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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 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

A quasi-linear analysis of the impurity effect on turbulent momentum transport and residual stress

We study the impact of impurities on turbulence driven intrinsic rotation (via residual stress) in the context of the quasi-linear theory. A two-fluid formulation for main and impurity ions is employed to study ion temperature gradient modes in sheared slab geometry modified by the presence of impurities. An effective form of the parallel Reynolds stress is derived in the center of mass frame of a coupled main ion-impurity system. Analyses show that the contents and the radial profile of impurities have a strong influence on the residual stress. In particular, an impurity profile aligned with that of main ions is shown to cause a considerable reduction of the residual stress, which may lead to the reduction of turbulence driven intrinsic rotation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

ELECTRON HEAT CONDUCTION IN THE SOLAR WIND: TRANSITION FROM SPITZER-HAeRM TO THE COLLISIONLESS LIMIT

We use a statistically significant set of measurements to show that the field-aligned electron heat flux q{sub Parallel-To} in the solar wind at 1 AU is consistent with the Spitzer-Haerm collisional heat flux q{sub sh} for temperature gradient scales larger than a few mean free paths L{sub T} {approx}> 3.5{lambda}{sub fp}. This represents about 65% of the measured data and corresponds primarily to high {beta}, weakly collisional plasma ({sup s}low solar wind{sup )}. In the more collisionless regime {lambda}{sub fp}/L{sub T} {approx}> 0.28, the electron heat flux is limited to q{sub Parallel-To }/q{sub 0} {approx} 0.3, independent of mean free path, where q{sub 0} is the ''free-streaming'' value; the measured q{sub Parallel-To} does not achieve the full q{sub 0}. This constraint q{sub Parallel-To }/q{sub 0} {approx} 0.3 might be attributed to wave-particle interactions, effects of an interplanetary electric potential, or inherent flux limitation. We also show a {beta}{sub e} dependence to these results that is consistent with a local radial electron temperature profile T{sub e} {approx} r {sup -{alpha}} that is a function of the thermal electron beta {alpha} = {alpha}({beta}{sub e}) and that the {beta} dependence of the collisionless regulation constraint is not obviously consistent with a whistler heat flux instability. It may be that the observed saturation of the measured heat flux is a simply a feature of collisional transport. We discuss the results in a broader astrophysical context.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Selective laser melting of Co-29Cr-6Mo alloy with laser power 180–360 W: Cellular growth, intercellular spacing and the related thermal condition

Highlights: • Increasing laser power increases the track size but not the track shape significantly. • Solidification mode and texture are similar over the range of laser power used. • Intercellular spacing is not affected by laser power but decreases slightly with growth angle. • Thermal gradient and cooling rate have been estimated to be ~ 2 × 10{sup 7} K/m and ~ 2 × 10{sup 6} K/s, respectively. - Abstract: A feature of solidification during selective laser melting (SLM) is cellular growth. Thus, how growth direction and cell size may be affected and possibly be controlled by SLM parameters are important to be understood. Furthermore, intercellular spacing (λ{sub 1}) is suggestive of the thermal condition during solidification which needs to be known for advancing the understanding of SLM. In this work, grain solidification orientation and λ{sub 1} of a Co-Cr-Mo alloy produced by SLM over a range of laser power (P = 180–360 W) with other parameters kept unchanged have been evaluated. It has been found that the overall preferred growth of cellular solidification is not significantly affected by P. This is because, over the range of P values used, the shape of track did not change sufficiently to result in a significant variation of growth direction during epitaxial growth. It has also been found that P does not affect λ{sub 1}. This suggests, since growth rate (R) relates to scan speed (v) which is unchanged, P does not significantly affect the temperature gradient of the solidification front (G). From the measured λ{sub 1} and estimated R, G and thus cooling rate ( T-dot ) can be estimated following the commonly used primary arm spacing model. The estimated G and T-dot which could not be directly measured will be compared with those predicted by simulation and the estimated T-dot will also be compared with those determined by using an emperical relationship from literature.

