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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 91 records · Page 5
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DOE OSTI2013

TURBULENCE IN THE OUTER REGIONS OF PROTOPLANETARY DISKS. I. WEAK ACCRETION WITH NO VERTICAL MAGNETIC FLUX

We use local numerical simulations to investigate the strength and nature of magnetohydrodynamic (MHD) turbulence in the outer regions of protoplanetary disks, where ambipolar diffusion is the dominant non-ideal MHD effect. The simulations include vertical stratification and assume zero net vertical magnetic flux. We employ a super time-stepping technique to ameliorate the Courant restriction on the diffusive time step. We find that in idealized stratified simulations, with a spatially constant ambipolar Elsasser number Am, turbulence driven by the magnetorotational instability (MRI) behaves in a similar manner as in prior unstratified calculations. Turbulence dies away for Am {<=} 1, and becomes progressively more vigorous as ambipolar diffusion is decreased. Near-ideal MHD behavior is recovered for Am {>=} 10{sup 3}. In the intermediate regime (10 {<=} Am {<=} 10{sup 3}) ambipolar diffusion leads to substantial increases in both the period of the MRI dynamo cycle and the characteristic scales of magnetic field structures. To quantify the impact of ambipolar physics on disk accretion, we run simulations at 30 AU and 100 AU that include a vertical Am profile based upon far-ultraviolet (FUV) ionized disk models. These models develop a vertically layered structure analogous to the Ohmic dead zone that is present at smaller radii. We find that, although the levels of surface turbulence can be strong (and consistent with constraints on turbulent line widths at these radii), the inferred accretion rates are at least an order of magnitude smaller than those observed in T Tauri stars. This discrepancy is very likely due to the assumption of zero vertical magnetic field in our simulations and suggests that vertical magnetic fields are essential for MRI-driven accretion in the outer regions of protoplanetary disks.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

ANGULAR MOMENTUM TRANSPORT BY ACOUSTIC MODES GENERATED IN THE BOUNDARY LAYER. II. MAGNETOHYDRODYNAMIC SIMULATIONS

We perform global unstratified three-dimensional magnetohydrodynamic simulations of an astrophysical boundary layer (BL)-an interface region between an accretion disk and a weakly magnetized accreting object such as a white dwarf-with the goal of understanding the effects of magnetic field on the BL. We use cylindrical coordinates with an isothermal equation of state and investigate a number of initial field geometries including toroidal, vertical, and vertical with zero net flux. Our initial setup consists of a Keplerian disk attached to a non-rotating star. In a previous work, we found that in hydrodynamical simulations, sound waves excited by shear in the BL were able to efficiently transport angular momentum and drive mass accretion onto the star. Here we confirm that in MHD simulations, waves serve as an efficient means of angular momentum transport in the vicinity of the BL, despite the magnetorotational instability (MRI) operating in the disk. In particular, the angular momentum current due to waves is at times larger than the angular momentum current due to MRI. Our results suggest that angular momentum transport in the BL and its vicinity is a global phenomenon occurring through dissipation of waves and shocks. This point of view is quite different from the standard picture of transport by a local anomalous turbulent viscosity. In addition to angular momentum transport, we also study magnetic field amplification within the BL. We find that the field is indeed amplified in the BL, but only by a factor of a few, and remains subthermal.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

DYNAMO EFFECTS IN MAGNETOROTATIONAL TURBULENCE WITH FINITE THERMAL DIFFUSIVITY

We investigate the saturation level of hydromagnetic turbulence driven by the magnetorotational instability in the case of vanishing net flux. Motivated by a recent paper of Bodo et al., we here focus on the case of a non-isothermal equation of state with constant thermal diffusivity. The central aim of the paper is to complement the previous result with closure parameters for mean-field dynamo models, and to test the hypothesis that the dynamo is affected by the mode of heat transport. We perform computer simulations of local shearing-box models of stratified accretion disks with approximate treatment of radiative heat transport, which is modeled via thermal conduction. We study the effect of varying the (constant) thermal diffusivity, and apply different vertical boundary conditions (BCs). In the case of impenetrable vertical boundaries, we confirm the transition from mainly conductive to mainly convective vertical heat transport below a critical thermal diffusivity. This transition is however much less dramatic when more natural outflow BCs are applied. Similarly, the enhancement of magnetic activity in this case is less pronounced. Nevertheless, heating via turbulent dissipation determines the thermodynamic structure of accretion disks and clearly affects the properties of the related dynamo. This effect, however, may have been overestimated in previous work, and a careful study of the role played by boundaries will be required.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

