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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 1,099 records · Page 61
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DOE OSTI2011

Microwave N{sub 2}-Ar plasma torch. I. Modeling

The spatial structure of a microwave plasma torch driven by an azimuthally symmetric surface wave operating in a N{sub 2}-Ar mixture at atmospheric pressure is investigated. A two-dimensional (2D) self-consistent theoretical model is developed to investigate the entire spatial structure of the source, including the discharge zone, sustained by the field of the surface TM{sub 00} mode, and the postdischarge plasma. Maxwell's equations, the rate balance equations for the most important excited species - vibrationally and electronically excited states, ions and nitrogen atoms N({sup 4}S) - and the Boltzmann equation for electrons are consistently solved. Model calculations of the 2D spatial distributions of species of interest such as charged particles (electrons and positive ions), N{sub 2}({Chi} {sup 1{Sigma}}{sub g}{sup +},v) vibrationally excited molecules, N{sub 2}(A {sup 3{Sigma}}{sub u}{sup +}) metastable molecules, and N({sup 4}S) ground state atoms are presented and discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Microwave N{sub 2}-Ar plasma torch. II. Experiment and comparison with theory

Spatially resolved emission spectroscopy techniques have been used to determine the gas temperature, the electron, and N{sub 2}{sup +} ion densities and the relative emission intensities of radiative species in a microwave (2.45 GHz) plasma torch driven by a surface wave. The experimental results have been analyzed in terms of a two-dimensional theoretical model based on a self-consistent treatment of particles kinetics, gas dynamics, and wave electrodynamics. The measured spatial variations in the various quantities agree well with the model predictions. The radially averaged gas temperature is around 3000 K and varies only slowly along the discharge zone of the source but it drops sharply down to about 400 K in the postdischarge. The experimental wave dispersion characteristics nearly follow the theoretical ones, thus confirming that this plasma source is driven by a surface wave.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Nonlinear Dynamics of Mirror Instability Revisited

A nonlinear dynamics of the mirror modes near the instability threshold is revisited. It is shown that the major saturation is provided by modification of the velocity distribution function in the vicinity of small parallel ion velocities. The final relaxation scenario is based on almost resonant particle interaction with mirror modes. The saturated plasma state can be considered as a magnetic counterpart to electrostatic Bernstein-Greene-Kruskal modes. Our analytical model is verified by relevant numerical simulations. Test particle and PIC simulations indeed show that it is a modification of distribution function at small parallel velocities that results in fading away of free energy driving mirror mode. The physical similarity of the mirror and Weibel instabilities is demonstrated. The multipoint satellite measurements in space plasma can be used to validate a proposed scenario.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Nonlinear quantum electrodynamics in vacuum and plasmas

We consider high field physics due to quantum electrodynamics, in particular those that can be studied in the next generation of laser facilities. Effective field theories based on the Euler-Heisenberg Lagrangian are briefly reviewed, and examples involving plasma- and vacuum physics are given.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Some New Aspects of Degenerate Quantum Plasma

Answers to some salient questions, which arise in quantum plasmas, are given. Starting from the Schroedinger equation for a single particle it is demonstrated how the Wigner-Moyal equation can be derived. It is shown that the Wigner-Moyal type of equation also exists in the classical field theory. As an example, from the Maxwell equations the Wigner-Moyal type of equation is obtained for a dense photon gas, which is classical, concluding that the Wigner-Moyal type of equation can be derived for any system, classical or quantum. A new type of quantum kinetic equations are presented. These novel kinetic equations allows to obtain a set of quantum hydrodynamic equations, which is impossible to derive by the Wigner-Moyal equation. The propagation of small perturbations and instabilities of these perturbations are then discussed, presenting new modes of quantum plasma waves. In the case of low frequency oscillations with ions, a new Bogolyubov type of spectrum is found. Furthermore, the Korteweg-de Vries (KdV) equation is derived and the contribution of the Madelung term in the formation of the KdV solitons is discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Amplitude modulation of electron plasma waves in a quantum plasma

Using the one dimensional quantum hydrodynamic model for a two-component electron-ion dense quantum plasma the linear and nonlinear properties of electron plasma waves are studied including ion motion. By using the standard method of multiple scales perturbation technique a nonlinear Schroedinger equation containing quantum effects is derived. From this equation it is shown that with immobile ions an electron plasma wave becomes modulationally unstable in two distinct regions of the wavenumber. Numerical calculation shows that the stability domain of the wavevector shrinks with the increase in quantum diffraction effect. It is also found that the growth rate of instability in the high wavenumber region increases with the increase in quantum effect. Ion motion is found to have significant effect in changing the stability/instability domains of the wavenumber in the low k-region.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Fractional dynamics in the light scattering intensity fluctuation in dusty plasma

