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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 1,243 records · Page 69
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Characterization of carrier transport properties in strained crystalline Si wall-like structures in the quasi-quantum regime

We report the transport characteristics of both electrons and holes through narrow constricted crystalline Si “wall-like” long-channels that were surrounded by a thermally grown SiO{sub 2} layer. The strained buffering depth inside the Si region (due to Si/SiO{sub 2} interfacial lattice mismatch) is where scattering is seen to enhance some modes of the carrier-lattice interaction, while suppressing others, thereby changing the relative value of the effective masses of both electrons and holes, as compared to bulk Si. In the narrowest wall devices, a considerable increase in conductivity was observed as a result of higher carrier mobilities due to lateral constriction and strain. The strain effects, which include the reversal splitting of light- and heavy-hole bands as well as the decrease of conduction-band effective mass by reduced Si bandgap energy, are formulated in our microscopic model for explaining the experimentally observed enhancements in both conduction- and valence-band mobilities with reduced Si wall thickness. Also, the enhancements of the valence-band and conduction-band mobilities are found to be associated with different aspects of theoretical model.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Increased magnetic damping of a single domain wall and adjacent magnetic domains detected by spin torque diode in a nanostripe

Spin torque resonance has been used to simultaneously probe the dynamics of a magnetic domain wall and of magnetic domains in a nanostripe magnetic tunnel junction. Due to the large associated resistance variations, we are able to analyze quantitatively the resonant properties of these single nanoscale magnetic objects. In particular, we find that the magnetic damping of both the domains and the domain wall is doubled compared to the damping value of the host magnetic layer. We estimate the contributions to the damping arising from the dipolar couplings between the different layers in the junction and from the intralayer spin pumping effect, and find that they cannot explain the large damping enhancement that we observe. We conclude that the measured increased damping is intrinsic to large amplitudes excitations of spatially localized modes or solitons such as vibrating or propagating domain walls.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Numerical validation of axial plasma momentum lost to a lateral wall induced by neutral depletion

Momentum imparted to a lateral wall of a compact inductively coupled plasma thruster is numerically investigated for argon and xenon gases by a particle-in-cell simulation with Monte Carlo collisions (PIC-MCC). Axial plasma momentum lost to a lateral wall is clearly shown when axial depletion of the neutrals is enhanced, which is in qualitative agreement with the result in a recent experiment using a helicon plasma source [Takahashi et al., Phys. Rev. Lett. 114, 195001 (2015)]. The PIC-MCC calculations demonstrate that the neutral depletion causes an axially asymmetric profile of the plasma density and potential, leading to axial ion acceleration and the non-negligible net axial force exerted to the lateral wall in the opposite direction of the thrust.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Dynamical effect of confining wall on diffusion of fluid at nanoscale

The dynamical effect of wall-fluid interaction on diffusion in direction perpendicular to confining walls has been studied, theoretically. The properties of fluids which incorporates the effect of static changes in confined fluid has been modified to include, directly, the dynamical effects of wall on the motion of particles. The results obtained for VACF in perpendicular direction of channel of width of two atomic diameters are presented and compared with the molecular dynamic (MD) simulation results as well as the results obtained without including the dynamical effect.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Origin of a Raman scattering peak generated in single-walled carbon nanotubes by X-ray irradiation and subsequent thermal annealing

We have found that a Raman scattering (RS) peak around 1870 cm{sup −1} was produced by the annealing of the X-ray irradiated film of single-walled carbon nanotubes (SWNTs) at 450 {sup o}C. The intensity of 1870-cm{sup −1} peak showed a maximum at the probe energy of 2.3 eV for the RS spectroscopy with various probe lasers. Both the peak position and the probe-energy dependence were almost identical to those of the one-dimensional carbon chains previously reported in multi-walled carbon nanotubes. Consequently, we concluded that the 1870-cm{sup −1} peak found in the present study is attributed to carbon chains. The formation of carbon chains by the annealing at temperature lower than 500 {sup o}C is firstly reported by the present study. The carbon chains would be formed by aggregation of the interstitial carbons, which are formed as a counterpart of carbon vacancies by X-ray irradiation diffused on SWNT walls. The result indicates that the combination of X-ray irradiation and subsequent thermal annealing is a feasible tool for generating new nanostructures in SWNT.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2017

Role of flexoelectric coupling in polarization rotations at the a-c domain walls in ferroelectric perovskites

