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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Impact of ideal MHD stability limits on high-beta hybrid operation

Here, the hybrid scenario is a candidate for stationary high-fusion gain tokamak operation in ITER and DEMO. To obtain such performance, the energy confinement and the normalized pressure ${{\beta}_{N}}$ must be maximized, which requires operating near or above ideal MHD no-wall limits. New experimental findings show how these limits can affect hybrid operation. Even if hybrids are mainly limited by tearing modes, proximity to the no-wall limit leads to 3D field amplification that affects plasma profiles, e.g. rotation braking is observed in ASDEX Upgrade throughout the plasma and peaks in the core. As a result, even the small ASDEX Upgrade error fields are amplified and their effects become visible. To quantify such effects, ASDEX Upgrade measured the response to 3D fields applied by $8\times 2$ non-axisymmetric coils as ${{\beta}_{N}}$ approaches the no-wall limit. The full n = 1 response profile and poloidal structure were measured by a suite of diagnostics and compared with linear MHD simulations, revealing a characteristic feature of hybrids: the n = 1 response is due to a global, marginally-stable n = 1 kink characterized by a large m = 1, n = 1 core harmonic due to q min being just above 1. A helical core distortion of a few cm forms and affects various core quantities, including plasma rotation, electron and ion temperature, and intrinsic W density. In similar experiments, DIII-D also measured the effect of this helical core on the internal current profile, providing information useful to understanding of the physics of magnetic flux pumping, i.e. anomalous current redistribution by MHD modes that keeps ${{q}_{\text{min}}}>1$ . Thanks to flux pumping, a broad current profile is maintained in DIII-D even with large on-axis current drive, enabling fully non-inductive operation at high ${{\beta}_{N}}$ up to 3.5–4.

3D equilibrium↗
DOE OSTI2022

Soil moisture thresholds explain a shift from light-limited to water-limited sap velocity in the Central Amazon during the 2015–16 El Niño drought

Transpiration is often considered to be light- but not water-limited in humid tropical rainforests due to abundant soil water, even during the dry seasons. The record-breaking 2015–16 El Niño drought provided a unique opportunity to examine whether transpiration is constrained by water under severe lack of rainfall. We measured sap velocity, soil water content, and meteorological variables in an old-growth upland forest in the Central Amazon throughout the 2015–16 drought. We found a rapid decline in sap velocity (-38 ± 21%, mean ± SD.) and in its temporal variability (-88%) during the drought compared to the wet season. Such changes were accompanied by a marked decline in soil moisture and an increase in temperature and vapor pressure deficit. Sap velocity was largely limited by net radiation during the wet and normal dry seasons; however, it shifted to be primarily limited by soil moisture during the drought. The threshold in which sap velocity became dominated by soil moisture was at 0.33 m 3 m -3 (around -150 kPa in soil matric potential), below which sap velocity dropped steeply. Our study provides evidence for a soil water threshold on transpiration in a moist tropical forest, suggesting a shift from light limitation to water limitation under future climate characterized by increased temperature and an increased frequency, intensity, duration and extent of extreme drought events.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2015

Global limits and interference patterns in dark matter direct detection

We compare the general effective theory of one-body dark matter nucleon interactions to current direct detection experiments in a global multidimensional statistical analysis. We derive exclusion limits on the 28 isoscalar and isovector coupling constants of the theory, and show that current data place interesting constraints on dark matter-nucleon interaction operators usually neglected in this context. We characterize the interference patterns that can arise in dark matter direct detection from pairs of dark matter-nucleon interaction operators, or from isoscalar and isovector components of the same operator. We find that commonly neglected destructive interference effects weaken standard direct detection exclusion limits by up to one order of magnitude in the coupling constants.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Enhanced Large-Signal Stability Method for Grid-Forming Inverters During Current Limiting: Preprint