36 MATERIALS SCIENCE↗
DOE OSTI2015

RF wave propagation and scattering in turbulent tokamak plasmas

Drift wave turbulence driven by the steep electron and ion temperature gradients in H-mode divertor tokamaks produce scattering of the RF waves used for heating and current drive. The X-ray emission spectra produced by the fast electrons require the turbulence broaden RF wave spectrum. Both the 5 GHz Lower Hybrid waves and the 170 GHz electron cyclotron [EC] RF waves experience scattering and diffraction by the electron density fluctuations. With strong LHCD there are bifurcations in the coupled turbulent transport dynamics giving improved steady-state confinement states. The stochastic scattering of the RF rays makes the prediction of the distribution of the rays and the associated particle heating a statistical problem. Thus, we introduce a Fokker-Planck equation for the probably density of the RF rays. The general frame work of the coupled system of coupled high frequency current driving rays with the low-frequency turbulent transport determines the profiles of the plasma density and temperatures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Collisionless microinstabilities in stellarators. II. Numerical simulations

Microinstabilities exhibit a rich variety of behavior in stellarators due to the many degrees of freedom in the magnetic geometry. It has recently been found that certain stellarators (quasi-isodynamic ones with maximum-J geometry) are partly resilient to trapped-particle instabilities, because fast-bouncing particles tend to extract energy from these modes near marginal stability. In reality, stellarators are never perfectly quasi-isodynamic, and the question thus arises whether they still benefit from enhanced stability. Here, the stability properties of Wendelstein 7-X and a more quasi-isodynamic configuration, QIPC, are investigated numerically and compared with the National Compact Stellarator Experiment and the DIII-D tokamak. In gyrokinetic simulations, performed with the gyrokinetic code GENE in the electrostatic and collisionless approximation, ion-temperature-gradient modes, trapped-electron modes, and mixed-type instabilities are studied. Wendelstein 7-X and QIPC exhibit significantly reduced growth rates for all simulations that include kinetic electrons, and the latter are indeed found to be stabilizing in the energy budget. These results suggest that imperfectly optimized stellarators can retain most of the stabilizing properties predicted for perfect maximum-J configurations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

The Biermann catastrophe in numerical magnetohydrodynamics

The Biermann battery effect is frequently invoked in cosmic magnetogenesis and studied in high-energy density laboratory physics experiments. Generation of magnetic fields by the Biermann effect due to misaligned density and temperature gradients in smooth flow behind shocks is well known. We show that a Biermann-effect magnetic field is also generated within shocks. Direct implementation of the Biermann effect in MHD codes does not capture this physical process, and worse, it produces unphysical magnetic fields at shocks whose value does not converge with resolution. We show that this convergence breakdown is due to naive discretization, which fails to account for the fact that discretized irrotational vector fields have spurious solenoidal components that grow without bound near a discontinuity. We show that careful consideration of the kinetics of ion viscous shocks leads to a formulation of the Biermann effect that gives rise to a convergent algorithm. We note two novel physical effects: a resistive magnetic precursor, in which a Biermann-generated field in the shock “leaks” resistively upstream, and a thermal magnetic precursor, in which a field is generated by the Biermann effect ahead of the shock front owing to gradients created by the shock’s electron thermal conduction precursor. Both effects appear to be potentially observable in experiments at laser facilities. We reexamine published studies of magnetogenesis in galaxy cluster formation and conclude that the simulations in question had inadequate resolution to reliably estimate the field generation rate. Corrected estimates suggest primordial field values in the range B∼10{sup −22}–10{sup −19} G by z = 3.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Electromagnetic gyrokinetic turbulence in finite-beta helical plasmas

A saturation mechanism for microturbulence in a regime of weak zonal flow generation is investigated by means of electromagnetic gyrokinetic simulations. The study identifies a new saturation process of the kinetic ballooning mode (KBM) turbulence originating from the spatial structure of the KBM instabilities in a finite-beta Large Helical Device (LHD) plasma. Specifically, the most unstable KBM in LHD has an inclined mode structure with respect to the mid-plane of a torus, i.e., it has a finite radial wave-number in flux tube coordinates, in contrast to KBMs in tokamaks as well as ion-temperature gradient modes in tokamaks and helical systems. The simulations reveal that the growth of KBMs in LHD is saturated by nonlinear interactions of oppositely inclined convection cells through mutual shearing as well as by the zonal flow. The saturation mechanism is quantitatively investigated by analysis of the nonlinear entropy transfer that shows not only the mutual shearing but also a self-interaction with an elongated mode structure along the magnetic field line.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Feasibility of bit patterned media for HAMR at 5 Tb/in{sup 2}