Ferroelectric and ferroelastic domain walls play important roles in ferroelectric properties. However, their couplings with flexoelectricity have been less understood. Here, we applied phase-field simulation to investigate the flexoelectric coupling with ferroelectric a/c twin structures in lead ziconate titanate thin films. Local stress gradients were found to exist near twin walls that created both lateral and vertical electric fields through the flexoelectric effect, resulting in polarization inclinations from either horizontal or normal orientation, polarization rotation angles deviated from 90°, and consequently highly asymmetric a/c twin walls. Furthermore, by tuning the flexoelectric strengths in a reasonable range from first-principles calculations, we found that the transverse flexoelectric coefficient has a larger influence on the polarization rotation than longitudinal and shear coefficients. And as polar rotations that commonly occur at compositional morphotropic phase boundaries contribute to the piezoelectric enhancement, this work calls for further exploration of alternative strain-engineered polar rotations via flexoelectricity in ferroelectric thin films.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Electrostatic potential mapping at ferroelectric domain walls by low-temperature photoemission electron microscopy

Low-temperature X-ray photoemission electron microscopy (X-PEEM) is used to measure the electric potential at domain walls in improper ferroelectric Er 0.99 Ca 0.01 MnO 3 . Here, by combining X-PEEM with scanning probe microscopy and theory, we develop a model that relates the detected X-PEEM contrast to the emergence of uncompensated bound charges, explaining the image formation based on intrinsic electronic domain-wall properties. In contrast to previously applied low-temperature electrostatic force microscopy (EFM), X-PEEM readily distinguishes between positive and negative bound charges at domain walls. Finally, our study introduces an X-PEEM-based approach for low-temperature electrostatic potential mapping, facilitating nanoscale spatial resolution and data acquisition times on the order of 0.1-1 s.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of reflections on 2D tomographic reconstructions of filtered cameras and on interpreting spectroscopic measurements in the JET ITER-like wall divertor

Considering reflections from metallic wall surfaces in generation of tomographic reconstructions of the tangentially viewing, visible-range spectroscopic divertor cameras in JET has been observed to yield enhanced spatial accuracy and significant reduction of emission artifacts in experimentally resolved 2D line emission distributions. Neglection of reflections in the tomography process was found to lead to overestimation of the emission near the wall surfaces by up to a factor of 4, as well as to formation of bright emission artifacts between the main emission regions and the wall surfaces, comprising locally up to 50% of the emission. Mimicking divertor spectroscopy measurements by integrating the tomographic reconstructions along vertical lines-of-sight implies that reflections comprise 15%–25% of the observed line-integrated emission peaks. Additionally, the spatial differences in the reflection contribution between the different lines-of-sight are less pronounced than in the 2D reconstructions due to the dominance of the brightest emission regions through which the spectroscopic lines-of-sight pass. However, postprocessing EDGE2D-EIRENE simulations using the CHERAB code and synthetic spectroscopy suggests a decrease of the spectroscopically inferred divertor electron temperature by up to 75%, when redistribution of the observed light due to reflections is considered.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Time-dependent modeling of coupled plasma-wall dynamics

Effects of dynamic wall outgassing on divertor plasma during an ELM-like heat pulse are modelled using newly coupled edge plasma code UEDGE and all reaction-diffusion code FACE. Different divertor regimes are simulated depending on the degree of the plasma detachment. It is shown that hydrogen outgassing from the divertor target plate induced by ELM pulse in the semi-detached regime can trigger deep plasma detachment. The induced detachment does not spontaneously reverse after the pulse due to ineffective implantation of low energy plasma hydrogen in the target material. Furthermore, we demonstrate that plasma neutrals play essential role in heat transport in a detached divertor. The modelled hydrogen dynamics in the wall material demonstrates that the heat pulse leads to enhanced hydrogen transport toward the wall bulk.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Cellulose synthase complexes act in a concerted fashion to synthesize highly aggregated cellulose in secondary cell walls of plants

Significance Plant cell walls are important in plant development and for textiles, wood products, and bioenergy. Cellulose, the microfibrillar component of primary cell walls (PCWs) and secondary cell walls (SCWs), is formed by cellulose synthase complexes (CSCs) at the plasma membrane. Here, we show that CSCs behave differently during PCW and SCW synthesis and form microfibrils with different organization. During PCW synthesis, dispersed CSCs synthesize cellulose microfibrils with low aggregation, whereas during SCW synthesis, densely arranged groups of CSCs move coherently to synthesize highly aggregated microfibrils. Our study suggests that controlled alterations in CSC distribution and orchestrated movements contribute to the high density and bundling of cellulose microfibrils in SCWs.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2025

Simulations of Heat Transfer Using Tight-Fitting Twisted Tape Inserts for First Wall Cooling in Molten Salt Breeder Blankets

