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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 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Substrate-Independent Light Confinement in Bioinspired All-Dielectric Surface Resonators

Traditionally, photonic crystal slabs can support resonances that are strongly confined to the slab but also couple to external radiation. However, when a photonic crystal slab is placed on a substrate, the resonance modes become less confined, and as the index contrast between slab and substrate decreases, they eventually disappear. Using the scale structure of the Dione juno butterfly wing as an inspiration, we present here a low-index zigzag surface structure that supports resonance modes even without index contrast with the substrate. The zigzag structure supports resonances that are contained away from the substrate, which reduces the interaction between the resonance and the substrate. We experimentally verify the existence of substrate-independent resonances in the visible wavelength regime. Potential applications include substrate-independent structural color and light guiding.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Tunable Light-Emitting Diodes Utilizing Quantum-Confined Layered Perovskite Emitters

Organic–inorganic perovskites have been shown to have excellent optoelectronic properties. Further, layered perovskites have been demonstrated, utilizing quantum confinement to achieve emission blue-shifted from the bulk band gap. Here, we tune this blue-shift to build LEDs that span the visible spectrum. We demonstrate that electroluminescence from red-shifted layers dominates emission from mixed-thickness devices and that the addition of excess ligand is necessary to drive emission toward blue-shifted layers. By tuning the thickness of the layers, we build LEDs with blue emission utilizing the lead bromide system and orange emission utilizing the lead iodide system. Lastly, we demonstrate that these materials suffer reversible degradation under an applied electric field. The spectrally narrow emission, combined with the favorable electronic properties of perovskite materials and access to shorter emission wavelengths through quantum confinement, demonstrates the promise of these materials as a new platform for low-cost, high-performance light-emitting devices.

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

Surfactant Two-Dimensional Self-Assembly under Confinement

Confinement-induced structural rearrangements in supported self-assembled surfactant layers in aqueous salt solutions are investigated using classical Density Functional Theory. The systematic study of the influence of the nature of electrolyte revealed that 2:1 electrolyte stabilizes the hemicylindrical configuration of ionic surfactant layers, while a confinement-induced transition to a tilted monolayer configuration was found in symmetric 1:1 and 2:2 electrolytes. On the basis of this study we formulate a general model for the energetics of structural rearrangements in supported surfactant layers.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Subtle Chemistry of Colloidal, Quantum-Confined Semiconductor Nanostructures

Nanoscale colloidal semiconductor structures with at least one dimension small enough to experience quantum confinement effects have captured the imagination and attention of scientists interested in controlling various chemical and photophysical processes. Aside from having desirable quantum confinement properties, colloidal nanocrystals are attractive because they are often synthesized in low-temperature, low-cost, and potentially scalable manners using simple benchtop reaction baths. Considerable progress in producing a variety of shapes, compositions, and complex structures has been achieved. However, there are challenges to overcome in order for these novel materials to reach their full potential and become new drivers for commercial applications. The final shape, composition, nanocrystal-ligand structure, and size can depend on a delicate interplay of precursors, surface ligands, and other compounds that may or may not participate in the reaction. In this Perspective, we discuss current efforts toward better understanding how the reactivity of the reagents can be used to produce unique and complex nanostructures.

14 SOLAR ENERGY↗
DOE OSTI2024

How turbulence spreading improves power handling in quiescent high confinement fusion plasmas

Viable magnetic fusion devices necessitate combining good confinement with effective power flux handling. A major concern for ITER, and devices beyond, is the divertor heat load width, which sets peak boundary heat loads on the plasma-facing materials. Current estimates of the heat flux width are narrow for future reactors. Here, we demonstrate how pedestal turbulence can expand into, or entrain, the stable scrape-off-layer and so broaden the heat flux width beyond these neoclassical predictions. Employing combined theoretical, computational, and experimental approaches, we focus on quiescent high confinement discharges on the DIII-D tokamak, but the results are of broader significance. Our findings uncover common trends in the edge turbulence intensity flux, the pressure perturbation skewness, and the turbulence mixing length, which together determine the heat flux width. This research demonstrates the physics of scrape-off-layer broadening by turbulence and highlights the promise of a turbulent pedestal for successful core-edge integration in ITER and future fusion devices.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Characterization of impurity confinement on Alcator C-Mod using a multi-pulse laser blow-off system

