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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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Bound-preserving discontinuous Galerkin methods for conservative phase space advection in curvilinear coordinates

In this study, we extend the positivity-preserving method of Zhang & Shu to simulate the advection of neutral particles in phase space using curvilinear coordinates. The ability to utilize these coordinates is important for non-equilibrium transport problems in general relativity and also in science and engineering applications with specific geometries. The method achieves high-order accuracy using Discontinuous Galerkin (DG) discretization of phase space and strong stability-preserving, Runge-Kutta (SSP-RK) time integration. Special care in taken to ensure that the method preserves strict bounds for the phase space distribution function $f$; i.e., $f\in[0,1]$. The combination of suitable CFL conditions and the use of the high-order limiter proposed in Zhang & Shu (2010) is sufficient to ensure positivity of the distribution function. However, to ensure that the distribution function satisfies the upper bound, the discretization must, in addition, preserve the divergence-free property of the phase space flow. Proofs that highlight the necessary conditions are presented for general curvilinear coordinates, and the details of these conditions are worked out for some commonly used coordinate systems (i.e., spherical polar spatial coordinates in spherical symmetry and cylindrical spatial coordinates in axial symmetry, both with spherical momentum coordinates). Results from numerical experiments --- including one example in spherical symmetry adopting the Schwarzschild metric --- demonstrate that the method achieves high-order accuracy and that the distribution function satisfies the maximum principle.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2015

Bound-Preserving Discontinuous Galerkin Methods for Conservative Phase Space Advection in Curvilinear Coordinates

We extend the positivity-preserving method of Zhang & Shu [49] to simulate the advection of neutral particles in phase space using curvilinear coordinates. The ability to utilize these coordinates is important for non-equilibrium transport problems in general relativity and also in science and engineering applications with specific geometries. The method achieves high-order accuracy using Discontinuous Galerkin (DG) discretization of phase space and strong stabilitypreserving, Runge-Kutta (SSP-RK) time integration. Special care in taken to ensure that the method preserves strict bounds for the phase space distribution function f; i.e., f ϵ [0, 1]. The combination of suitable CFL conditions and the use of the high-order limiter proposed in [49] is su cient to ensure positivity of the distribution function. However, to ensure that the distribution function satisfies the upper bound, the discretization must, in addition, preserve the divergencefree property of the phase space ow. Proofs that highlight the necessary conditions are presented for general curvilinear coordinates, and the details of these conditions are worked out for some commonly used coordinate systems (i.e., spherical polar spatial coordinates in spherical symmetry and cylindrical spatial coordinates in axial symmetry, both with spherical momentum coordinates). Results from numerical experiments - including one example in spherical symmetry adopting the Schwarzschild metric - demonstrate that the method achieves high-order accuracy and that the distribution function satisfies the maximum principle.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2017

The Helium Warm Breeze in IBEX Observations As a Result of Charge-exchange Collisions in the Outer Heliosheath

We simulated the signal due to neutral He atoms, observed by the Interstellar Boundary Explorer ( IBEX ), assuming that charge-exchange collisions between neutral He atoms and He{sup +} ions operate everywhere between the heliopause and a distant source region in the local interstellar cloud, where the neutral and charged components are in thermal equilibrium. We simulated several test cases of the plasma flow within the outer heliosheath (OHS) and investigated the signal generation for plasma flows both in the absence and in the presence of the interstellar magnetic field (ISMF). We found that a signal in the portion of IBEX data identified as being due to the Warm Breeze (WB) does not arise when a homogeneous plasma flow in front of the heliopause is assumed, but it appears immediately when any reasonable disturbance in its flow due to the presence of the heliosphere is assumed. We obtained a good qualitative agreement between the data selected for comparison and the simulations for a model flow with the velocity vector of the unperturbed gas and the direction and intensity of magnetic field adopted from recent determinations. We conclude that direct-sampling observations of neutral He atoms at 1 au from the Sun are a sensitive tool for investigating the flow of interstellar matter in the OHS, that the WB is indeed the secondary population of interstellar helium, which was hypothesized earlier, and that the WB signal is consistent with the heliosphere distorted from axial symmetry by the ISMF.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Revision of the Coulomb logarithm in the ideal plasma

