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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 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

The dominant micro-turbulence instabilities in the lower q 95 high β p plasmas on DIII-D and predict-first extrapolation

Large-radius internal transport barriers (ITB) are the signature of high β p scenarios on DIII-D. Previous studies show that a large Shafranov shift, rather the E x B shear, suppresses the turbulence and helps in the formation of the large-radius ITB. New gyrokinetic simulations suggest that the remaining micro-instabilities in lower q 95 (<7.0), high β p ITB plasmas are drift wave instabilities, including the collisionless trapped electron mode in the core and ITB peak gradient region, the electron temperature gradient mode in the ITB peak gradient region and at the ITB foot and the ion temperature gradient mode at the ITB foot. Gyrokinetic simulation results qualitatively agree with the density fluctuation analysis from beam emission spectroscopy, which suggests the existence of a low-k ion mode at the ITB foot. As a result, the gyrokinetic simulations also predict that a larger Shafranov shift can overwhelm the driving sources for turbulence from the profile gradients at higher β N , leading to stronger turbulence suppression and stronger ITBs in lower q 95 , high β p plasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Gyrotron with a sectioned cavity based on excitation of a far-from-cutoff operating mode

A typical problem of weakly relativistic low-power gyrotrons (especially in the case of operation at high cyclotron harmonics) is the use of long cavities ensuring extremely high diffraction Q-factors for the operating near-cutoff waves. As a result, a great share of the rf power radiated by electrons is spent in Ohmic losses. In this paper, we propose to use a sectioned cavity with π-shifts of the wave phase between sections. In such a cavity, a far-from-cutoff axial mode of the operating cavity having a decreased diffraction Q-factor is excited by the electron beam in a gyrotron-like regime.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

A Dual-Signaling Ferrocene-Pyrene Dyad: Triple-Mode Recognition of the Cu(II) Ions in Aqueous Medium

We report a structure of ferrocene-pyrene conjugate (1) comprising electro and photo-active dual-signaling units. In particular, 1 upon interaction with Cu(II), displays selectively one-photon fluorescence quenching, but it shows two-photon absorption (TPA) cross-section 1230 GM (at 780 nm). Further, 1 displayed two irreversible oxidative waves at 0.39 V and 0.80 V (vs Ag/AgCl), in the electrochemical analysis which upon addition of Cu{sup 2+}, led to the negative potential shift in both the oxidative waves to appear at 0.25 V and 0.68 V. The triple mode changes in presence of Cu(II) suggesting the possible application of 1 for the detection of Cu(II) in aqueous media. .

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2016

Global kinetic ballooning mode simulations in BOUT++

We report on simulation results of a 3+1 gyro-Landau-fluid (GLF) model in BOUT++ framework, which contributes to increasing the physics understanding of the edge turbulence. We find that there is no second stability region of kinetic ballooning modes (KBM) in the concentric circular geometry. The first unstable β of KBM decreases below the ideal ballooning mode threshold with increasing ${{\eta}_{i}}$. In order to study the KBM in the real tokamak equilibrium, we find that the approximation of shifted circular geometry ($\beta \ll {{\varepsilon}^{2}}$ ) is not valid for a high β global equilibrium near the second stability region of KBM. Thus we generate a series of real equilibria from a global equilibrium solver CORSICA, including both Shafranov shift and elongation effects, but not including bootstrap current. In these real equilibria, the second stability region of KBM are observed in our global linear simulations. The most unstable mode for different β are the same while the mode number spectrum near the second stability region is wider than the case near the first stability region. Here, the nonlinear simulations show that the energy loss of an ELM keeps increasing with β, because the linear drive of the turbulence remains strong for the case near the second stability region during profile evolution.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS2011

Common-Path Interferometric Wavefront Sensing for Space Telescopes

This paper presents an optical configuration for a common-path phase-shifting interferometric wavefront sensor.1 2 This sensor has a host of attractive features which make it well suited for space-based adaptive optics. First, it is strictly reflective and therefore operates broadband, second it is common mode and therefore does not suffer from systematic errors (like vibration) that are typical in other interferometers, third it is a phase-shifting interferometer and therefore benefits from both the sensitivity of interferometric sensors as well as the noise rejection afforded by synchronous detection. Unlike the Shack-Hartman wavefront sensor, it has nearly uniform sensitivity to all pupil modes. Optical configuration, theory and simulations for such a system will be discussed along with predicted performance.

wavefront sensors↗
NASA NTRS1989

High-frequency electrostatic waves near earth's bow shock

Electrostatic wave measurements from the Active Magnetospheric Particle Tracer Explorer Ion Release Module have been used to investigate the wave modes and their possible generation mechanisms in the earth's bow shock and magnetosheath. It is demonstrated that electrostatic waves are present in the bow shock and magnetosheath with frequencies above the maximum frequency for Doppler-shifted ion acoustic waves, yet below the plasma frequency. Waves in this frequency range are tentatively identified as electron beam mode waves. Data from 45 bow shock crossings are then used to investigate possible correlations between the electrostatic wave properties and the near-shock plasma parameters. The most significant relationships found are anticorrelations with Alfven Mach number and electron beta. Mechanisms which might produce electron beams in the shock and magnetosheath are discussed in terms of the correlation study results. These mechanisms include acceleration by the cross-shock electric field and by lower hybrid frequency waves. A magnetosheath 'time of flight' mechanism, in analogy to the electron foreshock region, is introduced as another possible beam generation mechanism.

