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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 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Effects of a random spatial variation of the plasma density on the mode conversion in cold, unmagnetized, and stratified plasmas

We study the effects of a random spatial variation of the plasma density on the mode conversion of electromagnetic waves into electrostatic oscillations in cold, unmagnetized, and stratified plasmas. Using the invariant imbedding method, we calculate precisely the electromagnetic field distribution and the mode conversion coefficient, which is defined to be the fraction of the incident wave power converted into electrostatic oscillations, for the configuration where a numerically generated random density variation is added to the background linear density profile. We repeat similar calculations for a large number of random configurations and take an average of the results. We obtain a peculiar nonmonotonic dependence of the mode conversion coefficient on the strength of randomness. As the disorder increases from zero, the maximum value of the mode conversion coefficient decreases initially, then increases to a maximum, and finally decreases towards zero. The range of the incident angle in which mode conversion occurs increases monotonically as the disorder increases. We present numerical results suggesting that the decrease of mode conversion mainly results from the increased reflection due to the Anderson localization effect originating from disorder, whereas the increase of mode conversion of the intermediate disorder regime comes from the appearance of many resonance points and the enhanced tunneling between the resonance points and the cutoff point. We also find a very large local enhancement of the magnetic field intensity for particular random configurations. In order to obtain high mode conversion efficiency, it is desirable to restrict the randomness close to the resonance region.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Study of microstructure and semiconductor to metallic conductivity transition in solid state sintered Li{sub 0.5}Mn{sub 0.5}Fe{sub 2}O{sub 4−δ} spinel ferrite

Li{sub 0.5}Mn{sub 0.5}Fe{sub 2}O{sub 4} ferrite has been prepared by solid state sintering route. XRD pattern showed single phased cubic spinel structure. The samples exhibited typical character of plastoferrite with ring shaped surface microstructure. New feature observed in the present ferrite is the frequency activated conductivity transition from semiconductor to metallic state above 800 K. The increase of conductivity with frequency in the semiconducting regime follows Jonscher power law, while decrease of conductivity in metallic regime obeys Drude equation. The conductivity in semiconductor regime has been understood by hopping mechanism of localized charge carriers among the cations in B sites of cubic spinel structure. At higher temperatures, overlapping of electronic orbitals from neighbouring ions and free particle like motion of lighter Li{sup +} ions among interstitial lattices contributed metallic conductivity. The samples provided evidence of localized nature of the charge carriers at lower temperatures and increasing delocalized character with the increase of measurement temperature. From application point of view, such ferrites behave as semiconductor at low temperature and allow electromagnetic wave to pass through, but transform into a metallic reflector with negative dielectric constant at high temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Theory of waves in pair-ion plasmas: Natural explanation of backward modes

Backward waves observed in the experiments by Oohara and Hatakeyama (Phys. Rev. Lett. 91, 205005 (2003)) are identified to be ion cyclotron harmonic waves inherent to the kinetic theory. The derived dispersion equation is based on exact solutions of the characteristic equations of the Vlasov equation in a bounded cylindrical coordinate system; it is different from its counterpart in unbounded plasmas, and it provides all the branches of the dispersion relations observed in the experiment. Positive and negative ions respond to a potential in the same time scale and cooperate to expose kinetic orbital behaviors to the macroscopic propagation characteristics. In addition, the experimental setting of the large Larmor radius makes higher harmonic ion cyclotron backward/forward waves observable. The large Larmor radius effects are naturally treated by a kinetic theory.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Quasi-linear heating and acceleration in bi-Maxwellian plasmas

Quasi-linear acceleration and heating rates are derived for drifting bi-Maxwellian distribution functions in a general nonrelativistic case for arbitrary wave vectors, propagation angles, and growth/damping rates. The heating rates in a proton-electron plasma due to ion-cyclotron/kinetic Alfvén and mirror waves for a wide range of wavelengths, directions of propagation, and growth or damping rates are explicitly computed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

High and low frequency instabilities driven by a single electron beam in two-electron temperature space plasmas