The standard picture of the Coulomb logarithm in the ideal plasma is controversial, the arguments for the lower cut off need revision. The two cases of far subthermal and of far superthermal electron drift motions are accessible to a rigorous analytical treatment. We show that the lower cut off b{sub min} is a function of symmetry and shape of the shielding cloud, it is not universal. In the subthermal case, shielding is spherical and b{sub min} is to be identified with the de Broglie wavelength; at superthermal drift the shielding cloud exhibits cylindrical (axial) symmetry and b{sub min} is the classical parameter of perpendicular deflection. In both situations, the cut offs are determined by the electron-ion encounters at large collision parameters. This is in net contrast to the governing standard interpretation that attributes b{sub min} to the Coulomb singularity at vanishing collision parameters b and, consequently, assigns it universal validity. The origin of the contradictions in the traditional picture is analyzed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Squeezed coherent states of motion for ions confined in quadrupole and octupole ion traps

Quasiclassical dynamics of trapped ions is characterized by applying the time dependent variational principle (TDVP) on coherent state orbits, in case of quadrupole and octupole combined (Paul and Penning) or radiofrequency (RF) traps. A dequantization algorithm is proposed, by which the classical Hamilton (energy) function associated to the system results as the expectation value of the quantum Hamiltonian on squeezed coherent states. We develop such method and particularize the quantum Hamiltonian for both combined and RF nonlinear traps, that exhibit axial symmetry. We also build the classical Hamiltonian functions for the particular traps we considered, and find the classical equations of motion.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Transition metal dichalcogenide monolayers in an ultrashort optical pulse: Femtosecond currents and anisotropic electron dynamics

We theoretically study the interaction of an ultrafast intense linearly polarized optical pulse with monolayers of transition metal dichalcogenides (TMDCs). Such a strong pulse redistributes electrons between the bands and generates femtosecond currents during the pulse. Due to the large bandwidth of the incident pulse, this process is completely an off-resonant. While in TMDCs, the time-reversal symmetry is conserved, the inversion symmetry is broken, and these monolayers have axial symmetry along the armchair direction but not along with the zigzag one. The pulse, polarized along with asymmetric directions of TMDC monolayer, generates both longitudinal, i.e., along the direction of polarization, and transverse, i.e., in the perpendicular direction, currents. Such currents result in charge transfer through the system. Finally, we study different TMDC materials and show how the femtosecond transport in TMDC monolayers depend on their parameters, such as lattice constant and bandgap.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Casimir–Polder energy for axially symmetric systems

We develop a formalism suitable for studying Maxwell’s equations in the presence of a medium that is axially symmetric, in particular with respect to Casimir–Polder interaction energies. As an application, we derive the Casimir–Polder interaction energy between an electric δ-function plate and an anisotropically polarizable molecule for arbitrary orientations of the principal axes of polarizabilities of the molecule. We show that in the perfect conductor limit for the plate the interaction is insensitive to the orientation of the polarizabilities of the molecule. We obtain the Casimir–Polder energy between an electric δ-function sphere and an anisotropically polarizable molecule, again for arbitrary orientations of the principal axes of polarizabilities of the molecule. We derive results when the polarizable molecule is either outside the sphere, or inside the sphere. We present the perfectly conducting limit for the δ-function sphere, and also the interaction energy for the special case when the molecule is at the center of the sphere. Our general proposition is that the Casimir–Polder energy between a dielectric body with axial symmetry and a unidirectionally polarizable molecule placed on the axis with its polarizability parallel to the axis gets non-zero contribution only from the m=0 azimuth mode. This feature in conjunction with the property that the m=0 mode separates into transverse electric and transverse magnetic modes allows the evaluation of Casimir–Polder energies for axially symmetric systems in a relatively easy manner.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Numerical simulation of transient moisture transfer into an electronic enclosure

