Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “General Physics”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,009 records · Page 56
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

The dynamics of ion with electrostatic waves in a sheared magnetic field

The interaction between an ion and multiple electrostatic waves propagating perpendicularly to an ambient magnetic field with shear is investigated. Based on the Lie transformation method, with the wave amplitude and the magnetic shear both as the perturbation parameters, the analytical formulas for the reduced Hamiltonian is derived and results are compared with numerical calculations of the complete equations of motion for the case of two on-resonance waves and the case of two off-resonance waves, respectively. It is found that the effect of magnetic shear drastically prevents the acceleration of an ion in both cases. This result will help us to understand the behaviors of ions in a magnetic sheared device, such as tokamak.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Extracting elements of molecular structure from the all-particle wave function

Structural information is extracted from the all-particle (non-Born-Oppenheimer) wave function by calculating radial and angular densities derived from n-particle densities. As a result, one- and two-dimensional motifs of classical molecular structure can be recognized in quantum mechanics. Numerical examples are presented for three- (H{sup -}, Ps{sup -}, H{sub 2}{sup +}), four- (Ps{sub 2}, H{sub 2}), and five-particle (H{sub 2}D{sup +}) systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Modulational instabilities in two-dimensional magnetized dust-lattice

The transverse oscillations of paramagnetic dust grains in hexagonal dusty plasma crystal are discussed. The nonlinearity, which is related to the sheath electric/magnetic field(s) and to the weak inter grain interactions (magnetic dipole), is shown to lead to the generation of phase harmonics. In the case of propagating two-dimensional transverse dust-lattice modes in an arbitrary direction, it is shown the modulational instability of the carrier wave is due to self-interaction. The stability depends on the form of the electric field, the magnetic field, and the magnetic permeability of dust particles. The long term evolution of the modulated wave packet is described by a discrete nonlinear Schrodinger equation and it is solved numerically by Adomian decomposition method.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Equation of motion with radiation reaction in ultrarelativistic laser-electron interactions

The intensity of the ultra-short pulse lasers has reached 10{sup 22} W/cm{sup 2} owing to the advancements of laser technology. When the motion of an electron becomes relativistic, bremsstrahlung accompanies it. The energy from this bremsstrahlung corresponds to the energy loss of the electron; therefore, the motion of the electron deviates from the case without radiation. The radiation behaves something like resistance. This effect called ''radiation reaction'' or ''radiation damping'' and the force converted from the radiation is named the ''radiation reaction force'' or the ''damping force''. The equation of motion with the reaction force is known as the Lorentz-Abraham-Dirac (LAD) equation, but the solution of this equation is not physical due to the fact that it has a ''run-away'' solution. As one solution of this problem, we have derived a new equation which takes the place of the Lorentz-Abraham-Dirac equation. We will show the validity of this equation with a simple theoretical analysis.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Computing the energy of a water molecule using multideterminants: A simple, efficient algorithm

Quantum Monte Carlo (QMC) methods such as variational Monte Carlo and fixed node diffusion Monte Carlo depend heavily on the quality of the trial wave function. Although Slater-Jastrow wave functions are the most commonly used variational ansatz in electronic structure, more sophisticated wave functions are critical to ascertaining new physics. One such wave function is the multi-Slater-Jastrow wave function which consists of a Jastrow function multiplied by the sum of Slater determinants. In this paper we describe a method for working with these wave functions in QMC codes that is easy to implement, efficient both in computational speed as well as memory, and easily parallelized. The computational cost scales quadratically with particle number making this scaling no worse than the single determinant case and linear with the total number of excitations. Additionally, we implement this method and use it to compute the ground state energy of a water molecule.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2011

Thermodynamic integration from classical to quantum mechanics

We present a new method for calculating quantum mechanical corrections to classical free energies, based on thermodynamic integration from classical to quantum mechanics. In contrast to previous methods, our method is numerically stable even in the presence of strong quantum delocalization. We first illustrate the method and its relationship to a well-established method with an analysis of a one-dimensional harmonic oscillator. We then show that our method can be used to calculate the quantum mechanical contributions to the free energies of ice and water for a flexible water model, a problem for which the established method is unstable.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Gyrokinetic determination of the electrostatic potential of rotating magnetic islands in tokamaks

The electrostatic potential related to a magnetic island structure with imposed width and rotation frequency is studied by means of gyrokinetic simulations, which allow its self-consistent determination via the Poisson equation. An adiabatic response of the trapped ions at the island separatrix leads to a significant smoothing of the potential with respect to analytic calculations based on a complete flattening of the pressure profile inside the island. As a consequence, the magnitude of the polarization current is drastically reduced. When the island size is comparable to the ion banana width, the adiabatic response covers the whole island region, leading to a reduced density flattening for islands rotating in the electron diamagnetic direction. This confirms previous results based on drift-kinetic simulations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Numerical investigation of pulse-modulated atmospheric radio frequency discharges in helium under different duty cycles

