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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 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Linear analyses of peeling-ballooning modes in high beta pedestal plasmas

We present the linear simulations of edge plasma instabilities using the 3-field peeling-ballooning model and gyro-Landau-fluid model under the BOUT++ framework. A series of realistic equilibria of shifted circular geometry are generated by a global equilibrium solver CORSICA, where the Shafranov shift, elongation effects, and bootstrap current are included. The linear growth rate spectrum of the peeling-ballooning modes is shown in a wide range of pressure gradient and parallel current density in the pedestal region. The results show that the bootstrap current stabilizes high beta ballooning modes. Here, the simulations with different fractions of bootstrap current indicate a trend for the existence of the high beta peeling-ballooning mode stability region. Taking the kinetic effects into account, the linear simulations of kinetic peeling-ballooning mode using the gyro-Landau-fluid model show that this region can be accessed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1998

Effects of Aging-Time Reference on the Long Term Behavior of the IM7/K3B Composite

An analytical study was undertaken to investigate the effects of the time-based shift reference on the long term behavior of the graphite reinforced thermoplastic polyimide composite IM7/K3B at elevated temperature. Creep compliance and the effects of physical aging on the time dependent response was measured for uniaxial loading at several isothermal conditions below the glass transition temperature (T(sub g). Two matrix dominated loading modes, shear and transverse, were investigated in tension and compression. The momentary sequenced creep/aging curves were collapsed through a horizontal (time) shift using the shortest, middle and longest aging time curve as the reference curve. Linear viscoelasticity was used to characterize the creep/recovery behavior and superposition techniques were used to establish the physical aging related material constants. The use of effective time expressions in a laminated plate model allowed for the prediction of long term creep compliance. The effect of using different reference curves with time/aging-time superposition was most sensitive to the physical aging shift rate at lower test temperatures. Depending on the loading mode, the reference curve used can result in a more accurate long term prediction, especially at lower test temperatures.

Veazie, David R.↗
DOE OSTI2019

Saturation Velocity Measurement of Al 0.7 Ga 0.3 N-Channel High Electron Mobility Transistors

Gate length dependent (80 nm–5000 mm) radio frequency measurements to extract saturation velocity are reported for Al 0.85 Ga 0.15 N/Al 0.7 Ga 0.3 N high electron mobility transistors fabricated into radio frequency devices using electron beam lithography. Direct current characterization revealed the threshold voltage shifting positively with increasing gate length, with devices changing from depletion mode to enhancement mode when the gate length was greater than or equal to 450 nm. Transconductance varied from 10 mS/mm to 25 mS/mm, with the 450 nm device having the highest values. Maximum drain current density was 268 mA/mm at 10 V gate bias. Scattering-parameter characterization revealed a maximum unity gain bandwidth ($f_T$) of 28 GHz, achieved by the 80 nm gate length device. A saturation velocity value of 3.8 × 10 6 cm/s, or 35% of the maximum saturation velocity reported for GaN, was extracted from the $f_T$ measurements

36 MATERIALS SCIENCE↗
DOE OSTI2026

A metastable tetragonal phase in two-dimensional halide perovskite lattices driven by a coherent Higgs mode

The optoelectronic properties of metal halide perovskites are defined by their coupled structural and photophysical properties, yet their lattice behaviour remains underexplored. Here, using impulsive stimulated Raman spectroscopy, we study light-induced phonon dynamics of two-dimensional butylammonium lead iodide ((BA) 2 PbI 4 ) films under varying excitation intensities, photon energies and temperatures. We reveal that, whereas (BA) 2 PbI 4 exhibits two thermally accessible orthorhombic phases, optically excited phonons transiently direct the lattice to a distinct, higher symmetry tetragonal phase. We show that bandgap oscillations arise from simultaneous distortions of in-plane and out-of-plane octahedral tilt angles with oscillations following a low-to-high symmetry pathway, marked by two vibrational frequencies independent of intensity—a signature of an optically excited Higgs mode. Notably, the Higgs mode at below-bandgap excitation induces a fourfold larger spectral shift than above-bandgap, where photogenerated charge carriers drive the system away from the optically induced tetragonal phase. This study illustrates how optomechanical coupling influences the optical properties of two-dimensional perovskites.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Cavity optomechanics with a nonlinear photonic-crystal nanomembrane

We have designed, fabricated and characterized a nanomembrane which could be used as a moving end mirror of a Fabry-Perot cavity. The high reflectivity and optimized mechanical properties of the membrane should allow us to demonstrate the mechanical ground state of the membrane. As any sub-micron mechanical resonator, our system demonstrates nonlinear dynamical effects. We characterize the mechanical response to a strong pump drive and observe a shift in the oscillation frequency and phase conjugation of the mechanical mode. Such nonlinear effects are expected to play a role in the quantum dynamics of the membrane as well.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Effect of scrape-off-layer current on reconstructed tokamak equilibrium