One of the major components in fusion energy systems is the fusion blanket, which has a vacuum vessel to contain the plasma. As part of the fusion blanket/vacuum vessel, the first wall and plasma-facing components require sufficient cooling to prevent material degradation during operation from the superheated plasma. Most fusion blanket concepts involve first wall and divertor coolant channels with heat transfer enhancements (HTEs) that are intended to withstand the incident high heat fluxes of 1 to 5 MW/m 2 . Twisted tape inserts are a proposed HTE that have been investigated previously for first wall cooling and monoblock divertor cooling channels and in other nonfusion heat transfer components. By inserting twisted tapes into straight pipes, the amount of turbulence in the system can be increased at lower Reynolds numbers by swirling the flow. This results in better heat transfer characteristics with marginal increases in frictional pressure losses. In particular, simulations of high-Prandtl-number fluids such as the proposed molten salt FLiBe in twisted tapes, which is prototypic to liquid immersion blankets, have not been previously explored. Here, in this study, we simulate various Prandtl numbers in pipes with twisted tape inserts using large eddy simulations to determine the effects of increasing Prandtl numbers on heat transfer performance. The quantities of particular interest are the Nusselt number and the friction factor, which were recovered using data reduction techniques to determine impacts on heat transfer and pressure losses. This work serves as a starting point for determining the feasibility of twisted tape inserts for liquid immersion blanket concepts.

LES↗
DOE OSTI2015

STRUCTURAL CHANGES OF THE SUBLIMATION WALL IN PROTOPLANETARY DISKS DUE TO VARYING ACCRETION ILLUMINATION: A MECHANISM FOR RAPID INFRARED VARIABILITY

We study the changes in the sublimation wall structure due to variable illumination of a stellar hot spot on the dusty surroundings of a young star. The model includes the settling of large grains toward the disk midplane and the effect of the vertical density profile on the shaping of the sublimation wall. From a survey of objects in the young cluster IC 348, we extract three objects (LRLL 32, 40, and 63) that present typical variability in the [3.6] and [4.5] IRAC bands. We use the Spitzer photometry and ground-based 2–5 μm spectra for comparison with the models. Even though there is a correlation between accretion luminosity and dust emission based on the observations, we conclude from the modeling that the systems with lower mass accretion rates (LRLL 32 and 63) cannot be explained simply by a variable hot spot illuminating a sublimation wall. The observed variability amplitude for LRLL 40 (the system with the largest value of the mass accretion rate) can be obtained using the mechanism presented here. When considering a wide range of hot spot sizes and temperatures, the models can reproduce the infrared fluctuations seen in recent surveys, but only with accretion rate fluctuations that are orders of magnitude larger than is typically observed. These results highlight the relevance of accretion as a variability mechanism as well as its limitations in producing the full extent of the observed infrared variability.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Anomalous feedback and negative domain wall resistance

Magnetic induction can be regarded as a negative feedback effect, where the motive-force opposes the change of magnetic flux that generates the motive-force. In artificial electromagnetics emerging from spintronics, however, this is not necessarily the case. In this paper, by studying the current-induced domain wall dynamics in a cylindrical nanowire, we show that the spin motive-force exerting on electrons can either oppose or support the applied current that drives the domain wall. The switching into the anomalous feedback regime occurs when the strength of the dissipative torque β is about twice the value of the Gilbert damping constant α. Finally, the anomalous feedback manifests as a negative domain wall resistance, which has an analogy with the water turbine.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Dominant two-fluid magnetohydrodynamic instabilities in CFETR upgrade phase-I scenario in presence of perfect conducting wall

Here, the dominant two-fluid magnetohydrodynamic (MHD) stabilities of China Fusion Engineering Test Reactor upgrade phase-I baseline equilibria in presence of perfect conducting wall have been evaluated using the initial value code NIMROD. Our MHD simulations show that for equilibrium with q axis < 1, high-n edge localized modes are slightly stabilized by toroidal shear flow, and n = 1 mode is found to be unstable in the core region with the characteristic of sawtooth instability. For q axis > 2 equilibrium, all modes are found to be unstable but edge localized both in single-fluid and two-fluid MHD models. Specifically for n ≤ 13 modes, the two-fluid MHD model gives a slightly higher growth rate than the single-fluid MHD model, while for n > 13 modes, this trend becomes the opposite, which means the two-fluid MHD model is needed for high-n mode analysis. In addition, n = 1–10 modes are found to be more unstable with increasing wall position and eventually their growth rates approach values in the no-wall limit.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Noninvasive THz-TDS measurements of plasma bounded and optically shielded by Hall thruster wall material