Grid-forming (GFM) inverters are a promising technology for the widespread integration of renewable energy sources in future power systems. As a key element of GFM inverter control, the primary controller governs the internal reference voltage and angle. During contingencies in the grid---such as faults, voltage drops, or frequency and phase jumps---an inverter can be forced into a current-limiting mode of operation modulating inverter dynamics, and, as a result, it is prone to losing synchronism with the grid. In this paper, we propose a novel GFM primary control method with an additional synchronization term that naturally activates during contingencies to improve the dynamic response. The method allows the inverter to remain synchronized with the grid, which improves the inverter's dynamic behavior both during and after current-limiting grid conditions and enhances grid support, including voltage support using full current capacity. The method is demonstrated for voltage, frequency, and phase jumps both in a single-machine-to-infinite-bus and a network-wide electromagnetic transient simulation of the IEEE 14-bus system with 5 GFM inverters. The simulations provide insights into the proposed synchronization method and confirm the high potential of the method, which robustly secures synchronism under severe contingencies.

current limiting↗
NASA NTRS2000

Force limit specifications vs. design limit loads in vibration testing

The purpose of the work presented herein is to discuss the results of force limit notching during vibration testing with respect to the traditional limit load design criteria. By using a single-degree-of-freedom (SDOF) system approach, this work shows that with an appropriate force specification the notched response due to force limiting will result in loads comparable with the structural design limit criteria.

force limiting vibration testing flight limit load↗
DOE OSTI2014

Evaluation of Cloud-resolving and Limited Area Model Intercomparison Simulations using TWP-ICE Observations. Part 2: Rain Microphysics

Ten 3D cloud-resolving model (CRM) simulations and four 3D limited area model (LAM) simulations of an intense mesoscale convective system observed on January 23-24, 2006 during the Tropical Warm Pool – International Cloud Experiment (TWP-ICE) are compared with each other and with observations and retrievals from a scanning polarimetric radar, co-located UHF and VHF vertical profilers, and a Joss-Waldvogel disdrometer in an attempt to explain published results showing a low bias in simulated stratiform rainfall. Despite different forcing methodologies, similar precipitation microphysics errors appear in CRMs and LAMs with differences that depend on the details of the bulk microphysics scheme used. One-moment schemes produce too many small raindrops, which biases Doppler velocities low, but produces rain water contents (RWCs) that are similar to observed. Two-moment rain schemes with a gamma shape parameter (μ) of 0 produce excessive size sorting, which leads to larger Doppler velocities than those produced in one-moment schemes, but lower RWCs than observed. Two moment schemes also produce a convective median volume diameter distribution that is too broad relative to observations and thus, may have issues balancing raindrop formation, collision coalescence, and raindrop breakup. Assuming a μ of 2.5 rather than 0 for the raindrop size distribution improves one-moment scheme biases, and allowing μ to have values greater than 0 may improve two-moment schemes. Under-predicted stratiform rain rates are associated with under-predicted ice water contents at the melting level rather than excessive rain evaporation, in turn likely associated with convective detrainment that is too high in the troposphere and mesoscale circulations that are too weak. In addition to stronger convective updrafts than observed, limited domain size prevents a large, well-developed stratiform region from developing in CRMs, while a dry bias in ECMWF analyses does the same to the LAMs.

cloud-resolving↗
DOE OSTI2018

The limiting layer of fish scales: structure and properties

Fish scales serve as a flexible natural armor that have received increasing attention across the materials community. Most efforts in this area have focused on the composite structure of the predominately organic elasmodine, and limited work addresses the highly mineralized external portion known as the Limiting Layer (LL). This coating serves as the first barrier to external threats and plays an important role in resisting puncture. In this investigation the structure, composition and mechanical behavior of the LL were explored for three different fish, including the arapaima (Arapaima gigas), the tarpon (Megalops atlanticus) and the carp (Cyprinus carpio). The scales of these three fish have received the most attention within the materials community. Features of the LL were evaluated with respect to anatomical position to distinguish site-specific functional differences. Results show that there are significant differences in the surface morphology of the LL from posterior and anterior regions in the scales, and between the three fish species. The calcium to phosphorus ratio and the mineral to collagen ratios of the LL are not equivalent among the three fish. Results from nanoindentation showed that the LL of tarpon scales is the hardest, followed by the carp and the arapaima and the differences in hardness are related to the apatite structure, possibly induced by the growth rate and environment of each fish.

