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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 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Two-Magnon Relaxation Processes in Nanocrystalline Thin Magnetic Films

Numerical analysis of the micromagnetic model was used to reveal the ‘resonance’ feature of relaxation processes in nanocrystalline thin magnetic films. This feature manifests itself in the form of sharp broadening of the ferromagnetic resonance (FMR) line at a certain frequency f{sub 1} depending on magnetic characteristics of the film, and is observed only in films the thickness of which exceeds some threshold value d{sub min}. Sharp broadening of the FMR line is accompanied by significant shift of the resonance field, whereas the shift value changes the sign at frequency ~ f{sub 1}. It was shown analytically that the nature of observed effects is associated with the two-magnon process of spin waves scattering on quasi-periodic magnetic microstructure – magnetization ‘ripple’. Obtained expressions for the threshold value of film thickness d{sub min} and frequency of maximum broadening of FMR line f{sub 1} agree well with the results of numerical computation of micromagnetic model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Small and arbitrary shock structures in spin 1/2 magnetohydrodynamic quantum plasma

The shock structures in spin-1/2 quantum plasma, in the presence of magnetic diffusivity, are studied in the framework of the quantum magnetohydrodynamic model. Linear dispersion relation for the system is carried out analytically, and the results are plotted numerically for several values of the plasma parameters. Numerical analysis for arbitrary amplitude waves is carried out, whereas for waves of small amplitude, the reductive perturbation technique is applied to obtain the Korteweg-de Vries-Burgers equation. Both the analyses are observed to give the same qualitative picture. Most importantly, the different plasma parameters are found to play significant roles in determining the nature of the shock waves. The parametric ranges for which monotonic shock and oscillatory shock solutions are observed, are found analytically.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Laser-pulse compression in a collisional plasma under weak-relativistic ponderomotive nonlinearity

We present theory and numerical analysis which demonstrate laser-pulse compression in a collisional plasma under the weak-relativistic ponderomotive nonlinearity. Plasma equilibrium density is modified due to the ohmic heating of electrons, the collisions, and the weak relativistic-ponderomotive force during the interaction of a laser pulse with plasmas. First, within one-dimensional analysis, the longitudinal self-compression mechanism is discussed. Three-dimensional analysis (spatiotemporal) of laser pulse propagation is also investigated by coupling the self-compression with the self-focusing. In the regime in which the laser becomes self-focused due to the weak relativistic-ponderomotive nonlinearity, we provide results for enhanced pulse compression. The results show that the matched interplay between self-focusing and self-compression can improve significantly the temporal profile of the compressed pulse. Enhanced pulse compression can be achieved by optimizing and selecting the parameters such as collision frequency, ion-temperature, and laser intensity.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1995

A numerical model for dynamic wave rotor analysis

A numerical model has been developed which can predict the dynamic (and steady state) performance of a wave rotor, given the geometry and time dependent boundary conditions. The one-dimensional, perfect gas, CFD based code tracks the gasdynamics in each of the wave rotor passages as they rotate past the various ducts. The model can operate both on and off-design, allowing dynamic behavior to be studied throughout the operating range of the wave rotor. The model accounts for several major loss mechanisms including finite passage opening time, fluid friction, heat transfer to and from the passage walls, and leakage to and from the passage ends. In addition, it can calculate the amount of work transferred to and from the fluid when the flow in the ducts is not aligned with the passages such as occurs in off-design operation. Since it is one-dimensional, the model runs reasonably fast on a typical workstation. This paper will describe the model and present the results of some transient calculations for a conceptual four port wave rotor designed as a topping cycle for a small gas turbine engine.