Experiments observed that the pulse duty cycle has effects on the plasma homogeneity in pulse-modulated radio frequency (rf) discharges. In this paper, pulse-modulated rf (13.56 MHz) helium discharges are theoretically investigated using a two dimensional fluid model. With the pulse period being fixed to 15 {mu}s, it is found that when the pulse-on duration is over 4 {mu}s, i.e., the duty cycle is larger than approximately 27%, the discharge transits from an inhomogeneous to a homogeneous mode in every specific part of each pulse cycle under currently-used simulation parameters. More quantitative analysis shows that the discharge becomes more homogeneous as the duty cycle is increased but does not reach complete homogeneity. Possible reasons for the homogeneity improvement are discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Onset conditions for positive direct current corona discharges in air under the action of photoionization

This paper presents a numerical model for the inception of positive dc corona discharges in air near cylindrical anodes, which plays a bridge role between the classic positive corona onset criterion and the photoionization model considering the effective radiation wavelength. The predicted onset voltages agree well with Peek's experimental data in a wide range of conductor radii and relative air densities. The influence of the collisional quenching of emitting excited states on the surface onset field is significant with low air density or small conductor radius. Within the effective radiation wavelength, numerical expressions for the photon absorption coefficient in air and Townsend's second coefficient due to photoionization are deduced on the basis of the new model. A different perspective on the classic coefficients is given.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Effect of guide field on lower-hybrid drift instabilities in current sheet containing energetic particles

The effect of a guide field on the linear lower-hybrid drift instability (LHDI) in a thin current sheet containing energetic particles is investigated using kinetic theory. It is found that the symmetry properties of the LHD modes are destroyed by the guide field. The LHDI growth rate decreases with the strength of the latter, and the perturbed magnetic field is much higher than that of the guide-field free case.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Laser heating of finite two-dimensional dust clusters: A. Experiments

Laser manipulation allows to control the kinetic particle temperature in dusty plasmas. Different methods of laser heating for plasma crystals are benchmarked experimentally. The methods are analyzed with respect to homogeneity and isotropy in a spatial, temporal, and statistical sense. It is shown that it is possible to achieve particle dynamics very close to thermal equilibrium and that laser heating methods allow for a detailed study of phase transitions in finite size systems.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Dynamic procedure for filtered gyrokinetic simulations

Large eddy simulations (LES) of gyrokinetic plasma turbulence are investigated as interesting candidates to decrease the computational cost. A dynamic procedure is implemented in the gene code, allowing for dynamic optimization of the free parameters of the LES models (setting the amplitudes of dissipative terms). Employing such LES methods, one recovers the free energy and heat flux spectra obtained from highly resolved direct numerical simulations. Systematic comparisons are performed for different values of the temperature gradient and magnetic shear, parameters which are of prime importance in ion temperature gradient driven turbulence. Moreover, the degree of anisotropy of the problem, which can vary with parameters, can be adapted dynamically by the method that shows gyrokinetic large eddy simulation to be a serious candidate to reduce numerical cost of gyrokinetic solvers.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Three-dimensional model and simulation of vacuum arcs under axial magnetic fields

In this paper, a three-dimensional (3d) magneto-hydro-dynamic (MHD) model of axial magnetic field vacuum arcs (AMFVAs) is established. Based on this model, AMFVAs are simulated and analyzed. Three-dimensional spatial distributions of many important plasma parameters and electric characteristics in AMFVAs can be obtained, such as ion number density, ion temperature, electron temperature, plasma pressure, current densities along different directions (x, y, and z), ion velocities along different directions, electric fields strength along different directions, and so on. Simulation results show that there exist significant spiral-shaped rotational phenomena in the AMFVAs, this kind of rotational phenomenon also can be verified by the many related experiments (AMFVAs photographs, especially for stronger AMF strength). For current simulation results of AMFVAs, the maximal rotational velocity at anode side is about 1100 m/s. Radial electric field is increased from arc center to arc edge; axial electric field is decreased from cathode side to anode side. Radial electric field at arc edge can be larger than axial electric field. Azimuthal electric field in most regions is much smaller than radial and axial electric field, but it can reach about 1.19 kV/m. Radial magnetic field is the smallest one compared with other components, it reaches to maximum value at the position near to anode, it can influence arc characteristics.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Trajectory-guided configuration interaction simulations of multidimensional quantum dynamics

We propose an approach to modelling multidimensional quantum systems which uses direct-dynamics trajectories to guide wavefunction propagation. First, trajectory simulations are used to generate a sample of dynamically relevant configurations on the potential energy surface (PES). Second, the sampled configurations are used to construct an n-mode representation of the PES using a greedy algorithm. Finally, the time-dependent Schroedinger equation is solved using a configuration interaction expansion of the wavefunction, with individual basis functions derived directly from the 1-mode contributions to the n-mode PES. This approach is successfully demonstrated by application to a 20-dimensional benchmark problem describing tunnelling in the presence of coupled degrees of freedom.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Non-Born-Oppenheimer electronic and nuclear densities for a Hooke-Calogero three-particle model: Non-uniqueness of density-derived molecular structure