Arola, Dwayne D.↗
DOE OSTI2026

Density limit in peeling-limited pedestals at and above the Greenwald value in DIII-D high poloidal beta plasmas

High pressure, peeling limited pedestals with pedestal normalized beta β N,ped >2 and pedestal top density n ped at or above the Greenwald density nG have been achieved in DIII-D high poloidal beta plasmas, with high global normalized beta β N >3 and energy confinement H 98 ~1.2-1.7. Higher β N allows higher pedestal density above the Greenwald value and higher pedestal pressure, even with a low injected torque. MHD modeling confirms that the experimental profiles lie near the peeling-mode unstable boundary with high normalized pressure gradient βMHD and high edge current density. Experimental analysis and stability calculations indicate that the high poloidal beta with strong Shafranov shift, high βMHD and weak/negative magnetic shear improves the pedestal stability by decoupling the peeling and ballooning modes and stabilizing the ballooning modes, thus facilitating access to the second stable region of peeling-ballooning mode. The access to the second stable peeling-ballooning stability region opens Super-H-like channels without extremely strong shaping or strong torque injection. The high-pressure peeling pedestal allows the pedestal density to go beyond the Greenwald limit with strong ExB shear maintained: pedestal pressure increases with pedestal density even when n ped >n G , until reaching the ideal MHD instability boundary, where giant ELMs occur. The giant ELMs are dominated by a strong n=1 component and cause a large reduction of the edge pressure, but a negligible change of the core pressure, consistent with kink/peeling-mode induced instability. The pedestal recovers from the collapse and typically sustains a high baseline density, around the Greenwald limit, during the whole discharge duration. Experiments also found that internal transport barriers and n ped ~0.9nG, peeling limited pedestals could be simultaneously achieved in high β N plasmas, while an internal feedback between ITB strength and pedestal performance is found.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Environmental Constraints that Limit Methanogenesis

Methanogens are active in many different ecosystems, including habitats with biologically-derived organic matter as substrates such as aquatic sediments, wetlands, agricultural or natural soils subject to inundation, sewage digesters, and the anoxic portions of animal digestive tracts. Methanogens are also present in habitats with geochemically-supplied substrates such as hot springs, hydrothermal vents, volcanically-influenced habitats, and, potentially, the deep crustal subsurface. Methanogens as a group tolerate a broad range of physicochemical conditions, including temperatures from -2 to 122°C, pH values of 3.0 to 10.2, salinities up to halite saturation, and pressures of at least 75 MPa. Globally, variations in methane emissions can be explained to a large degree by variations in temperature and water availability. The distribution and activity of methanogens are constrained by ecological interactions that can be stimulatory or competitive, and by physicochemical factors that act at the biochemical or bioenergetic levels. In addition to the constraints placed on methanogens by physicochemical extremes, methanogen distribution and activity are constrained by the availability of energy and nutrients, the presence of inhibitory molecules (most notably oxygen), and the seawater anion, sulfate, due to competitive ecological interactions. Although methanogen tolerances to individual extremes are documented in culture, and the corresponding biochemical adaptations are understood to varying degrees, the natural environment frequently presents combinations of extreme conditions and energy limitations that may limit methanogen distribution to less than the optimally tolerated range of a single parameter. Little is understood about the compound effects of such extremes, nor the commonalities among them that will ultimately form the basis for predictive models of environmental methanogen population distribution. Future work that targets these questions, through a combination of culture work, “omic” analyses, in situ studies, and conceptual and quantitative models, will be needed to better understand the physiological ecology of methanogens.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2017