Paxson, D. E.↗
DOE OSTI2016

Overmoded subterahertz surface wave oscillator with pure TM{sub 01} mode output

Overmoded O-type Cerenkov generators using annular electron beams are facing the problem of multi-modes output due to the inevitable structural discontinuities. A simple but effective method to achieve the pure TM{sub 01} mode output is applied on the 0.14 THz overmoded surface wave oscillator (SWO) in this paper. In spite of still using an overmoded slow wave structure to ensure the easy fabrication, the followed smooth circular waveguide is shrinkingly tapered to the output waveguide with appropriate radius that it cuts off other higher modes except TM{sub 01} mode. Moreover, the modified device here has the same power capacity as the previous one according to the numerical analysis. By optimized lengths of the transition waveguide and tapered waveguide, particle-in-cell simulation results indicate that the subterahertz wave with output power increased 14.2% at the same frequency is obtained from the proposed SWO under the previous input conditions, and importantly, the output power is all carried by TM{sub 01} mode as expected. Further simulation results in the pulse regime confirm the feasibility of the optimized structure in the actual experiments. This simple and viable design is also applicable to overmoded devices in the lower frequency band of subterahertz wave.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Propagation of femtosecond pulses in a hollow-core revolver fibre

We have studied for the first time the propagation of femtosecond pulses through an optical fibre with an air-filled hollow core and a cladding in the form of one ring of noncontacting cylindrical capillaries for high-power radiation transmission in the 1.55-μm telecom range. Numerical analysis results demonstrate that the parameters of the fibre enable radiation transmission in the form of megawatt-power Raman solitons through up to a 25-m length of the fibre and tuning of the emission wavelength over 130 nm. We have experimentally demonstrated femtosecond pulse transmission through fibres up to 5 m in length in the linear propagation regime, without distortions of the pulse spectrum, with a dispersion-induced temporal pulse broadening within 20%. (fiber optics)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Numerically study on a new hybrid photovoltaic thermal (PVT) collectors with natural circulation

Traditional design of Hybrid photovoltaic thermal (PV/T) system has the solar cells fixed on the top of the absorber. A new PV/T system in which the solar cells are pasted on the bottom of the glass cover is suggested with the aim of realizing higher electricity output considering the lower temperature of glass cover compared with that of absorber. A numerical analysis model is set up to compare the performances of the traditional PV/T and the new PV/T in this study. It is found that compared to the traditional PV/T, the new PV/T shows higher daily electric efficiency. But this superiority is not as apparent as expected. The key point to increase the daily electric efficiency of a PV/T lies in increasing the solar energy transfer efficiency of the solar cell. The total energy gain of the new PV/T is about 7% lower than that of the traditional PV/T because of smaller mass rate and more energy loss from glass cover with higher temperature.

14 SOLAR ENERGY↗
DOE OSTI2018

Anomalous Internal Conversion as a Clue to Solving the {sup 209}Bi Puzzle

A numerical analysis of the specific-difference (SD) method for eliminating the Bohr–Weisskopf effect in the theory of hyperfine splitting is performed within a more general theory of anomalous nuclear moments. The limiting accuracy of the SD method is demonstrated. In analyzing experimental data on the hyperfine splitting in H- and Li-like ions of {sup 209}Bi, it is found that the disagreement between theoretical and experimental results that was reported recently was due to going beyond this theoretical accuracy. Prospects of further experimental and theoretical studies in these realms are discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Wave propagation in fractal-inspired self-similar beam lattices

We combine numerical analysis and experiments to investigate the effect of hierarchy on the propagation of elastic waves in triangular beam lattices. While the response of the triangular lattice is characterized by a locally resonant band gap, both Bragg-type and locally resonant gaps are found for the hierarchical lattice. Therefore, our results demonstrate that structural hierarchy can be exploited to introduce an additional type of band gaps, providing a robust strategy for the design of lattice-based metamaterials with hybrid band gap properties (i.e., possessing band gaps that arises from both Bragg scattering and localized resonance)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Zoomed high-resolution simulations of Multi-coupled Dark Energy: cored galaxy density profiles at high redshift