Electronic systems are sometimes exposed to harsh environmental conditions of temperature and humidity. Moisture transfer into electronic enclosures and condensation can cause several problems such as corrosion and alteration in thermal stresses. It is therefore essential to study the local climate inside the enclosures to be able to protect the electronic systems. In this work, moisture transfer into a typical electronic enclosure is numerically studied using CFD. In order to reduce the CPU-time and make a way for subsequent factorial design analysis, a simplifying modification is applied in which the real 3D geometry is approximated by a 2D axial symmetry one. The results for 2D and 3D models were compared in order to calibrate the 2D representation. Furthermore, simulation results were compared with experimental data and good agreement was found.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Spatial distribution of the plasma parameters in the RF negative ion source prototype for fusion

A numerical model, based on the fluid plasma theory, has been used for description of the spatial distribution of the plasma parameters (electron density and temperature, plasma potential as well as densities of the three types of positive hydrogen ions) in the IPP prototype RF negative hydrogen ion source. The model covers the driver and the expansion plasma region of the source with their actual size and accounts for the presence of the magnetic filter field with its actual value and location as well as for the bias potential applied to the plasma grid. The obtained results show that without a magnetic filter the two 2D geometries considered, respectively, with an axial symmetry and a planar one, represent accurately the complex 3D structure of the source. The 2D model with a planar symmetry (where the E×B and diamagnetic drifts could be involved in the description) has been used for analysis of the influence, via the charged-particle and electron-energy fluxes, of the magnetic filter and of the bias potential on the spatial structure of the plasma parameters in the source. Benchmarking of results from the code to experimental data shows that the model reproduces the general trend in the axial behavior of the plasma parameters in the source.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Note: Triggering behavior of a vacuum arc plasma source

Axial symmetry of discharge is very important for application of vacuum arc plasma. It is discovered that the triggering method is a significant factor that would influence the symmetry of arc discharge at the final stable stage. Using high-speed multiframe photography, the transition processes from cathode-trigger discharge to cathode-anode discharge were observed. It is shown that the performances of the two triggering methods investigated are quite different. Arc discharge triggered by independent electric source can be stabilized at the center of anode grid, but it is difficult to achieve such good symmetry through resistance triggering. It is also found that the triggering process is highly correlated to the behavior of emitted electrons.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Interface defects in SiC power MOSFETs - An electrically detected magnetic resonance study based on spin dependent recombination

This study presents electrically detected magnetic resonance (EDMR) measurements on a silicon carbide (SiC) MOSFET having the structure of a double-diffused silicon MOSFET (DMOS). The resonance pattern of a SiC DMOS was measured by monitoring the change of the recombination current between the source/body and the drain. The amplitude of the response has a maximum when the device is biased in depletion due to the equal concentrations of electrons and holes at the interface resulting in the most efficient recombination. The measured anisotropic g-tensor has axial symmetry with g{sub ∥} = 2.0051(4) (B ‖ c-axis), and g{sub ⊥} = 2.0029(4) (B⊥ c-axis) and the pattern shows several hyperfine (HF) peaks. We tentatively identify the observed defect as a silicon vacancy located directly at the interface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Group III Acceptors with Shallow and Deep Levels in Silicon Carbide: ESR and ENDOR Studies

Results of investigations of Group III acceptors (B, Al, and Ga) in crystals of silicon carbide using the most informative electron spin resonance and electron nuclear double resonance methods are presented. Structural models of the acceptors with shallow and deep levels are considered. In addition to the data obtained earlier, studies using high-frequency magnetic resonance were obtained, which allowed revealing orthorhombic deviations from the axial symmetry for the deep acceptors; theoretical analysis explains experimentally found shifts of g factors for the deep acceptors arising due to the orthorhombic deviations, which appear probably due to the Jahn–Teller effect.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Dynamic plasmonic beam shaping by vector beams with arbitrary locally linear polarization states