Methods are described that extend fields from reconstructed equilibria to include scrape-off-layer current through extrapolated parametrized and experimental fits. The extrapolation includes both the effects of the toroidal-field and pressure gradients which produce scrape-off-layer current after recomputation of the Grad-Shafranov solution. To quantify the degree that inclusion of scrape-off-layer current modifies the equilibrium, the χ-squared goodness-of-fit parameter is calculated for cases with and without scrape-off-layer current. The change in χ-squared is found to be minor when scrape-off-layer current is included however flux surfaces are shifted by up to 3 cm. Here the impact on edge modes of these scrape-off-layer modifications is also found to be small and the importance of these methods to nonlinear computation is discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Generation of Intense Low-Divergence Isolated Soft-X-Ray Attosecond Pulses in a Gas-Filled Waveguide Using Three-Color Synthesized Laser Pulses

Isolated attosecond pulses (IAPs) in the soft-x-ray (SXR) region have been successfully produced via high-order harmonic generation in a gas medium at several laboratories, but pulse energies are much lower compared to the IAPs in the extreme ultraviolet. Here, we show that in a gas-filled hollow waveguide, an efficient gating method using a few-cycle three-color synthesizer is able to generate stronger SXR IAPs. We show that an 104-as IAP in the SXR region (central energy of 210 eV) with a higher pulse energy (0.9 nJ) and a lower divergence (within 1.5 mrad) can be generated with a three-color laser beam for input pulse energy at 0.62 mJ, under the condition of optimal gas pressure and waveguide length. The enhancement of the IAP is attributed to favorable phase matching of SXR high harmonics caused by the evolution of the three-color waveform during its propagation in the waveguide, which can be understood by analyzing the time shift of the waveform due to the interplay of the waveguide mode, atomic dispersion, and plasma dispersion. We check that the gating method is quite robust with respect to the phase jitter in the waveform. The advantage of this method is further demonstrated by comparing the generation of SXR high harmonics and attosecond pulses with two other schemes: the three-color waveform in a gas cell and a previously optimized two-color waveform in a gas-filled waveguide.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2023

Physically motivated analytical expression for the temperature dependence of the zero-field splitting of the nitrogen-vacancy center in diamond

The temperature dependence of the zero-field splitting (ZFS) between the | $m_s$ = 0 $\rangle$ and | $m_s$ = ±1 $\rangle$ levels of the nitrogen-vacancy (NV) center's electronic ground-state spin triplet can be used as a robust nanoscale thermometer in a broad range of environments. However, despite numerous measurements of this dependence in different temperature ranges, to our knowledge no analytical expression has been put forward that captures the scaling of the ZFS of the NV center across all relevant temperatures. Here we present a simple, analytical, and physically motivated expression for the temperature dependence of the NV center's ZFS that matches all experimental observations, in which the ZFS shifts in proportion to the occupation numbers of two representative phonon modes. In contrast to prior models our expression does not diverge outside the regions of fitting. In conclusion, we show that our model quantitatively matches experimental measurements of the ZFS from 15 to 500 K in single NV centers in ultrapure bulk diamond, and we compare our model and measurements to prior models and experimental data.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Transition to collapsed tetragonal phase in CaFe 2 As 2 single crystals as seen by 57 Fe Mössbauer spectroscopy

Temperature dependent measurements of 57 Fe Mössbauer spectra on CaFe 2 As 2 single crystals in the tetragonal and collapsed tetragonal phases are reported. Clear features in the temperature dependencies of the isomer shift, relative spectra area, and quadrupole splitting are observed at the transition from the tetragonal to the collapsed tetragonal phase. From the temperature dependent isomer shift and spectral area data, an average stiffening of the phonon modes in the collapsed tetragonal phase is inferred. The quadrupole splitting increases by ~25% on cooling from room temperature to ~100 K in the tetragonal phase and is only weakly temperature dependent at low temperatures in the collapsed tetragonal phase, in agreement with the anisotropic thermal expansion in this material. In order to gain microscopic insight about these measurements, we perform ab initio density functional theory calculations of the electric field gradient and the electron density of CaFe 2 As 2 in both phases. By comparing the experimental data with the calculations we are able to fully characterize the crystal structure of the samples in the collapsed-tetragonal phase through determination of the As z coordinate. Furthermore, based on the obtained temperature dependent structural data we are able to propose charge saturation of the Fe-As bond region as the mechanism behind the stabilization of the collapsed-tetragonal phase at ambient pressure.

