to explore a more extensive laser-target parameter space than previously reported in the literature as well as to study the laser pulse coupling to the target, the structure of the fields, and the properties of the accelerated ion beam in the MVA scheme. The efficiency of acceleration depends on the coupling of the laser energy to the self-generated channel in the target. The accelerated proton beams demonstrate a high level of collimation with achromatic angular divergence, and carry a significant amount of charge. For petawatt-class lasers, this acceleration regime provides a favorable scaling of the maximum ion energy with the laser power for the optimized interaction parameters. Lastly, the megatesla-level magnetic fields generated by the laser-driven coaxial plasma structure in the target are a prerequisite for accelerating protons to the energy of several hundred mega-electron-volts.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

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