We perform for the first time high-resolution zoom-in re-simulations of individual halos in the context of the Multi-coupled Dark Energy (McDE) scenario, which is characterised by the existence of two distinct Dark Matter particle species with opposite couplings to a Dark Energy scalar field. We compare the structural properties of the simulated halos to the standard ΛCDM results. The zoomed-in initial conditions are set up using a specifically designed code called ZInCo that we publicly release along with the present paper. Our numerical results allow to investigate in detail and with unprecedented resolution the halo segregation process that characterises McDE cosmologies from its very early stages. In particular, we find that in contrast to what could be inferred from previous numerical analysis at lower resolution, the segregation process is already in place at redshifts as high as z ∼ 7. Most remarkably, we find that the subsequent evolution of the segregation leads to the formation of cored total matter density profiles with a core size that progressively increases in time. The shape of the cored profiles can be accurately predicted as the superposition of two NFW profiles with an increasing offset, thereby confirming the interpretation of the simulations results in terms of the segregation of the two dark matter components of the halo as a consequence of their different coupling to the Dark Energy field.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Study on lower hybrid current drive efficiency at high density towards long-pulse regimes in Experimental Advanced Superconducting Tokamak

Significant progress on both L- and H-mode long-pulse discharges has been made recently in Experimental Advanced Superconducting Tokamak (EAST) with lower hybrid current drive (LHCD) [J. Li et al., Nature Phys. 9, 817 (2013) And B. N. Wan et al., Nucl. Fusion 53, 104006 (2013).]. In this paper, LHCD experiments at high density in L-mode plasmas have been investigated in order to explore possible methods of improving current drive (CD) efficiency, thus to extend the operational space in long-pulse and high performance plasma regime. It is observed that the normalized bremsstrahlung emission falls much more steeply than 1/n{sub e-av} (line-averaged density) above n{sub e-av} = 2.2 × 10{sup 19} m{sup −3} indicating anomalous loss of CD efficiency. A large broadening of the operating line frequency (f = 2.45 GHz), measured by a radio frequency (RF) probe located outside the EAST vacuum vessel, is generally observed during high density cases, which is found to be one of the physical mechanisms resulting in the unfavorable CD efficiency. Collisional absorption of lower hybrid wave in the scrape off layer (SOL) may be another cause, but this assertion needs more experimental evidence and numerical analysis. It is found that plasmas with strong lithiation can improve CD efficiency largely, which should be benefited from the changes of edge parameters. In addition, several possible methods are proposed to recover good efficiency in future experiments for EAST.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Beam-plasma instability in charged plasma in the absence of ions

We report on the possibility of the beam-plasma instability development in the system with electron beam interacting with the single-component hot electron plasma without ions. As considered system, we analyse the interaction of the low-current relativistic electron beam (REB) with squeezed state in the high-current REB formed in the relativistic magnetically insulated two-section vircator drift space. The numerical analysis is provided by means of 3D electromagnetic simulation in CST Particle Studio. We have conducted an extensive study of characteristic regimes of REB dynamics determined by the beam-plasma instability development in the absence of ions. As a result, the dependencies of instability increment and wavelength on the REB current value have been obtained. The considered process brings the new mechanism of controlled microwave amplification and generation to the device with a virtual cathode. This mechanism is similar to the action of the beam-plasma amplifiers and oscillators.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Feasibility of a 90° electric sector energy analyzer for low energy ion beam characterization

A simple formula to calculate refocusing by locating the output slit at a specific distance away from the exit of 90° ion deflecting electric sector is given. Numerical analysis is also performed to calculate the ion beam trajectories for different values of the initial angular deviation of the beam. To validate the theory, a compact (90 mm × 5.5 mm × 32 mm) 90° sector ESA is fabricated which can fit through the inner diameter of a conflat 70 vacuum flange. Experimental results show that the dependence of resolution upon the distance between the sector exit and the Faraday cup agrees with the theory. The fabricated 90° sector electrostatic energy analyzer was then used to measure the space resolved ion energy distribution functions of an ion beam with the energy as low as 600 eV.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