Predicting Deformation Limits of Dual-Phase Steels Under Complex Loading Paths

The deformation limits of various DP980 steels are examined in this study with deformation instability theory. Under uniaxial tension, overall stress-strain curves of the material are estimated based on simple rule of mixture (ROM) with both iso-strain and iso-stress assumptions. Under complex loading paths, actual microstructure-based finite element (FE) method is used to explicitly resolve the deformation incompatibilities between the soft ferrite and hard martensite phases. The results show that, for uniaxial tension, the deformation instability theory with iso-strain-based ROM can be used to provide the lower bound estimate of the uniform elongation (UE) for the various DP980 considered. Under complex loading paths, the deformation instability theory with microstructure-based FE method can be used in examining the effects of various microstructural features on the deformation limits of DP980 steels.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Tendency of a rotating electron plasma to approach the Brillouin limit

In this study, a neutral plasma is considered to be immersed in an axial magnetic field together with a radial electric field. If the electrons are magnetized, but the ions are not magnetized, then the electrons will rotate but the ions will not rotate, leading to current generation. The currents, in turn, weaken the axial magnetic field, leading to an increase in the rotation frequency of the slow Brillouin mode. This produces a positive feedback effect, further weakening the magnetic field. The operating point thus tends to drift towards the Brillouin limit, possibly finding stability only in proximity to the limit itself. An example of this effect might be the cylindrical Hall thruster configuration.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

K{sub α} x-ray imaging of laser-irradiated, limited-mass zirconium foils

X-ray fluorescence measurements to determine the effect of target heating on imaging efficiency, at a photon energy of 15.7 keV corresponding to the K{sub α} line of zirconium, have been carried out using limited-mass foils irradiated by the Texas Petawatt Laser. Zirconium foils that ranged in volume from 3000 × 3000 × 21 μm{sup 3} to 150 × 150 × 6 μm{sup 3} were irradiated with 100 J, 8 ps-long pulses and a mean intensity of 4 × 10{sup 19} W/cm{sup 2}. The K{sub α} emission was measured simultaneously using a highly ordered pyrolytic graphite crystal spectrometer and a curved quartz imaging crystal. The measured ratio of the integrated image signal to the integrated spectral signal was, within the experimental error, constant, indicating that the imaging efficiency's dependence on temperature is weak throughout the probed range. Based on our experience of target heating under similar conditions, we estimate a temperature of ∼200 eV for the smallest targets. The successful imaging of K{sub α} emission for temperatures this high represents an important proof of concept for Zr K{sub α} imaging. At these temperatures, the imaging of K{sub α} emission from lower-Z materials (such as Cu) is limited by temperature-dependent shifts in the K{sub α} emission energy.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Contribution of alloy clustering to limiting the two-dimensional electron gas mobility in AlGaN/GaN and InAlN/GaN heterostructures: Theory and experiment

The influence of alloy clustering on fluctuations in the ground state energy of the two-dimensional electron gas (2DEG) in AlGaN/GaN and InAlN/GaN heterostructures is studied. We show that because of these fluctuations, alloy clustering degrades the mobility even when the 2DEG wavefunction does not penetrate the alloy barrier unlike alloy disorder scattering. A comparison between the results obtained for AlGaN/GaN and InAlN/GaN heterostructures shows that alloy clustering limits the 2DEG mobility to a greater degree in InAlN/GaN heterostructures. Our study also reveals that the inclusion of an AlN interlayer increases the limiting mobility from alloy clustering. Moreover, Atom probe tomography is used to demonstrate the random nature of the fluctuations in the alloy composition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Orbital-motion-limited theory of dust charging and plasma response

The foundational theory for dusty plasmas is the dust charging theory that provides the dust potential and charge arising from the dust interaction with a plasma. The most widely used dust charging theory for negatively charged dust particles is the so-called orbital motion limited (OML) theory, which predicts the dust potential and heat collection accurately for a variety of applications, but was previously found to be incapable of evaluating the dust charge and plasma response in any situation. Here, we report a revised OML formulation that is able to predict the plasma response and hence the dust charge. Numerical solutions of the new OML model show that the widely used Whipple approximation of dust charge-potential relationship agrees with OML theory in the limit of small dust radius compared with plasma Debye length, but incurs large (order-unity) deviation from the OML prediction when the dust size becomes comparable with or larger than plasma Debye length. This latter case is expected for the important application of dust particles in a tokamak plasma.