In an attempt to understand the excitation mechanisms of broadband electrostatic noise, beam-generated electrostatic instabilities are investigated using kinetic theory in a four-component magnetised plasma model composed of beam electrons (magnetic field-aligned), background hot and cool electrons and ions. All species are fully magnetised and considered to be Maxwellian. The dependence of the instability growth rates and real frequencies on various plasma parameters such as beam speed, particle densities and temperatures, magnetic field strength, wave propagation angle, and temperature anisotropy of the beam are examined. In this study we have found that the electron-acoustic, electron beam-resonant and ion-acoustic instabilities are excited. Our studies have focused on three velocity regimes, namely, the low (v{sub dbz} 2 C{sub h}) regimes, where v{sub dbz} (C{sub h}) is the electron beam drift speed (thermal speed of the hot electrons). Plasma parameters from satellite measurements are used where applicable to provide realistic predictions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Cyclotron resonance in plasma flow

This paper is devoted to the mechanism of particle acceleration via resonant interaction with the electromagnetic circular wave propagating along the inhomogeneous background magnetic field in the presence of a plasma flow. We consider the system where the plasma flow velocity is large enough to change the direction of wave propagation in the rest frame. This system mimics a magnetic field configuration typical for inner structure of a quasi-parallel shock wave. We consider conditions of gyroresonant interaction when the force corresponding to an inhomogeneity of the background magnetic field is compensated by the Lorentz force of the wave-magnetic field. The wave-amplitude is assumed to be about 10% of the background magnetic field. We show that particles can gain energy if kv{sub sw}>ω>kv{sub sw}−Ω{sub c} where k is the wave number, v{sub sw} is a plasma flow velocity, and ω and Ω{sub c} are the wave frequency and the particle gyrofrequency, respectively. This mechanism of acceleration resembles the gyrosurfing mechanism, but the effect of the electrostatic field is replaced by the effect of the magnetic field inhomogeneity.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Resonant cavity mode dependence of anomalous and inverse spin Hall effect

The direct current electric voltage induced by the Inverse Spin Hall Effect (ISHE) and Anomalous Hall Effect (AHE) was investigated in the TE{sub 011} and TE{sub 102} cavities. The ISHE and AHE components were distinguishable through the fitting of the voltage spectrum. The unwanted AHE was minimized by placing the DUT (Device Under Test) at the center of both the TE{sub 011} and TE{sub 102} cavities. The voltage of ISHE in the TE{sub 011} cavity was larger than that in the TE{sub 102} cavity due to the higher quality factor of the former. Despite optimized centering, AHE voltage from TE{sub 011} cavity was also higher. The reason was attributed to the E-field distribution inside the cavity. In the case of the TE{sub 011} cavity, the DUT was easily exposed to the E-field in all directions. Therefore, the parasitic AHE voltage in the TE{sub 102} cavity was less sensitive than that in the TE{sub 011} cavity to decentering problem.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Self-focusing and stimulated Brillouin back-scattering of a long intense laser pulse in a finite temperature relativistic plasma

The nonlinear dynamics of electromagnetic waves propagating through a plasma considering the effects of relativistic mass and ponderomotive nonlinearities is investigated. The modified electron density distribution, the dispersion relation, and the spatial profiles of electromagnetic wave amplitude in the plasma are obtained. It is shown that the cut-off frequency decreases, and there is an intensity range in which the ponderomotive self-focusing takes place. In the upper limit of this range, the laser beam is defocused due to the relativistic ponderomotive force. In addition, the stability of electromagnetic waves to stimulated Brillouin scattering is studied, and the backscattered wave resulting from decay of high power electromagnetic beam is resolved in relativistic regime. The study of effects of electron density and temperature on the growth rate of backscattered wave has been shown that by increasing these effects, the growth rate of instability increases.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Effect of laser polarization on quantum electrodynamical cascading

Development of quantum electrodynamical (QED) cascades in a standing electromagnetic wave for circular and linear polarizations is simulated numerically with a 3D PIC-MC code. It is demonstrated that for the same laser energy the number of particles produced in a circularly polarized field is greater than in a linearly polarized field, though the acquiring mean energy per particle is larger in the latter case. The qualitative model of laser-assisted QED cascades is extended by including the effect of polarization of the field. It turns out that cascade dynamics is notably more complicated in the case of linearly polarized field, where separation into the qualitatively different “electric” and “magnetic” regions (where the electric field is stronger than the magnetic field and vice versa) becomes essential. In the “magnetic” regions, acceleration is suppressed, and moreover the high-energy electrons are even getting cooled by photon emission. The volumes of the “electric” and “magnetic” regions evolve periodically in time and so does the cascade growth rate. In contrast to the linear polarization, the charged particles can be accelerated by circularly polarized wave even in “magnetic region.” The “electric” and “magnetic” regions do not evolve in time, and cascade growth rate almost does not depend on time for circular polarization.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Generation of electromagnetic waves in the very low frequency band by velocity gradient