New perspectives on superparameterization for geophysical turbulence

This is a research expository paper regarding superparameterization, a class of multi-scale numerical methods designed to cope with the intermittent multi-scale effects of inhomogeneous geophysical turbulence where energy often inverse-cascades from the unresolved scales to the large scales through the effects of waves, jets, vortices, and latent heat release from moist processes. Original as well as sparse space–time superparameterization algorithms are discussed for the important case of moist atmospheric convection including the role of multi-scale asymptotic methods in providing self-consistent constraints on superparameterization algorithms and related deterministic and stochastic multi-cloud parameterizations. Test models for the statistical numerical analysis of superparameterization algorithms are discussed both to elucidate the performance of the basic algorithms and to test their potential role in efficient multi-scale data assimilation. The very recent development of grid-free seamless stochastic superparameterization methods for geophysical turbulence appropriate for “eddy-permitting” mesoscale ocean turbulence is presented here including a general formulation and illustrative applications to two-layer quasigeostrophic turbulence, and another difficult test case involving one-dimensional models of dispersive wave turbulence. This last test case has randomly generated solitons as coherent structures which collapse and radiate wave energy back to the larger scales, resulting in strong direct and inverse turbulent energy cascades.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Simulation of an intracavity Q -switched optical parametric oscillator using rate equations

Based on rate equations we have refined the model of an intracavity Q -switched optical parametric oscillator (IOPO) by taking into account the influence of self-Raman stimulated scattering. The rate equations are solved and analysed in the plane-wave approximation for a Gaussian spatial beam distribution profile under various conditions. The spatial rate equations are also solved numerically for a beam with a field distribution of the TEM{sub 00} mode. The numerical analysis shows that stimulated self-Raman scattering leads to a decrease in the output signal power and an increase in the signal pulse width. This model allows one to predict that the output signal power can be increased with increasing ratio of Raman to fundamental photon lifetimes. This suggests a practical method for determining the self-Raman gain coefficient experimentally. In addition, a multi-pulse process for the signal beam of the IOPO can be regenerated by the presented model. However, in comparison with the previous research, the presented model is more accurate and allows one to design and optimise single-resonator IOPOs.The bibliography includes 25 references. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Ion-cyclotron instability in plasmas described by product-bi-kappa distributions

The dispersion relation for parallel propagating waves in the ion-cyclotron branch is investigated numerically by considering that the velocity distribution of the ion population is a function of type product-bi-kappa. We investigate the effects of the non-thermal features and of the anisotropy associated with this type of distribution on the ion-cyclotron instability, as well as the influence of different forms of the electron distribution, by considering Maxwellian distributions, bi-kappa distributions, and product-bi-kappa distributions. The cases of ions described by either Maxwellian or bi-kappa distributions are also considered, for comparison. The results of the numerical analysis show that the increase in the non-thermal character associated with the anisotropic kappa distributions for ions contributes to enhance the instability as compared to that obtained in the Maxwellian case, in magnitude and in wave number range, with more significant enhancement for the case of ion product-bi-kappa distributions than for the case of ion bi-kappa distributions. It is also shown that the ion-cyclotron instability is decreased if the electrons are described by product-bi-kappa distributions, while electrons described by bi-kappa distributions lead to growth rates which are very similar to those obtained considering a Maxwellian distribution for the electron population.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1987

VSAERO analysis of structurally decoupled tip planforms for a semispan wing

A numerical analysis has been conducted with the three-dimensional panel code VSAERO for two interacting lifting surfaces that are separated in the spanwise direction by a narrow gap, with the angle of attack of the outboard section being set independently of the inboard section, as in the 'free tip' rotor blade system proposed for helicopters. Computed values of tip surface lift and pitching moment coefficients are correlated with experimental data to determine the most suitable method for modeling the gap region between the surfaces. It is shown that the induced drag of the tip surface is reduced for negative incidence angles relative to the inboard section.

Martin, Daniel M.↗