Light scattering intensity fluctuation in dusty plasma system is studied. The scattered electric field of the laser light is treated as a stationary stochastic process. A nonstandard form of fractional Langevin equation is solved using Green's function approach to obtain the so-called fractional Ornstein-Uhlenbeck process. The empirical correlation function of light intensity fluctuation is fitted with four model correlation functions which are representative of different mechanisms for monodisperse particle transport, namely, the kinetic (ballistic) model, the hydrodynamical (diffusive) model, the hybrid kinetic-hydrodynamic model, and the fractional kinetic model for polydisperse particles. The shifted fractional derivative index is found to be related to power-law exponent of polydisperse dust mass distribution. It is shown that the correlation model based on fractional Ornstein-Uhlenbeck process may provide a novel insight into the complex transport behaviors in dusty plasma.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Effects of magnetic field strength on the low frequency oscillation in Hall thrusters

In order to study the effect of magnetic field strength on low frequency oscillation in Hall thrusters, experiments were carried out with different operating parameters. Experimental results show that the effect of magnetic field strength on the low frequency oscillation changes with operating parameters. In the decline zone of magnetoampere characteristic curve, low frequency oscillation increases with the increase of magnetic field strength at low mass flow rate, while decreases with the increase of magnetic field strength at high mass flow rate. With further experiments and numerical simulations, it is found that the change of electron current at low mass flow rate and the change of ion current at high mass flow rate account for the variations of low frequency oscillation. Finally, the physical analysis is performed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Three-dimensional theory of weakly nonlinear Compton scattering

Nonlinear effects are known to occur in light sources when the wiggler parameter, or normalized 4-potential, A=e{radical}(-A{sub {mu}}A{sup {mu}})/m{sub 0}c, approaches unity. In this paper, it is shown that nonlinear spectral features can appear at arbitrarily low values of A if the fractional bandwidth of the undulator, {Delta}{phi}{sup -1}, is sufficiently small and satisfies the condition A{sup 2{Delta}{phi}{approx}}1. Consequences for the spectral brightness of Compton scattering light sources are outlined. Compton and Thomson scattering theories are compared with the Klein-Nishina cross-section formula to highlight differences in the case of narrow band gamma-ray operation. A weakly nonlinear Compton scattering theory is developed in one (plane wave) and three (local plane wave approximation) dimensions. Analytical models are presented and benchmarked against numerical calculations solving the Lorentz force equation with a fourth-order Runge-Kutta algorithm. Finally, narrow band gamma-ray spectra are calculated for realistic laser and electron beams.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2011

Influence of annular magnet on discharge characteristics in enhanced glow discharge plasma immersion ion implantation

A permanent annular magnet positioned at the grounded anode alters the discharge characteristics in enhanced glow discharge plasma immersion ion implantation (EGD-PIII). The nonuniform magnetic field increases the electron path length and confines electron motion due to the magnetic mirror effect and electron-neutral collisions thus occur more frequently. The plasma potential and ion density measured by a Langmuir probe corroborate that ionization is improved near the grounded anode. This hybrid magnetic field EGD-PIII method is suitable for implantation of gases with low ionization rates.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

A multidimensional pseudospectral method for optimal control of quantum ensembles

In our previous work, we have shown that the pseudospectral method is an effective and flexible computation scheme for deriving pulses for optimal control of quantum systems. In practice, however, quantum systems often exhibit variation in the parameters that characterize the system dynamics. This leads us to consider the control of an ensemble (or continuum) of quantum systems indexed by the system parameters that show variation. We cast the design of pulses as an optimal ensemble control problem and demonstrate a multidimensional pseudospectral method with several challenging examples of both closed and open quantum systems from nuclear magnetic resonance spectroscopy in liquid. We give particular attention to the ability to derive experimentally viable pulses of minimum energy or duration.

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

Debye screening and injection of positrons across the magnetic surfaces of a pure electron plasma in a stellarator

Just as quasineutral plasmas, pure electron plasmas display collective behavior and screen externally imposed electrostatic potentials. However, for pure electron plasmas, the screening of large negative potentials is fully nonlinear and deviates significantly from the textbook Debye screening. Such potentials create an evacuated region that may penetrate a long distance away from the source of the potential perturbation. An analytic model for this case is presented and compared to numerical solutions of the pure electron plasma equilibrium equation in a stellarator. Debye screening in non-neutral plasmas is asymmetric; in a pure electron plasma, negative potentials are not screened as effectively as positive potentials. Because external potentials can penetrate deep into the plasma, they can have a strong effect on particle orbit quality. Importantly, the assessment of transport driven by internal objects, e.g., diagnostic rods, depends on this. Numerical calculations are presented, showing that the poor shielding of negative potentials can be used not only to perturb and worsen confinement of a pure electron plasma, but can also be used to improve confinement modestly and can be used for injection of positrons across the magnetic surfaces in an initially pure electron plasma.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Making a geometrically asymmetric capacitive rf discharge electrically symmetric