36 MATERIALS SCIENCE↗
DOE OSTI2015

SU-E-I-11: A New Cone-Beam CT System for Bedside Head Imaging

Purpose: To design and develop a new mobile cone-beam CT (CBCT) system for head imaging with good soft-tissue visibility, to be used bedside in ICU and neurosurgery department to monitor treatment and operation outcome in brain patients. Methods: The imaging chain consists of a 30cmx25cm amorphous silicon flat panel detector and a pulsed, stationary anode monoblock x-ray source of 100kVp at a maximal tube current of 10mA. The detector and source are supported on motorized mechanisms to provide detector lateral shift and source angular tilt, enabling a centered digital radiographic imaging mode and half-fan CBCT, while maximizing the use of the x-ray field and keep the source to detector distance short. A focused linear anti-scatter grid is mounted on the detector, and commercial software with scatter and other corrective algorithms is used for data processing and image reconstruction. The gantry rotates around a horizontal axis, and is able to adjust its height for different patient table positions. Cables are routed through a custom protective sleeve over a large bore with an in-plane twister band, facilitating single 360-degree rotation without a slip-ring at a speed up to 5 seconds per rotation. A UPS provides about 10 minutes of operation off the battery when unplugged. The gantry is on locked casters, whose brake is control by two push handles on both sides for easy reposition. The entire system is designed to have a light weight and a compact size for excellent maneuverability. Results: System design is complete and main imaging components are tested. Initial results will be presented and discussed later in the presentation. Conclusion: A new mobile CBCT system for head imaging is being developed. With its compact size, a large bore, and quality design, it is expected to be a useful imaging tool for bedside uses. The work is supported by a grant from Chinese Academy of Sciences.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2023

Projecting Future Energy Production from Operating Wind Farms in North America. Part II: Statistical Downscaling

Abstract Capacity factors (CFs) derived from daily expected power at 22 operating wind farms in different regions of North America are used as predictands to train statistical downscaling algorithms using output from ERA5. The statistical downscaling models are then used to make CF projections for a suite of CMIP6 Earth System Models (ESMs). Downscaling is performed using a hybrid statistical approach that employs synoptic types derived using k -means clustering applied to sea level pressure fields with variance corrections applied as a function of the pressure gradient intensity. ESMs exhibit marked variability in terms of the skill with which the frequency of synoptic types and pressure gradients are reproduced relative to ERA5, and that differential skill is used to infer differential credibility in the associated CF projections. Projections of median annual mean CF [P50(CF)] in each 20-yr period from 1980 to 2099 show evidence of declines at most wind farms except in parts of the southern Great Plains, although the magnitude of the changes is strongly dependent on the ESM. For example, P50(CF) in 2080–99 deviate from those in 1980–99 by from −3.1 to +0.2 percentage points in the Northeast. The largest-magnitude declines in P50(CF) ranging from −3.9 to −2 percentage points are projected for the southern West Coast. CF trends exhibit marked seasonality and are strongly linked to changes in the relative intensity of future synoptic patterns, with much less impact from shifts in the occurrence of synoptic types over time. Internal climate modes continue to play a significant role in inducing interannual variability in wind power production, even under high radiative forcing scenarios. Significance Statement We describe how future climate changes may affect wind resources and wind power generation. Near-term changes in projected wind power electricity generation potential at operating wind farms over North America are small, but by the end of the current century electricity production is projected to decrease in many areas but may increase in parts of the southern Great Plains. The amount of change in projected wind power production is a strong function of the Earth system model that is downscaled and also depends on the continued presence of internally forced climate variability. An additional dependence on the amount of greenhouse gas–induced global warming indicates the transition of the energy sector to low-carbon sources may assist in maintaining the abundant U.S. wind resource.

Meteorology & Atmospheric Sciences↗
NASA NTRS1980

On failure modes in finite width angle ply laminates

The results of a failure analysis of finite width (+ or - theta)s T300/5208 graphite-epoxy laminates subjected to thermal curing loads and applied tensile strain are presented. Stress distributions obtained from linear elastic and linear thermoelastic finite element stress analyses are used in conjunction with the tensor polynomial failure criterion to predict the mode and location of first failure in laminates with fiber angles ranging from five to seventy-five degrees. It is shown that the mode of failure is predominantly interlaminar shear for small fiber angles, shifts to combined interlaminar normal, in-plane shear and in-plane normal for intermediate angles, and is primarily transverse tension for large fiber angles. The location of initial failure shifts from the (+ or - theta)s interface for small fiber angles to the midplane for large fiber angles.