It is shown that a magnetized plasma layer with a velocity gradient in the flow perpendicular to the ambient magnetic field is unstable to waves in the Very Low Frequency band that spans the ion and electron gyrofrequencies. The waves are formally electromagnetic. However, depending on wave vector k{sup ¯}=kc/ω{sub pe} (normalized by the electron skin depth) and the obliqueness, k{sub ⊥}/k{sub ||}, where k{sub ⊥,||} are wave vectors perpendicular and parallel to the magnetic field, the waves are closer to electrostatic in nature when k{sup ¯}≫1 and k{sub ⊥}≫k{sub ||} and electromagnetic otherwise. Inhomogeneous transverse flows are generated in plasma that contains a static electric field perpendicular to the magnetic field, a configuration that may naturally arise in the boundary layer between plasmas of different characteristics.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Electrodynamics of a ring-shaped spiral resonator

We present analytical, numerical, and experimental investigations of electromagnetic resonant modes of a compact monofilar Archimedean spiral resonator shaped in a ring, with no central part. Planar spiral resonators are interesting as components of metamaterials for their compact deep-subwavelength size. Such resonators couple primarily to the magnetic field component of the incident electromagnetic wave, offering properties suitable for magnetic meta-atoms. Surprisingly, the relative frequencies of the resonant modes follow the sequence of the odd numbers as f{sub 1}:f{sub 2}:f{sub 3}:f{sub 4}… = 1:3:5:7…, despite the nearly identical boundary conditions for electromagnetic fields at the extremities of the resonator. In order to explain the observed spectrum of resonant modes, we show that the current distribution inside the spiral satisfies a particular Carleman type singular integral equation. By solving this equation, we obtain a set of resonant frequencies. The analytically calculated resonance frequencies and the current distributions are in good agreement with experimental data and the results of numerical simulations. By using low-temperature laser scanning microscopy of a superconducting spiral resonator, we compare the experimentally visualized ac current distributions over the spiral with the calculated ones. Theory and experiment agree well with each other. Our analytical model allows for calculation of a detailed three-dimensional magnetic field structure of the resonators.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Stimulated Raman scattering of beat wave of two counter-propagating X-mode lasers in a magnetized plasma

Effects of transverse static magnetic field on stimulated Raman scattering (SRS) of the beat wave excited by two counter-propagating lasers are studied. Two counter-propagating lasers with frequency difference, ω{sub 1}∼ω{sub 2}≥2ω{sub p}, drive a non resonant space charge beat mode at wave number k{sup →}{sub 0}≈k{sup →}{sub 1}+k{sup →}{sub 2} in a plasma, where k{sup →}{sub 1} and k{sup →}{sub 2} are wave vectors of lasers having frequencies ω{sub 1} and ω{sub 2}, respectively. The driven beat wave acts as a pump for SRS and excites parametrically a pair of plasma wave (ω,k{sup →}) and side band electromagnetic wave (ω{sub 3},k{sup →}{sub 3}) propagating in the sideward direction in such a way that momentum remains conserved. The growth rate of Raman process is maximum for side scattering at θ{sub s}=π/2 for lower values of applied magnetic field (∼1 kG), which can be three fold by applying magnetic field ∼5.0 kG. Thus, optimum value of magnetic field can be utilized to achieve maximum electron acceleration in counter propagating geometry of beat wave acceleration by reducing the growth rate of Raman process.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Propagation of terahertz electromagnetic wave in plasma with inhomogeneous collision frequency

In this paper, we investigate the absorption spectra of terahertz electromagnetic wave in plasma with inhomogeneous collision frequency. Profiles are introduced to describe the non-uniformity of collision frequency. It is interesting to find that when the plasma is collision frequency inhomogeneous, the absorption spectrum would decreases faster than that in uniform plasma. And the rate of decreasing would be different when the profile changes. Two parameters are set up to predict how the profiles affect the absorption spectra. Furthermore, the effects of electron density are also considered.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Study of the effects of corrugated wall structures due to blanket modules around ICRH antennas