The electrical asymmetry effect in a spherical, geometrically asymmetric capacitive argon discharge driven by two consecutive harmonics is investigated using particle in cell simulations and analytical modeling. We find that the discharge asymmetry can be reduced electrically by tuning the phase shift between the driving frequencies, i.e., the absolute value of the dc self-bias voltage can be completely reduced and the mean ion energies at both electrodes can be adapted.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Temperatures following x-ray free-electron-laser heating of thin low- and medium-Z solid targets

The absorption of ultrashort (100 fs or shorter) bursts of x-ray free-electron laser (XFEL) radiation by thin (less than an attenuation length) solid carbon and iron target is modeled. Photon energies of several hundred eV to several keV and intensities of 10{sup 16} to 10{sup 18} W cm{sup -2} are considered. We calculate carbon and iron temperatures at sufficient times after XFEL irradiation that local thermodynamic equilibrium has been established and electron-ion thermalization has occurred, assuming classical heat capacities. It is shown that there is an optimum photon energy for maximizing temperatures. Constant irradiation at 10{sup 18} W cm{sup -2} for 100 fs with photons of 2 and 3 keV, for example, is predicted to result in a temperature of over 500 eV in solid density carbon and over 1 keV in solid density iron, respectively.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Current sheath behavior and its velocity enhancement in a low energy Mather-type plasma focus device

The dynamics of the plasma sheath layer and its velocity enhancement have been studied in a low energy (4.9 kJ) Mather-type plasma focus device. Experiments were performed to study the effect of the Lorentz force variation on the current sheath expansion and movement, as well as the existence of traction between all parts of the sheath layer. Two different shape of anodes (cylindrical and step) along with an axial magnetic probe were used to investigate the effects of various experimental conditions, namely different charging voltages and gas pressures. In order to explore the upper limit of the current sheath velocity, a comparison has been made between the experimental data gathered by the probe and the Lee's computational model. The limitations governing the enhancement of the current sheath velocity that can lead to the deterioration of a good focusing phenomenon were also investigated. The increase of the current sheath velocity due to the usage of the step anode on ion generation and hard x-ray emissions have been demonstrated by means of an ion collector and a hard x-ray detector.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Using nonproduct quadrature grids to solve the vibrational Schroedinger equation in 12D

In this paper we propose a new quadrature scheme for computing vibrational spectra and apply it, using a Lanczos algorithm, to CH{sub 3}CN. All 12 coordinates are treated explicitly. We need only 157'419'523 quadrature points. It would not be possible to use a product Gauss grid because 33 853 318 889 472 product Gauss points would be required. The nonproduct quadrature we use is based on ideas of Smolyak, but they are extended so that they can be applied when one retains basis functions {theta}{sub n{sub 1}}(r{sub 1}){center_dot}{center_dot}{center_dot}{theta}{sub n{sub D}}(r{sub D}) that satisfy the condition {alpha}{sub 1}n{sub 1} + {center_dot}{center_dot}{center_dot} + {alpha}{sub D}n{sub D}{<=}b, where the {alpha}{sub k} are integers. We demonstrate that it is possible to exploit the structure of the grid to efficiently evaluate the matrix-vector products required to use the Lanczos algorithm.

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

Biparametric equilibria bifurcations of the Pierce diode: A one-dimensional plasma-filled device

The equilibria bifurcations of the biparametric version of the classical Pierce diode, a one-dimensional plasma-filled device, are analyzed in detail. Our investigation reveals that this spatiotemporal model is not structurally stable in relation to a second control parameter, the ratio of the plasma ion density to the injected electron beam density. For the first time, we relate the existence of one-fluid chaotic regions with specific biparametric equilibria bifurcations, identifying the restricted regions in the parametric plane where they occur. We show that the system presents several biparametric scenarios involving codimension-two transcritical bifurcations. Finally, we provide the spatial profile of the stable and unstable one-fluid equilibria in order to describe their metamorphoses.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Universal asymptotics of poloidal spectra of magnetic perturbations of saddle coils in tokamaks

Universal and generic features of poloidal spectra of external magnetic perturbations created by a set of saddle coils in tokamak plasmas are studied in a vacuum approximation. It is found that the poloidal mode spectra with high-accuracy are described by a linear combination of three universal asymptotical formulas which depend only on the safety factor of the equilibrium plasma and the geometry of perturbation coils. The validity of these formulas is confirmed by numerical calculations of the mode spectra in the DIII-D plasma [G. L. Jackson et al., Europhys. Conf. Abstr. 27A, P-4.47 (2007)] and a spherical tokamak, National Spherical Tokamak Experiment [D. A. Gates, et al., Nucl. Fusion 49, 104016 (2009)], plasmas perturbed by external control coils.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