A SECOND-ORDER GODUNOV METHOD FOR MULTI-DIMENSIONAL RELATIVISTIC MAGNETOHYDRODYNAMICS

We describe a new Godunov algorithm for relativistic magnetohydrodynamics (RMHD) that combines a simple, unsplit second-order accurate integrator with the constrained transport (CT) method for enforcing the solenoidal constraint on the magnetic field. A variety of approximate Riemann solvers are implemented to compute the fluxes of the conserved variables. The methods are tested with a comprehensive suite of multi-dimensional problems. These tests have helped us develop a hierarchy of correction steps that are applied when the integration algorithm predicts unphysical states due to errors in the fluxes, or errors in the inversion between conserved and primitive variables. Although used exceedingly rarely, these corrections dramatically improve the stability of the algorithm. We present preliminary results from the application of these algorithms to two problems in RMHD: the propagation of supersonic magnetized jets and the amplification of magnetic field by turbulence driven by the relativistic Kelvin-Helmholtz instability (KHI). Both of these applications reveal important differences between the results computed with Riemann solvers that adopt different approximations for the fluxes. For example, we show that the use of Riemann solvers that include both contact and rotational discontinuities can increase the strength of the magnetic field within the cocoon by a factor of 10 in simulations of RMHD jets and can increase the spectral resolution of three-dimensional RMHD turbulence driven by the KHI by a factor of two. This increase in accuracy far outweighs the associated increase in computational cost. Our RMHD scheme is publicly available as part of the Athena code.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

MAGNETIC KELVIN-HELMHOLTZ INSTABILITY AT THE SUN

Flows and instabilities play a major role in the dynamics of magnetized plasmas including the solar corona, magnetospheric and heliospheric boundaries, cometary tails, and astrophysical jets. The nonlinear effects, multi-scale and microphysical interactions inherent to the flow-driven instabilities, are believed to play a role, e.g., in plasma entry across a discontinuity, generation of turbulence, and enhanced drag. However, in order to clarify the efficiency of macroscopic instabilities in these processes, we lack proper knowledge of their overall morphological features. Here we show the first observations of the temporally and spatially resolved evolution of the magnetic Kelvin-Helmholtz instability in the solar corona. Unprecedented high-resolution imaging observations of vortices developing at the surface of a fast coronal mass ejecta are taken by the new Solar Dynamics Observatory, validating theories of the nonlinear dynamics involved. The new findings are a cornerstone for developing a unifying theory on flow-driven instabilities in rarefied magnetized plasmas, which is important for understanding the fundamental processes at work in key regions of the Sun-Earth system.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

SDO/AIA OBSERVATION OF KELVIN-HELMHOLTZ INSTABILITY IN THE SOLAR CORONA

We present observations of the formation, propagation, and decay of vortex-shaped features in coronal images from the Solar Dynamics Observatory associated with an eruption starting at about 2:30 UT on 2010 April 8. The series of vortices were formed along the interface between an erupting (dimming) region and the surrounding corona. They ranged in size from several to 10 arcsec and traveled along the interface at 6-14 km s{sup -1}. The features were clearly visible in six out of the seven different EUV wave bands of the Atmospheric Imaging Assembly. Based on the structure, formation, propagation, and decay of these features, we identified the event as the first observation of the Kelvin-Helmholtz instability (KHI) in the corona in EUV. The interpretation is supported by linear analysis and by a nonlinear 2.5-dimensional magnetohydrodynamic model of KHI. We conclude that the instability is driven by the velocity shear between the erupting and closed magnetic field of the coronal mass ejection. The shear-flow-driven instability can play an important role in energy transfer processes in coronal plasma.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