Tang, Xian-Zhu↗
DOE OSTI2016

Extracting electrode space charge limited current: Charge injection into conjugated polyelectrolytes with a semiconductor electrode

Conjugated polyelectrolytes and related mixed ionic-electronic conductors (MIECs) are being explored for energy applications including solid-state lighting and photovoltaics. Fundamental models of charge injection into MIECs have been primarily developed for MIECs contacted with highly conductive or metal electrodes (MEs), despite many potential applications involving semiconductors. We theoretically and experimentally demonstrate that an appropriate semiconductor electrode (SE), n-type for electron or p-type of hole injection, can limit injection into MIECs. When the SE is the injecting electrode and is under accumulation, there is little difference from a ME. When the SE acts as the extracting electrode, however, injection into the MIEC can be limited because a fraction of any applied bias must support charge depletion in the semiconductor rather than charge injection into the MIEC. In a ME/MIEC/SE system, this can lead to significant asymmetry in current-voltage and injected charge-voltage behavior.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Structural and thermodynamic limits of layer thickness in 2D halide perovskites

In the fast-evolving field of halide perovskite semiconductors, the 2D perovskites (A') 2 (A) n -1 M n X 3 n +1 [where A = Cs + , CH 3 NH 3 + , HC(NH 2 ) 2 + ; A' = ammonium cation acting as spacer; M = Ge 2+ , Sn 2+ , Pb 2+ ; and X = Cl - , Br - , I - ] have recently made a critical entry. The n value defines the thickness of the 2D layers, which controls the optical and electronic properties. The 2D perovskites have demonstrated preliminary optoelectronic device lifetime superior to their 3D counterparts. They have also attracted fundamental interest as solution-processed quantum wells with structural and physical properties tunable via chemical composition, notably by the n value defining the perovskite layer thickness. The higher members ( n > 5) have not been documented, and there are important scientific questions underlying fundamental limits for n . To develop and utilize these materials in technology, it is imperative to understand their thermodynamic stability, fundamental synthetic limitations, and the derived structure–function relationships. We report the effective synthesis of the highest iodide n -members yet, namely (CH 3 (CH 2 ) 2 NH 3 ) 2 (CH 3 NH 3 ) 5 Pb 6 I 19 ( n = 6) and (CH 3 (CH 2 ) 2 NH 3 ) 2 (CH 3 NH 3 ) 6 Pb 7 I 22 ( n = 7), and confirm the crystal structure with single-crystal X-ray diffraction, and provide indirect evidence for “(CH 3 (CH 2 ) 2 NH 3 ) 2 (CH 3 NH 3 ) 8 Pb 9 I 28 ” (“ n = 9”). Direct HCl solution calorimetric measurements show the compounds with n > 7 have unfavorable enthalpies of formation (Δ H f ), suggesting the formation of higher homologs to be challenging. Finally, we report preliminary n -dependent solar cell efficiency in the range of 9–12.6% in these higher n -members, highlighting the strong promise of these materials for high-performance devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

A novel approach to derive halo-independent limits on dark matter properties

We propose a method that allows to place an upper limit on the dark matter elastic scattering cross section with nucleons which is independent of the velocity distribution. Our approach combines null results from direct detection experiments with indirect searches at neutrino telescopes, and goes beyond previous attempts to remove astrophysical uncertainties in that it directly constrains the particle physics properties of the dark matter. The resulting halo-independent upper limits on the scattering cross section of dark matter are remarkably strong and reach σ{sub SI}{sup p}≲10{sup −43} (10{sup −42}) cm{sup 2} and σ{sub SD}{sup p}≲10{sup −37} (3×10{sup −37}) cm{sup 2}, for dark matter particles of m{sub DM}∼1 TeV annihilating into W{sup +}W{sup −} (bb-bar), assuming ρ{sub loc}=0.3 GeV/cm{sup 3}.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