Herakovich, C. T.↗
NASA NTRS1984

The Fourier Tachometer 2: an Instrument for Measuring Global Solar Velocity Fields

The High Altitude Observatory and Sacramento Peak Observatory have jointly constructed a second version of the Fourier Tachometer, which is now undergoing final integration and testing. This is an interferometric instrument for measuring the Doppler shift of solar spectrum lines. The principal features and performance goals of this instrument are: simultaneous velocity observations over a 2-dimensional, 100 x 100 pixel field of view; measurement of absolute Doppler shifts with 1 m/s accuracy; noise level for moderate-1 oscillation modes of 1 cm/s for a 1-day observing run; flexibility and ease of use. Early (though incomplete) testing suggests that these goals should be attainable with the current instrument.

Brown, T. M.↗
NASA NTRS1992

On the detectability of Jovian oscillations with infrared heterodyne measurements

A method is proposed for detecting Jovian global oscillations using IR heterodyne measurements in the lower stratosphere of the planet. Measurements of the S(0) and S(1) lines over a period of a few nights permitted the detection of modes with periods of 5 to 20 min and of Doppler shifts corresponding to velocities as low as 1 m/sec (which is less than the value reported by Schmider and Mosser, 1990 from observations of the Jovian troposphere in the visible range. Possible limitations of the method are pointed out.

Mosser, Benoit↗
NASA NTRS2012

Liquid Droplet Dynamics in Gravity Compensating High Magnetic Field

Numerical models are used to investigate behavior of liquid droplets suspended in high DC magnetic fields of various configurations providing microgravity-like conditions. Using a DC field it is possible to create conditions with laminar viscosity and heat transfer to measure viscosity, surface tension, electrical and thermal conductivities, and heat capacity of a liquid sample. The oscillations in a high DC magnetic field are quite different for an electrically conducting droplet, like liquid silicon or metal. The droplet behavior in a high magnetic field is the subject of investigation in this paper. At the high values of magnetic field some oscillation modes are damped quickly, while others are modified with a considerable shift of the oscillating droplet frequencies and the damping constants from the non-magnetic case.

Bojarevics, V.↗
DOE OSTI2022

Vibrational properties of CuInP 2 S 6 across the ferroelectric transition

In order to explore the properties of a two-sublattice ferroelectric, we measured the infrared and Raman scattering response of CuInP 2 S 6 across the ferroelectric and glassy transitions and compared our findings to a symmetry analysis, calculations of phase stability, and lattice dynamics. In addition to uncovering displacive character and a large hysteresis region surrounding the ferroelectric transition temperature T C , we identify the vibrational modes that stabilize the polar phase and confirm the presence of two ferroelectric variants with opposite polarizations. Below T C , a poorly understood relaxational or glassy transition at T g is characterized by local structure changes in the form of subtle peak shifting and activation of low frequency out-of-plane Cu- and In-containing modes. Here, the latter are due to changes in the Cu/In coordination environments and associated order-disorder processes. Moreover, T g takes place in two steps with another large hysteresis region and significant underlying scattering. Combined with imaging of the room temperature phase separation, this effort lays the groundwork for studying CuInP 2 S 6 under external stimuli and in the ultrathin limit.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Role of microtearing mode in DIII-D and future high- β p core plasmas

This paper provides modeling evidence for a new variant of microturbulence instability that regulates the electron thermal fluxes in the transport barrier region of DIII-D "high β p " discharges. This scenario is being investigated as a possible scenario for future steady-state reactors, and thus it is essential to understand the physics processes which govern its performance in order to confidently extrapolate to future devices. Additional analysis examining the extrapolation of this scenario to ITER shows that while the new microtearing mode identified here is unlikely to be unstable in that regime (due to increased temperatures and reduced collisionality), the large shift in magnetic axis (Shafranov shift) that arises in these plasmas may still favor stabilization of common ballooning instabilities which peak on the low field side, but destabilize electrostatic slab modes on the high field side.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Enhancement-mode AlGaN channel high electron mobility transistor enabled by p-AlGaN gate

Here, this work exhibits the ability to shift the threshold voltage of an Al 0.45 Ga 0.55 N/Al 0.3 Ga 0.7 N high electron mobility transistor through the implementation of a 100 nm thick p-Al 0.3 Ga 0.7 N gate. A maximum threshold voltage of +0.3 V was achieved with a 3 μm gate length. In addition to achieving enhancement-mode operation, this work also shows the capability to obtain high saturated drain current (>50 mA/mm), no gate hysteresis, high I ON,MAX /I OFF,MIN ratio of >10 9 , and exceptionally low gate leakage current of 10 -6 mA/mm even under high forward bias of V gs = 8 V.

36 MATERIALS SCIENCE↗