A new laser blow-off system for use in impurity transport studies on Alcator C-Mod was developed and installed for the 2009 run campaign. Its design included capabilities for multiple impurity injections during a single plasma pulse and remote manipulation of the ablated spot size. The system uses a 0.68 J, Nd:YAG laser operating at up to 10 Hz coupled with the fast beam steering via a 2D piezoelectric mirror mount able to move spot locations in the 100 ms between laser pulses and a remote controllable optical train that allow ablated spot sizes to vary from {approx}0.5 to 7 mm. The ability to ablate a wide range in target Z along with Alcator C-Mod's extensive diagnostic capabilities (soft x-ray, vacuum ultraviolet (VUV), charge exchange spectroscopy, etc.) allows for detailed studies of the impurity transport dependencies and mechanisms. This system has demonstrated the achievement of all its design goals including the ability for non-perturbative operation allowing for insight into underlying impurity transport processes. A detailed overview of the laser blow-off system and initial results of operation are presented. This includes an investigation into the characterization of impurity confinement in the I-mode confinement regime recently investigated on C-Mod.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2011

Magnetic confinement of a high-density cylindrical plasma

The stationary structure of a weakly collisional plasma column, confined by an axial magnetic field and a cylindrical vessel, is studied for the high-density case, when the diamagnetic azimuthal current is large enough to demagnetize partially the plasma. The plasma response is characterized mainly by two dimensionless parameters: the ratios of the electron gyroradius and the electron skin-depth to the plasma radius, and each of them measures the independent influence of the applied magnetic field and the plasma density on the plasma response. The strong magnetic confinement regime, characterized by very small wall losses, is limited to the small gyroradius and large skin-depth ranges. In the high-density case, when the electron skin-depth is smaller than the electron gyroradius, the skin-depth turns out to be the magnetic screening length, so that the bulk of the plasma behaves as unmagnetized.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Inertial confinement fusion implosions with imposed magnetic field compression using the OMEGA Laser

Experiments applying laser-driven magnetic-flux compression to inertial confinement fusion (ICF) targets to enhance the implosion performance are described. Spherical plastic (CH) targets filled with 10 atm of deuterium gas were imploded by the OMEGA Laser, compare Phys. Plasmas 18, 056703 or Phys. Plasmas 18, 056309. Before being imploded, the targets were immersed in an 80-kG magnetic seed field. Upon laser irradiation, the high implosion velocities and ionization of the target fill trapped the magnetic field inside the capsule, and it was amplified to tens of megagauss through flux compression. At such strong magnetic fields, the hot spot inside the spherical target was strongly magnetized, reducing the heat losses through electron confinement. The experimentally observed ion temperature was enhanced by 15%, and the neutron yield was increased by 30%, compared to nonmagnetized implosions [P. Y. Chang et al., Phys. Rev. Lett. 107, 035006 (2011)]. This represents the first experimental verification of performance enhancement resulting from embedding a strong magnetic field into an ICF capsule. Experimental data for the fuel-assembly performance and magnetic field are compared to numerical results from combining the 1-D hydrodynamics code LILAC with a 2-D magnetohydrodynamics postprocessor.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

A statistical analysis of avalanching heat transport in stationary enhanced core confinement regimes

We present a statistical analysis of heat transport in stationary enhanced confinement regimes obtained from flux-driven gyrofluid simulations. The probability density functions of heat flux in improved confinement regimes, characterized by the Nusselt number, show significant deviation from Gaussian, with a markedly fat tail, implying the existence of heat avalanches. Two types of avalanching transport are found to be relevant to stationary states, depending on the degree of turbulence suppression. In the weakly suppressed regime, heat avalanches occur in the form of quasi-periodic (QP) heat pulses. Collisional relaxation of zonal flow is likely to be the origin of these QP heat pulses. This phenomenon is similar to transient limit cycle oscillations observed prior to edge pedestal formation in recent experiments. On the other hand, a spectral analysis of heat flux in the strongly suppressed regime shows the emergence of a 1/f (f is the frequency) band, suggesting the presence of self-organized criticality (SOC)-like episodic heat avalanches. This episodic 1/f heat avalanches have a long temporal correlation and constitute the dominant transport process in this regime.