HYDROMAGNETIC INSTABILITIES IN RELATIVISTIC NEUTRON STARS

We model the nonlinear ideal magnetohydrodynamics of poloidal magnetic fields in neutron stars in general relativity assuming a polytropic equation of state. We identify familiar hydromagnetic modes, in particular the 'sausage/varicose' mode and 'kink' instability inherent to poloidal magnetic fields. The evolution is dominated by the kink instability, which causes a cataclysmic reconfiguration of the magnetic field. The system subsequently evolves to new, non-axisymmetric, quasi-equilibrium end states. The existence of this branch of stable quasi-equilibria may have consequences for magnetar physics, including flare generation mechanisms and interpretations of quasi-periodic oscillations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

Self-similarity and optical kinks in resonant nonlinear media

We show that self-similar optical waves with a kink structure exist in a wide class of resonant nonlinear media, adequately treated in the two-level approximation. The self-similar structure of the present kinks is reflected in the time evolution of the field profile, atomic dipole moment, and one-atom inversion. We develop an analytical theory of such kinks. We show that the discovered kinks are accelerating nonlinear waves, asymptotically attaining their shape and the speed of light. We also numerically explore the formation and eventual disintegration of our kinks due to energy relaxation processes. Thus, the present kinks can be viewed as intermediate asymptotics of the system.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2010

Vortex lattices generated by the Kelvin-Helmholtz instability in the Gross-Pitaevskii equation

Vortex streets are formed from sheared initial conditions in classical fluids even without viscosity, which is called the Kelvin-Helmholtz instability. We demonstrate that similar vortex streets are generated from sheared initial conditions by the direct numerical simulation of the Gross-Pitaevskii (GP) equation which describes the dynamics of the Bose-Einstein condensates. Furthermore, we show the vortex-lattice formation from sheared initial conditions analogous to the rigid-body rotation in the GP equation under a rotating harmonic potential. The vortex-lattice formation by the dynamical instability in the system without energy dissipation differs from the vortex-lattice formation process by the imaginary time evolution of the GP equation where the lowest energy state is obtained.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Crossover between Kelvin-Helmholtz and counter-superflow instabilities in two-component Bose-Einstein condensates

Dynamical instabilities at the interface between two Bose-Einstein condensates that are moving relative to each other are investigated using mean-field and Bogoliubov analyses. Kelvin-Helmholtz instability is dominant when the interface thickness is much smaller than the wavelength of the unstable interface mode, whereas the counter-superflow instability becomes dominant in the opposite case. These instabilities emerge not only in an immiscible system but also in a miscible system where an interface is produced by external potential. Dynamics caused by these instabilities are numerically demonstrated in rotating trapped condensates.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Anderson localization of light near boundaries of disordered photonic lattices

We study numerically the effect of boundaries on Anderson localization of light in truncated two-dimensional photonic lattices in a nonlinear medium. We demonstrate suppression of Anderson localization at the edges and corners, so that stronger disorder is needed near the boundaries to obtain the same localization as in the bulk. We find that the level of suppression depends on the location in the lattice (edge vs corner), as well as on the strength of disorder. We also discuss the effect of nonlinearity on various regimes of Anderson localization.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Interface dynamics of a two-component Bose-Einstein condensate driven by an external force

The dynamics of an interface in a two-component Bose-Einstein condensate driven by a spatially uniform time-dependent force is studied. Starting from the Gross-Pitaevskii Lagrangian, the dispersion relation for linear waves and instabilities at the interface is derived by means of a variational approach. A number of diverse dynamical effects for different types of driving force is demonstrated, which includes the Rayleigh-Taylor instability for a constant force, the Richtmyer-Meshkov instability for a pulse force, dynamic stabilization of the Rayleigh-Taylor instability and onset of the parametric instability for an oscillating force. Gaussian Markovian and non-Markovian stochastic forces are also considered. It is found that the Markovian stochastic force does not produce any average effect on the dynamics of the interface, while the non-Markovian force leads to exponential perturbation growth.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Nuclear 'pasta phase' and its consequences on neutrino opacities