In future fusion reactors, and in ITER, the first wall will be covered by blanket modules. These blanket modules, whose dimensions are of the order of the ICRF wavelengths, together with the clearance gaps between them will constitute a corrugated structure which will interact with the electromagnetic waves launched by ICRF antennas. The conditions in which the grooves constituted by the clearance gaps between the blanket modules can become resonant are studied. Simple analytical models and numerical simulations show that mushroom type structures (with larger gaps at the back than at the front) can bring down the resonance frequencies, which could lead to large voltages in the gaps between the blanket modules and perturb the RF properties of the antenna if they are in the ICRF operating range. The effect on the wave propagation along the wall structure, which is acting as a spatially periodic (toroidally and poloidally) corrugated structure, and hence constitutes a slow wave structure modifying the wall boundary condition, is examined.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Full wave simulation of waves in ECRIS plasmas based on the finite element method

This paper describes the modeling and the full wave numerical simulation of electromagnetic waves propagation and absorption in an anisotropic magnetized plasma filling the resonant cavity of an electron cyclotron resonance ion source (ECRIS). The model assumes inhomogeneous, dispersive and tensorial constitutive relations. Maxwell's equations are solved by the finite element method (FEM), using the COMSOL Multiphysics{sup ®} suite. All the relevant details have been considered in the model, including the non uniform external magnetostatic field used for plasma confinement, the local electron density profile resulting in the full-3D non uniform magnetized plasma complex dielectric tensor. The more accurate plasma simulations clearly show the importance of cavity effect on wave propagation and the effects of a resonant surface. These studies are the pillars for an improved ECRIS plasma modeling, that is mandatory to optimize the ion source output (beam intensity distribution and charge state, especially). Any new project concerning the advanced ECRIS design will take benefit by an adequate modeling of self-consistent wave absorption simulations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Progress in quantitative GPR development at CNDE

Ground penetrating radar (GPR) uses electromagnetic (EM) radiation pulses to locate and map embedded objects. Commercial GPR instruments are generally geared toward producing images showing the location and extent of buried objects, and often do not make full use of available absolute amplitude information. At the Center for Nondestructive Evaluation (CNDE) at Iowa State University efforts are underway to develop a more quantitative approach to GPR inspections in which absolute amplitudes and spectra of measured signals play a key role. Guided by analogous work in ultrasonic inspection, there are three main thrusts to the effort. These focus, respectively, on the development of tools for: (1) analyzing raw GPR data; (2) measuring the EM properties of soils and other embedding media; and (3) simulating GPR inspections. This paper reviews progress in each category. The ultimate goal of the work is to develop model-based simulation tools that can be used assess the usefulness of GPR for a given inspection scenario, to optimize inspection choices, and to determine inspection reliability.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Tunable and angle-insensitive plasmon resonances in graphene ribbon arrays with multispectral diffraction response

Plasmon resonances in graphene ribbon arrays are investigated numerically by means of the Finite Element Method. Numerical analysis shows that a series of multipolar resonances take place when graphene ribbon arrays are illuminated by a TM polarized electromagnetic wave. Moreover, these resonances are angle-independent, and can be tuned greatly by the width and the doping level of the graphene ribbons. Specifically, we demonstrate that for graphene arrays with several sets of graphene ribbons, which have different widths or doping levels, each of these multipolar resonances will be split into several ones. In addition, as plasmon resonances can confine electromagnetic field at the ribbon edges, graphene ribbons with different widths or doping levels offer intriguing application for electrically tunable spectral imaging.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Left-handed properties of manganite-perovskites La{sub 1-x}Sr{sub x}MnO{sub 3} at various dopant concentrations

The experimental study of Double Negative (DNG) state of electromagnetic wave propagating in lanthanum manganite-perovskites doped with strontium La{sub 1-x}Sr{sub x}MnO{sub 3} is provided firstly below individual Curie temperatures (in ferromagnetic metal state (FM) for La{sub 1-x}Sr{sub x}MnO{sub 3}). Various dopant concentrations are considered for ceramic specimens: x = 0.15; 0.225; 0.3; 0.45; 0.6. It is shown that dependence of the DNG-peak intensity on dopant concentration is sharply non-monotone with maximum at the dopant concentrations x = 0.225 – 0.3. This behaviour follows the change of Curie temperature with increase of dopant concentration in such substances. The obtained dependence of DNG peak intensity supports the opinion concerning the role of disorder in highly doped manganite-perovskite magnetic ceramics under study.

36 MATERIALS SCIENCE↗