42 ENGINEERING↗
DOE OSTI2012

Plasma confinement regimes and collective modes characterizing them

A unified theory is presented for the modes that are excited at the edge of the plasma column and are important signatures of the advanced confinement regimes into which magnetically confined plasmas can be driven. In particular, the so-called EDA H-Regime, the Elmy H-Regime, and the I-Regime are considered. The modes that are identified theoretically have characteristics that are consistent with or have anticipated those of the modes observed experimentally for each of the investigated regimes. The phase velocities, the produced transport processes, the frequencies, the wavelengths, and the consistency with the direction of spontaneous rotation are the factors considered for comparison with the relevant experiments. The quasi-coherent mode [I. Cziegler, Ph.D. dissertation, Massachusetts Institute of Technology, Cambridge, MA, 2011] that is present in the EDA H-Regime has a phase velocity in the direction of the ion diamagnetic velocity in the plasma reference frame. Consequently, this is identified as a ballooning mode near finite Larmor radius marginal stability involving the effects of transverse ion viscosity and other dissipative effects. In this regime, impurities are driven outward by the combined effects of the local temperature gradients of the impurities and their thermal conductivity, while in the Elmy H-Regime impurities are driven toward the center of the plasma column. In the I-Regimes, the excited 'Heavy Particle' modes [B. Coppi and T. Zhou, Phys. Plasmas 19, 012302 (2012); Phys. Lett. A 375, 2916 (2011)] are not of the ballooning kind and are shown to expel the impurities toward the plasma edge in the presence of significant fluctuations. These modes can have a finite frequency of oscillation with a phase velocity in the direction of the electron diamagnetic velocity or they can be nearly purely growing, explaining why there are I-Regimes where fluctuations are not observed. Instead, the modes considered for the Elmy H-Regime are of the ballooning kind. They are driven by the combined effects of the plasma pressure gradient and the magnetic field curvature, are close to conditions under which the acquired growth rates are proportional to half power of the relevant dissipation parameters, involve the effects of finite magnetic diffusivity and finite electron thermal conductivity, and can have phase velocities in either direction.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Measuring time of flight of fusion products in an inertial electrostatic confinement fusion device for spatial profiling of fusion reactions

A new diagnostic has been developed that uses the time of flight (TOF) of the products from a nuclear fusion reaction to determine the location where the fusion reaction occurred. The TOF diagnostic uses charged particle detectors on opposing sides of the inertial electrostatic confinement (IEC) device that are coupled to high resolution timing electronics to measure the spatial profile of fusion reactions occurring between the two charged particle detectors. This diagnostic was constructed and tested by the University of Wisconsin-Madison Inertial Electrostatic Confinement Fusion Group in the IEC device, HOMER, which accelerates deuterium ions to fusion relevant energies in a high voltage ({approx}100 kV), spherically symmetric, electrostatic potential well [J. F. Santarius, G. L. Kulcinski, R. P. Ashley, D. R. Boris, B. B. Cipiti, S. K. Murali, G. R. Piefer, R. F. Radel, T. E. Radel, and A. L. Wehmeyer, Fusion Sci. Technol. 47, 1238 (2005)]. The TOF diagnostic detects the products of D(d,p)T reactions and determines where along a chord through the device the fusion event occurred. The diagnostic is also capable of using charged particle spectroscopy to determine the Doppler shift imparted to the fusion products by the center of mass energy of the fusion reactants. The TOF diagnostic is thus able to collect spatial profiles of the fusion reaction density along a chord through the device, coupled with the center of mass energy of the reactions occurring at each location. This provides levels of diagnostic detail never before achieved on an IEC device.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

Spherical ion oscillations in a positive polarity gridded inertial-electrostatic confinement device

A pulsed, positive polarity gridded inertial electrostatic confinement device has been investigated experimentally, using a differential emissive probe and potential traces as primary diagnostics. Large amplitude oscillations in the plasma current and plasma potential were observed within a microsecond of the discharge onset, which are indicative of coherent ion oscillations about a temporarily confined excess of recirculating electron space charge. The magnitude of the depth of the potential well in the established virtual cathode was determined using a differential emissive Langmuir probe, which correlated well to the potential well inferred from the ion oscillation frequency for both hydrogen and argon experiments. It was found that the timescale for ion oscillation dispersion is strongly dependent on the neutral gas density, and weakly dependent on the peak anode voltage. The cessation of the oscillations was found to be due to charge exchange processes converting ions to high velocity neutrals, causing the abrupt de-coherence of the oscillations through an avalanche dispersion in phase space.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

First-principles study of quantum confinement and surface effects on the electronic properties of InAs nanowires