We have investigated the feasibility of BPM for HAMR via Finite Difference Time Domain and atomistic simulation and we have substantiated the feasibility of 5 Tb/in{sup 2} with two filling factors 25% and 56% even when the maximum on-track bit temperature is below the Curie temperature. The success of this underheated switching is attributed to sufficiently low anisotropy instead of reduction of Curie temperature. The temperature gradient in the cross-track direction is almost doubled if the optical head width is reduced by half, indicating the possibility of higher areal densities. Moreover, contrary to continuous media, we also found that the power absorption peaks at the bottom of the bit patterned FePt when the media is illuminated from above, which is probably due to stronger coupling there between FePt and the surrounding materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Multi-channel transport experiments at Alcator C-Mod and comparison with gyrokinetic simulations

Multi-channel transport experiments have been conducted in auxiliary heated (Ion Cyclotron Range of Frequencies) L-mode plasmas at Alcator C-Mod [Marmar and Alcator C-Mod Group, Fusion Sci. Technol. 51(3), 3261 (2007)]. These plasmas provide good diagnostic coverage for measurements of kinetic profiles, impurity transport, and turbulence (electron temperature and density fluctuations). In the experiments, a steady sawtoothing L-mode plasma with 1.2 MW of on-axis RF heating is established and density is scanned by 20%. Measured rotation profiles change from peaked to hollow in shape as density is increased, but electron density and impurity profiles remain peaked. Ion or electron heat fluxes from the two plasmas are the same. The experimental results are compared directly to nonlinear gyrokinetic theory using synthetic diagnostics and the code GYRO [Candy and Waltz, J. Comput. Phys. 186, 545 (2003)]. We find good agreement with experimental ion heat flux, impurity particle transport, and trends in the fluctuation level ratio (T(tilde sign){sub e}/T{sub e})/(ñ{sub e}/n{sub e}), but underprediction of electron heat flux. We find that changes in momentum transport (rotation profiles changing from peaked to hollow) do not correlate with changes in particle transport, and also do not correlate with changes in linear mode dominance, e.g., Ion Temperature Gradient versus Trapped Electron Mode. The new C-Mod results suggest that the drives for momentum transport differ from drives for heat and particle transport. The experimental results are inconsistent with present quasilinear models, and the strong sensitivity of core rotation to density remains unexplained.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Thermal energy and charge currents in multi-terminal nanorings

We study in experiment and theory thermal energy and charge transfer close to the quantum limit in a ballistic nanodevice, consisting of multiply connected one-dimensional electron waveguides. The fabricated device is based on an AlGaAs/GaAs heterostructure and is covered by a global top-gate to steer the thermal energy and charge transfer in the presence of a temperature gradient, which is established by a heating current. The estimate of the heat transfer by means of thermal noise measurements shows the device acting as a switch for charge and thermal energy transfer. The wave-packet simulations are based on the multi-terminal Landauer-Büttiker approach and confirm the experimental finding of a mode-dependent redistribution of the thermal energy current, if a scatterer breaks the device symmetry.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2012

CONSEQUENCES OF MAGNETIC FIELD STRUCTURE FOR HEAT TRANSPORT IN MAGNETOHYDRODYNAMICS

Interfaces between hot and cold magnetized plasmas exist in various astrophysical contexts, for example, where hot outflows impinge on an ambient interstellar medium. It is of interest to understand how the structure of the magnetic field spanning the interface affects the temporal evolution of the temperature gradient. Here, we explore the relation between the magnetic field topology and the heat transfer rate by adding various fractions of tangled versus ordered field across a hot-cold interface that allows the system to evolve to a steady state. We find a simple mathematical relation for the rate of heat conduction as a function of the initial ratio of ordered-to-tangled field across the interface. We discuss potential implications for the astrophysical context of magnetized wind blown bubbles around evolved stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Formation of Vortex Flows in Liquid Crystal Phases Encapsulated in Microliter Volumes under the Action of Focused Laser Radiation

A theoretical description of the process of formation of vortex flows v(t, r) and the evolution of the director field n̂ in microliter liquid crystal (LC) volumes with a free surface under the influence of a temperature gradient ∇T(t, r), which is initiated by focused laser radiation, has been proposed. Thermomechanical contributions to both the stress tensor and viscous moment which are acting per unit volume of the LC phase were taken into account in the framework of the nonlinear generalization of the classical Ericksen−Leslie theory, which allowed describing the origin and formation of vortex flows in nematics formed by 4-n-pentyl- 4'-cyanobiphenyl molecules. Various hydrodynamic modes of vortex formation in microsized LC volumes under the action of focused laser radiation have been investigated by numerical methods.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