Onsager, T. G.↗
DOE OSTI2015

Probing the interaction of ionic liquids with graphene using surface-enhanced Raman spectroscopy

We report an in situ measurement of the interaction of an imidazolium-based room temperature ionic liquid with both pure silver and a graphene-over-silver electrode under an applied electrochemical potential. At a negative applied potential, overall signal intensity increased indicating enhanced ionic liquid concentration at both silver and graphene electrodes. Vibrational modes associated with the imidazolium ring exhibited greater intensity enhancements and larger peak shifts compared with the anion indicating that the cation adsorbs with the ring and alkyl chain parallel to the electrode surface for both silver and graphene. In contrast to the silver, the surface enhanced Raman spectra of the ionic liquid near graphene showed shifts in the cation peaks even at no applied potential because of the strong π–π interaction between the ionic liquid and the graphene. Furthermore, the intensity of the graphene peak decreased in the presence of ionic liquid possibly due to the interaction between the ionic liquid and graphene. In conclusion, these results illustrate the effectiveness of surface-enhanced Raman spectroscopy to investigate electrolyte interactions with graphene at the liquid/electrode interface.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Multiparticle Simulations of Quadrupolar Ion Detection in an Ion Cyclotron Resonance Cell with Four Narrow Aperture Detection Electrodes

The current paradigm in FT-ICR cell design is to approximate the ideal three-dimensional quadratic trapping potential as closely as possible to maintain ion cloud spatial coherence and achieve long transients, either with hyperbolically shaped electrodes, shimming electrodes, or by dynamic harmonization. In sharp contrast, the FT-ICR analyzer cell with four narrow aperture detection electrodes (NADEL) introduces significant anharmonic terms to the trapping potential. This analyzer cell is capable of quadrupolar detection by which one can measure a signal that is close to the unperturbed cyclotron frequency. This is far less sensitive to trapping potential and space charge shifts than the reduced cyclotron frequency measured in conventional ICR cells. The quadrupolar mode of ion detection in NADEL cells has been examined previously by SIMION simulations of ion clouds with up to 500 ions per simulation. Here, the behavior of the NADEL analyzer cell is examined through particle-in-cell (PIC) simulations, which allows us to examine the behavior of large populations (tens of thousands) of ions with space charge considerations, and to calculate the induced charge on the NADEL detection electrodes, and thus the transient signal. PIC simulations confirm a unique spatial distribution of the ions, with a coherent motion that results in long transient signals. Dependence of the ion cloud and image current signal on cell design, ion energy, and magnetron radius are examined. Coalescence effects are compared with those found in a dynamically harmonized cell. The insensitivity of the measured cyclotron frequency to space-charge is demonstrated both with simulations and experimentally. .

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Longitudinal oscillations in a non-uniform spatially dispersive plasma

Longitudinal oscillations of the electron fluid in the hydrodynamic model of a metal are examined with pressure effects taken into account. It is well-known that this entails spatial dispersion. The equilibrium electron number density is taken to be non-uniform and a non-self-adjoint fourth order differential equation obeyed by the electric potential is derived. A velocity potential necessary for the description of sound waves is introduced in the standard fashion and the generalized version of Bloch orthogonality appropriate to a non-uniform background is deduced. We observe a duality between electric and velocity potentials in the sense that the respective differential operators are adjoint to each other. The spectrum is calculated in the special case of an exponential profile for the equilibrium electron number density. The surface plasmons are connected with the analytic properties of the scattering amplitude in the complex plane. The phase shift at threshold is expressed in terms of the number of surface plasmon modes via an expression reminiscent of Levinson’s statement in quantum mechanics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Quasielastic and Inelastic Neutron Scattering Study of Ultraconfined Water in Natural Mordenite ((Ca,Na 2 ,K 2 )Al 2 Si 10 O 24 ·7H 2 O)

Mordenite ((Ca,Na 2 ,K 2 )Al 2 Si 10 O 24 ·7H 2 O) is a natural and synthetic nanoporous zeolite containing several channels of different sizes in its structure. Because of this, its structure provides an important opportunity to study the relationship between confined and ultraconfined water as these channels have sizes between those typical of these water environments. In this study, the properties of water molecules in these environments were analyzed using inelastic and quasielastic neutron spectroscopy of a natural mordenite. The quasielastic spectra showed the presence of nonfreezing, mobile water molecules through the entire temperature range of the measurements, but very little anisotropy of the dynamics measured along and perpendicular to the channels. Faster and slower quasi-elastic neutron scattering (QENS) components may be consistent with the presence of two separate classes of water molecules in mordenite. Inelastic neutron spectroscopy also found no evidence of directional anisotropy. In conclusion, the strong intensity characteristic of neutron recoil on protons from highly mobile water molecules at low temperature (5 K), along with a significant shift of water librational band to lower energies and the O–H stretching modes to high energies, indicate that the hydrogen bonds acting on these water molecules in mordenite resemble those in liquid water rather than ice.