In this paper, we calculate the diffusion coefficients that are related to the neutrino opacities considering the formation of nuclear pasta and homogeneous matter at low densities. Our results show that the mean-free paths are significantly altered by the presence of nuclear pasta in stellar matter when compared with the results obtained with homogeneous matter. These differences in neutrino opacities certainly influence the Kelvin-Helmholtz phase of protoneutron stars and consequently the results of supernova explosion simulations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Effects of laser-plasma interactions on terahertz radiation from solid targets irradiated by ultrashort intense laser pulses

Interactions of 100-fs laser pulses with solid targets at intensities of 10{sup 18} W/cm{sup 2} and resultant terahertz (THz) radiation are studied under different laser contrast ratio conditions. THz emission is measured in the specular reflection direction, which appears to decrease as the laser contrast ratio varies from 10{sup -8} to 10{sup -6}. Correspondingly, the frequency spectra of the reflected light are observed changing from second harmonic dominant, three-halves harmonic dominant, to vanishing of both harmonics. Two-dimensional particle-in-cell simulation also suggests that this observation is correlated with the plasma density scale length change. The results demonstrate that the THz emission is closely related to the laser-plasma interaction processes. The emission is strong when resonance absorption is a key feature of the interaction, and becomes much weaker when parametric instabilities dominate.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Influence of Energetic Ions on Tearing Modes

In contrast with the stability effects of trapped energetic ions on tearing modes, the effects of circulating energetic ions (CEI) on tearing modes depend on the toroidal circulating direction, and are closely related to the momentum of energetic ions. CEI provide an additional source or sink of momentum to affect tearing modes. For co-CEI, tearing modes can be stabilized if the momentum of energetic ions is large enough. On the other hand, the growth of tearing modes can be enhanced by counter-CEI. Further, a possibility to suppress the island growth of neoclassical tearing modes by co-CEI is pointed out.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Direct Observation of Current in Type-I Edge-Localized-Mode Filaments on the ASDEX Upgrade Tokamak

Magnetically confined plasmas in the high confinement regime are regularly subjected to relaxation oscillations, termed edge localized modes (ELMs), leading to large transport events. Present ELM theories rely on a combined effect of edge current and the edge pressure gradients which result in intermediate mode number (n congruent with 10-15) structures (filaments) localized in the perpendicular plane and extended along the field lines. By detailed localized measurements of the magnetic field perturbation associated to type-I ELM filaments, it is shown that these filaments carry a substantial current.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Plasma Jet Braking: Energy Dissipation and Nonadiabatic Electrons

We report in situ observations by the Cluster spacecraft of wave-particle interactions in a magnetic flux pileup region created by a magnetic reconnection outflow jet in Earth's magnetotail. Two distinct regions of wave activity are identified: lower-hybrid drift waves at the front edge and whistler-mode waves inside the pileup region. The whistler-mode waves are locally generated by the electron temperature anisotropy, and provide evidence for ongoing betatron energization caused by magnetic flux pileup. The whistler-mode waves cause fast pitch-angle scattering of electrons and isotropization of the electron distribution, thus making the flow braking process nonadiabatic. The waves strongly affect the electron dynamics and thus play an important role in the energy conversion chain during plasma jet braking.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

First Observation of Edge Localized Modes Mitigation with Resonant and Nonresonant Magnetic Perturbations in ASDEX Upgrade

First experiments with nonaxisymmetric magnetic perturbations, toroidal mode number n=2, produced by newly installed in-vessel saddle coils in the ASDEX Upgrade tokamak show significant reduction of plasma energy loss and peak divertor power load associated with type-I edge localized modes (ELMs) in high-confinement mode plasmas. ELM mitigation is observed above an edge density threshold and is obtained both with magnetic perturbations that are resonant and not resonant with the edge safety factor profile. Compared with unperturbed type-I ELMy reference plasmas, plasmas with mitigated ELMs show similar confinement, similar plasma density, and lower tungsten impurity concentration.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