We have used first principles methods to systematically investigate the quantum confinement effect on the electronic properties of zinc-blende (ZB) and wurtzite (WZ) InAs nanowires (NWs) with different orientations and diameters, and compared their electronic properties before and after pseudo-hydrogen passivation. The results show that the calculated carrier effective masses are dependent on the NW diameter, except for [110] ZB NWs, and the hole effective masses of [111] ZB NWs are larger than the electron effective masses when the NW diameter is ≥26 Å. The band alignments of [111] ZB and [0001] WZ NWs reveal that the effect of quantum confinement on the conduction bands is greater than on the valence bands, and the position of the valence band maximum level changes little with increasing NW diameter. The pseudo-hydrogen passivated NWs have larger band gaps than the corresponding unpassivated NWs. The carrier effective masses and mobilities can be adjusted by passivating the surface dangling bonds.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

An x-ray setup to investigate the atomic order of confined liquids in slit geometry

A setup has been designed to investigate thin films of confined liquids with the use of X-ray scattering methods. The confinement is realized between the flat culets of a pair of diamonds by positioning and orienting the lower diamond with nanometer and micro radian accuracy. We routinely achieve gaps between 5 and 50 nm at culet diameters of 200 μm. With this setup and a micro focused X-ray beam we have investigated the in-plane and the out-off-plane atomic order of benzene with atomic resolution.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Role of quantum confinement in luminescence efficiency of group IV nanostructures

Experimental results obtained previously for the photoluminescence efficiency (PL{sub eff}) of Ge quantum dots (QDs) are theoretically studied. A log-log plot of PL{sub eff} versus QD diameter (D) resulted in an identical slope for each Ge QD sample only when E{sub G}∼(D{sup 2}+D){sup −1}. We identified that above D ≈ 6.2 nm: E{sub G}∼D{sup −1} due to a changing effective mass (EM), while below D ≈ 4.6 nm: E{sub G}∼D{sup −2} due to electron/hole confinement. We propose that as the QD size is initially reduced, the EM is reduced, which increases the Bohr radius and interface scattering until eventually pure quantum confinement effects dominate at small D.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Effect of phonon confinement on lattice thermal conductivity of lead Telluride quantum well structure

The paper examines the effect of spatial confinement of acoustic phonons on average group velocity and consequently the lattice thermal conductivity of a free-standing PbTe quantum well structure and their temperature dependence. The average group velocity at 100 Å decreases 30% to the bulk value and falls more rapidly on reducing the width of quantum well. Moreover, the lattice thermal conductivity of 100 Å wide PbTe quantum well with value of 0.60 W/mK shows considerable decrease of 70% compared to it’s bulk value. It is observed that the effect of reduction in well width is less pronounce as temperature increases. This appears mainly due to dominance of umklapp processes over the confinement effects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Transitions to improved confinement regimes induced by changes in heating in zero-dimensional models for tokamak plasmas

It is shown that rapid substantial changes in heating rate can induce transitions to improved energy confinement regimes in zero-dimensional models for tokamak plasma phenomenology. We examine for the first time the effect of step changes in heating rate in the models of Kim and Diamond [Phys. Rev. Lett. 90, 185006 (2003)] and Malkov and Diamond [Phys. Plasmas 16, 012504 (2009)], which nonlinearly couple the evolving temperature gradient, micro-turbulence, and a mesoscale flow; and in the extension of Zhu et al. [Phys. Plasmas 20, 042302 (2013)], which couples to a second mesoscale flow component. The temperature gradient rises, as does the confinement time defined by analogy with the fusion context, while micro-turbulence is suppressed. This outcome is robust against variation of heating rise time and against introduction of an additional variable into the model. It is also demonstrated that oscillating changes in heating rate can drive the level of micro-turbulence through a period-doubling path to chaos, where the amplitude of the oscillatory component of the heating rate is the control parameter.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Geodesic least squares regression for scaling studies in magnetic confinement fusion

In regression analyses for deriving scaling laws that occur in various scientific disciplines, usually standard regression methods have been applied, of which ordinary least squares (OLS) is the most popular. However, concerns have been raised with respect to several assumptions underlying OLS in its application to scaling laws. We here discuss a new regression method that is robust in the presence of significant uncertainty on both the data and the regression model. The method, which we call geodesic least squares regression (GLS), is based on minimization of the Rao geodesic distance on a probabilistic manifold. We demonstrate the superiority of the method using synthetic data and we present an application to the scaling law for the power threshold for the transition to the high confinement regime in magnetic confinement fusion devices.

97 MATHEMATICS AND COMPUTING↗