Vector beams, which have space-variant state of polarization (SOP) comparing with scalar beams with spatially homogeneous SOP, are used to manipulate surface plasmon polarizations (SPPs). We find that the excitation, orientation, and distribution of the focused SPPs excited in a high numerical aperture microscopic configuration highly depend on the space-variant polarization of the incident vector beam. When it comes to vector beam with axial symmetry, multi-foci of SPPs with the same size and uniform intensity can be obtained, and the number of foci is depending on the polarization order n. Those properties can be of great value in biological sensor and plasmonic tweezers applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Paramagnetic Cu{sup II} complexes with 1-(hetarylmethyl)silatranes

The structures of paramagnetic complexes CuCl{sub 2}•L with L = HetCH{sub 2}Si(OCH{sub 2}CH{sub 2}){sub 3}N [Het = pyrrol-1-yl (1), indol-1-yl (2), carbazol-9-yl (3), imidazole-1-yl (4), 3,5-dimethylpyrazol-1-yl (5), 1,2,4-triazol-1-yl (6), benzimidazol-1-yl (7), and 1,2,3-benzotriazol-1-yl (8)] were studied by the ESR and quantum chemical methods in terms of the density functional theory (DFT) approximation. The difference in structures of complexes CuCl{sub 2}•1—CuCl{sub 2}•8 is mainly determined by the nature of the five-membered heterocycle. The ESR spectra at room temperature are typical of the mononuclear complexes of Cu{sup II} of axial symmetry with various types of distortions. In complexes CuCl{sub 2}•1—CuCl{sub 2}•3, narrow symmetric signals of zero-valent copper are recorded. These signals remain in the spectra for at least six months, which can characterize them as efficient stabilizing matrices of nanoparticles.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2017

Rayleigh convective instability in a cloud medium

The problem of convective instability of an atmospheric layer containing a horizontally finite region filled with a cloud medium is considered. Solutions exponentially growing with time, i.e., solitary cloud rolls or spatially localized systems of cloud rolls, have been constructed. In the case of axial symmetry, their analogs are convective vortices with both ascending and descending motions on the axis and cloud clusters with ring-shaped convective structures. Depending on the anisotropy of turbulent exchange, the scale of vortices changes from the tornado scale to the scale of tropical cyclones. The solutions with descending motions on the axis can correspond to the formation of a tornado funnel or a hurricane eye in tropical cyclones.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Quality Parameters of Zero-Order Bessel Light Beams

A collimated light beam at the output of a semiconductor laser module was studied experimentally. The intensity distribution in the transverse cross section of this beam did not have infinite-order axial symmetry and varied with the longitudinal coordinate. The experimental spatial intensity distributions in light fields formed by a conical lens (axicon) from radiation of He–Ne and semiconducting lasers were compared. Criteria were proposed for objective numerical quality assessment of zero-order Bessel light beams. The quality parameters of Bessel light fields formed by the axicon from radiation of He–Ne and semiconducting lasers were obtained from trial calculations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

The Mg impurity in nitride alloys

Although several magnetic resonance studies address the Mg acceptor in GaN, there are few reports on Mg doping in the alloys, where hole production depends strongly on the Al or In content. Our electron paramagnetic resonance (EPR) measurements of the p-type alloys suggest that the Mg impurity retains the axial symmetry, characteristic of a p-type dopant in both alloys; however, In and Al produce additional, different characteristics of the acceptor. In InGaN, the behavior is consistent with a lowering of the acceptor level and increasing hole density as In concentration increases. For AlGaN, the amount of neutral Mg decreases with increasing Al content, which is attributed to different kinetics of hydrogen diffusion thought to occur in samples with higher Al mole fraction.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