A New Generation of Standard Solar Models

We compute a new generation of standard solar models (SSMs) that includes recent updates on some important nuclear reaction rates and a more consistent treatment of the equation of state. Models also include a novel and flexible treatment of opacity uncertainties based on opacity kernels, required in light of recent theoretical and experimental works on radiative opacity. Two large sets of SSMs, each based on a different canonical set of solar abundances with high and low metallicity (Z), are computed to determine model uncertainties and correlations among different observables. We present detailed comparisons of high- and low-Z models against different ensembles of solar observables, including solar neutrinos, surface helium abundance, depth of the convective envelope, and sound speed profile. A global comparison, including all observables, yields a p-value of 2.7σ for the high-Z model and 4.7σ for the low-Z one. When the sound speed differences in the narrow region of 0.65<r/R{sub ⊙}<0.70 are excluded from the analysis, results are 0.9σ and 3.0σ for high- and low-Z models respectively. These results show that high-Z models agree well with solar data but have a systematic problem right below the bottom of the convective envelope linked to steepness of molecular weight and temperature gradients, and that low-Z models lead to a much more general disagreement with solar data. We also show that, while simple parametrizations of opacity uncertainties can strongly alleviate the solar abundance problem, they are insufficient to substantially improve the agreement of SSMs with helioseismic data beyond that obtained for high-Z models due to the intrinsic correlations of theoretical predictions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Properties of high-β microturbulence and the non-zonal transition

The physics underlying the non-zonal transition [M. J. Pueschel et al., Phys. Rev. Lett. 110, 155005 (2013)] are explored in detail, and various studies are presented which support the theory that critically weakened zonal flows are indeed responsible for the failure of ion-temperature-gradient-driven turbulence at high plasma β to saturate at typical transport values. Regarding flux-surface-breaking magnetic fluctuations and their impact on zonal flows, numerical approaches to obtaining zonal flow residuals are elaborated on, and simulation results are shown to agree with analytical predictions, corroborating the interpretation that flux-surface-breaking magnetic fluctuations cause the transition. Consistently, the zonal-flows-related energetics of the turbulence are found to change fundamentally when exceeding the threshold.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Response of microscale turbulence and transport to the evolution of resistive magnetohydrodynamic magnetic island

Nonlinear evolution of microscale turbulence interacting with a naturally growing MHD magnetic island is simulated based on a Landau-fluid model. Here, we report on a new short wavelength magnetic-island-induced ion temperature gradient (ITG) instability triggered by a critical threshold of magnetic island width in multiscale turbulence, which is referred to as sw-MITG mode. The sw-MITG mode is characterized by a substantially low stability threshold and a global structure propagating along the ion diamagnetic drift direction. Its generation results from the response of microscale fluctuations to turbulent cross-field heat transport associated with increasing boundary layer width about the island separatrix. An intermittency of heat transport is caused by the sw-MITG mode interacting with dynamical magnetic island and microturbulence.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Multiscale Modeling of Microstructural Evolution in Fused-Coating Additive Manufacturing

A quantitative phase field model with adaptive-computational-domain-expansion (ACDE) method was developed to simulate the microstructural evolution in fused-coating additive manufacturing process. A heat transfer and fluid flow model was established to investigate the solidification behavior of the molten pool in a single deposited layer. The solidification parameters such as temperature gradient and solidification rate were calculated from the macroscopic model and then fed into the phase field model to get the microstructure at different positions within a single deposited layer. Columnar dendrites were formed at the bottom of the deposited layer, and equiaxed dendrites appeared in the upper part of the layer. The ACDE method is used to improve the simulation efficiency. The metallographic experiments and energy-dispersive spectrometer analysis for composition were conducted on the fused-coating specimen, and the results were in good accordance with the simulations.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Photothermally controlled Marangoni flow around a micro bubble

We have experimentally investigated the control of Marangoni flow around a micro bubble using photothermal conversion. Using a focused laser spot acting as a highly localized heat source on Au nanoparticles/dielectric/Ag mirror thin film enables us to create a micro bubble and to control the temperature gradient around the bubble at a micrometer scale. When we irradiate the laser next to the bubble, a strong main flow towards the bubble and two symmetric rotation flows on either side of it develop. The shape of this rotation flow shows a significant transformation depending on the relative position of the bubble and the laser spot. Using this controllable rotation flow, we have demonstrated sorting of the polystyrene spheres with diameters of 2 μm and 0.75 μm according to their size.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