We consider the calculation of non-Born-Oppenheimer, nBO, one-particle densities for both electrons and nuclei. We show that the nBO one-particle densities evaluated in terms of translationally invariant coordinates are independent of the wavefunction describing the motion of center of mass of the whole system. We show that they depend, however, on an arbitrary reference point from which the positions of the vectors labeling the particles are determined. We examine the effect that this arbitrary choice has on the topology of the one-particle density by selecting the Hooke-Calogero model of a three-body system for which expressions for the one-particle densities can be readily obtained in analytic form. We extend this analysis to the one-particle densities obtained from full Coulomb interaction wavefunctions for three-body systems. We conclude, in view of the fact that there is a close link between the choice of the reference point and the topology of one-particle densities that the molecular structure inferred from the topology of these densities is not unique. We analyze the behavior of one-particle densities for the Hooke-Calogero Born-Oppenheimer, BO, wavefunction and show that topological transitions are also present in this case for a particular mass value of the light particles even though in the BO regime the nuclear masses are infinite. In this vein, we argue that the change in topology caused by variation of the mass ratio between light and heavy particles does not constitute a true indication in the nBO regime of the emergence of molecular structure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Simulations of ionization in a hot cavity surface ion source

A new numerical Monte Carlo method based model of a hot cavity surface ionization ion source is presented in this paper. The model, intended to support the studies on ionization phenomena in a widely used class of ion sources, takes into account geometry of the ion source and extraction system, ionizer temperature and other features. The results of ion source efficiency calculations for various configurations of the extraction field are reviewed. The dominant role of the ionizer region near the extraction opening is described. Simulated dependences of ionization efficiency on the working parameters like ionizer length and temperature, ionization potential of the substance, and extraction voltage are discussed. A good agreement of the experimental data (e.g., influence of ionizer temperature, current-voltage curve) and the predictions of the model is found. It is also shown that the contribution to the ionization yield from impact of thermionic electrons accelerated by the extraction field may be significant, especially for the substances of small surface ionization coefficient. The simulation results are compared to the predictions of different theoretical models of the ion source--the obtained simulation data are in accordance both with a well-known Kirchner formula and the so called spherical ionizer model.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Ultrafast structural dynamics and isomerization in Rydberg-exited Quadricyclane

The quadricyclane - norbornadiene system is an important model for the isomerization dynamics between highly strained molecules. In a breakthrough observation for a polyatomic molecular system of that complexity, we follow the photoionization from Rydberg states in the time-domain to derive a measure for the time-dependent structural dynamics and the time-evolving structural dispersion even while the molecule is crossing electronic surfaces. The photoexcitation to the 3s and 3p Rydberg states deposits significant amounts of energy into vibrational motions. We observe the formation and evolution of the vibrational wavepacket on the Rydberg surface and the internal conversion from the 3p Rydberg states to the 3s state. In that state, quadricyclane isomerizes to norbornadiene with a time constant of {tau}{sub 2} = 136(45) fs. The lifetime of the 3p Rydberg state in quadricyclane is {tau}{sub 1} = 320(31) and the lifetime of the 3s Rydberg state in norbornadiene is {tau}{sub 3} = 394(32).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Optimal control of open quantum systems: A combined surrogate Hamiltonian optimal control theory approach applied to photochemistry on surfaces

In this paper, control of open quantum systems with emphasis on the control of surface photochemical reactions is presented. A quantum system in a condensed phase undergoes strong dissipative processes. From a theoretical viewpoint, it is important to model such processes in a rigorous way. In this work, the description of open quantum systems is realized within the surrogate Hamiltonian approach [R. Baer and R. Kosloff, J. Chem. Phys. 106, 8862 (1997)]. An efficient and accurate method to find control fields is optimal control theory (OCT) [W. Zhu, J. Botina, and H. Rabitz, J. Chem. Phys. 108, 1953 (1998); Y. Ohtsuki, G. Turinici, and H. Rabitz, J. Chem. Phys. 120, 5509 (2004)]. To gain control of open quantum systems, the surrogate Hamiltonian approach and OCT, with time-dependent targets, are combined. Three open quantum systems are investigated by the combined method, a harmonic oscillator immersed in an ohmic bath, CO adsorbed on a platinum surface, and NO adsorbed on a nickel oxide surface. Throughout this paper, atomic units, i.e., ({Dirac_h}/2{pi})=m{sub e}=e=a{sub 0}= 1, have been used unless otherwise stated.

36 MATERIALS SCIENCE↗