We experimentally demonstrate the capability of terahertz time-domain spectroscopy (THz-TDS) to noninvasively measure the electron density and collision frequency of plasma bounded and optically shielded by Hall thruster wall material. This paper augments the standard THz-TDS plasma diagnostic theory to account for plasma boundaries, presents THz optical property measurements of three different wall materials (grades M, M26, and HP boron nitride composite), and provides electron density and collision frequency measurements of an inductively coupled plasma bounded and optically shielded by each wall material. We find that the electron density measurement capability is weakly impacted by the boundaries, whereas the electron collision frequency measurement capability is strongly reduced by the boundaries. Here, the bounded plasma electron density trends deviate substantially from those of the unbounded plasma.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Domain walls and gravitational waves in the Standard Model

We study domain walls which can be created in the Standard Model under the assumption that it is valid up to very high energy scales. We focus on domain walls interpolating between the physical electroweak vacuum and the global minimum appearing at very high field strengths. The creation of the network which ends up in the electroweak vacuum percolating through the Universe is not as difficult to obtain as one may expect, although it requires certain tuning of initial conditions. Our numerical simulations confirm that such domain walls would swiftly decay and thus cannot dominate the Universe. We discuss the possibility of detection of gravitational waves produced in this scenario. We have found that for the standard cosmology the energy density of these gravitational waves is too small to be observed in present and planned detectors.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Direct gyrokinetic comparison of pedestal transport in JET with carbon and ITER-like walls

Our work compares the gyrokinetic instabilities and transport in two representative JET pedestals, one (pulse 78697) from the JET configuration with a carbon wall (C) and another (pulse 92432) from after the installation of JET's ITER-like Wall (ILW). The discharges were selected for a comparison of JET-ILW and JET-C discharges with good confinement at high current (3 MA, corresponding also to low ρ * ) and retain the distinguishing features of JET-C and JET-ILW, notably, decreased pedestal top temperature for JET-ILW. A comparison of the profiles and heating power reveals a stark qualitative difference between the discharges: the JET-ILW pulse (92432) requires twice the heating power, at a gas rate of 1.9 × 10 22 e s -1 , to sustain roughly half the temperature gradient of the JET-C pulse (78697), operated at zero gas rate. This points to heat transport as a central component of the dynamics limiting the JET-ILW pedestal and reinforces the following emerging JET-ILW pedestal transport paradigm, which is proposed for further examination by both theory and experiment. ILW conditions modify the density pedestal in ways that decrease the normalized pedestal density gradient a/L n , normally via an outward shift in relation to the temperature pedestal. This is attributable to some combination of direct metal wall effects and the need for increased fueling to mitigate tungsten contamination. The modification to the density profile increases η=L n /L γ , thereby producing more robust ion temperature gradient (ITG) and electron temperature gradient driven instability. The decreased pedestal gradients for JET-ILW (92432) also result in a strongly reduced E × B shear rate, further enhancing the ion scale turbulence. Collectively, these effects limit the pedestal temperature and demand more heating power to achieve good pedestal performance. Our simulations, consistent with basic theoretical arguments, find higher ITG turbulence, stronger stiffness, and higher pedestal transport in the ILW plasma at lower ρ * .

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Amorphization and siliconization of silicon carbide as a first wall material

The understanding and prediction of silicon carbide (SiC) material evolution exposed to SOL plasma conditions is of prime interest because SiC represents a promising main chamber wall plasma-facing material for next-step fusion devices (low hydrogenic diffusion, good mechanical and thermal properties under neutron irradiation). Gross and net Si erosion rates from SiC surfaces in contact with a well-diagnosed L-mode plasma in the DIII-D tokamak have been simulated and the surface concentrations of impurities have been tracked as a function of time. Coupled simulation of surface model and impurity transport demonstrates amorphization of crystalline SiC exposed to L-mode plasma due to the accumulation of displacement damages under ion irradiation. This affects the lifetime of SiC plasma facing components. Surface evolution is tightly coupled to impurity transport in the plasma and therefore needs to be integrated with impurity transport simulations to effectively predict Si erosion rates and sub-surface concentrations as a function of time. The simulation workflow couples a semi-analytical surface model to the impurity transport code GITR. The surface model is a homogeneous mixed-material model that tracks physical & chemical sputtering and reflection of impurities. Gross erosion is primarily influenced by the background plasma parameters and redeposition patterns are mainly influenced by the prompt redeposition due to the gyro-orbits of impurity ions. Although crystalline form of SiC is preferable for fusion wall applications because of resistance to neutron irradiation, this work indicates that crystalline SiC will undergo amorphization under D plasma contact with implications of higher sputtering and fuel retention. These results direct us to explore the effects of amorphization on crystalline SiC and further the physics basis of SiC usage as first wall material for fusion environments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