Crystal structure↗
DOE OSTI2015

Enhanced spin-phonon-electronic coupling in a 5d oxide

Enhanced coupling of material properties offers new fundamental insights and routes to multifunctional devices. In this context 5d oxides provide new paradigms of cooperative interactions that drive novel emergent behaviour. This is exemplified in osmates that host metal insulator transitions where magnetic order appears intimately entwined. Here we consider such a material, the 5d perovskite NaOsO 3 , and observe a coupling between spin and phonon manifested in a frequency shift of 40 cm 1, the largest measured in any material. The anomalous modes are shown to involve solely Os O interactions and magnetism is revealed as the driving microscopic mechanism for the phonon renormalization. The magnitude of the coupling in NaOsO 3 is primarily due to a property common to all 5d materials: the large spatial extent of the ion. This allows magnetism to couple to phonons on an unprecedented scale and in general offers multiple new routes to enhanced coupled phenomena in 5d materials.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Viscoelastic optical nonlocality of low-loss epsilon-near-zero nanofilms

Optical nonlocalities are elusive and hardly observable in traditional plasmonic materials like noble and alkali metals. Here we report experimental observation of viscoelastic nonlocalities in the infrared optical response of epsilon-near-zero nanofilms made of low-loss doped cadmium-oxide. The nonlocality is detectable thanks to the low damping rate of conduction electrons and the virtual absence of interband transitions at infrared wavelengths. We describe the motion of conduction electrons using a hydrodynamic model for a viscoelastic fluid, and find excellent agreement with experimental results. The electrons’ elasticity blue-shifts the infrared plasmonic resonance associated with the main epsilon-near-zero mode, and triggers the onset of higher-order resonances due to the excitation of electron-pressure modes above the bulk plasma frequency. We also provide evidence of the existence of nonlocal damping, i.e., viscosity, in the motion of optically-excited conduction electrons using a combination of spectroscopic ellipsometry data and predictions based on the viscoelastic hydrodynamic model.

42 ENGINEERING↗
DOE OSTI2014

Graphene-assisted near-field radiative heat transfer between corrugated polar materials

Graphene has attracted great attention in nanoelectronics, optics, and energy harvesting. Here, the near-field radiative heat transfer between graphene-covered corrugated silica is investigated based on the exact scattering theory. It is found that graphene can improve the radiative heat flux between silica gratings by more than one order of magnitude and alleviate the performance sensitivity to lateral shift. The underlying mechanism is mainly attributed to the improved photon tunneling of modes away from phonon resonances. Besides, coating with graphene leads to nonlocal radiative transfer that breaks Derjaguin's proximity approximation and enables corrugated silica to outperform bulk silica in near-field radiation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Ion-dust streaming instability with non-Maxwellian ions

The influence of non-Maxwellian ions on the ion-dust streaming instability in a complex plasma is investigated. The ion susceptibility employed for the calculations self-consistently accounts for the acceleration of the ions by a homogeneous background electric field and their collisions with neutral gas particles via a Bhatnagar-Gross-Krook collision term [e.g., A. V. Ivlev et al., Phys. Rev. E 71, 016405 (2005)], leading to significant deviations from a shifted Maxwellian distribution. The dispersion relation and the properties of the most unstable mode are studied in detail and compared with the Maxwellian case. The largest deviations occur at low to intermediate ion-neutral damping. In particular, the growth rate of the instability for ion streaming below the Bohm speed is found to be lower than in the case of Maxwellian ions, yet remains on a significant level even for fast ion flows above the Bohm speed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Resonant shattering flares as asteroseismic tests of chiral effective field theory

Chiral effective field theory (𝜒⁢EFT) has proved to be a powerful microscopic framework for predicting the properties of neutron-rich nuclear matter with quantified theoretical uncertainties up to about twice the nuclear saturation density. Tests of 𝜒⁢EFT predictions are typically performed at low densities using nuclear experiments, with neutron star (NS) constraints only being considered at high densities. In this work, we discuss how asteroseismic quasinormal modes within NSs could be used to constrain specific matter properties at particular densities not just the integrated quantities to which bulk NS observables are sensitive. We focus on the crust-core interface mode, showing that measuring this mode's frequency would provide a meaningful test of 𝜒⁢EFT at densities around half the saturation density. Conversely, we use nuclear matter properties predicted by 𝜒⁢EFT to estimate that this mode's frequency is around 185 ± 50Hz. Asteroseismic observables such as resonant phase shifts in gravitational-wave signals and multimessenger resonant shattering flare timings, therefore, have the potential to provide useful tests of 𝜒⁢EFT.

effective field theory↗
DOE OSTI2026

Effect of stress and temperature on zero group velocity Lamb modes

Zero group velocity (ZGV) Lamb wave resonances are highly localized and sensitive to changes in material properties, making them a viable option for probing residual stresses and elastic nonlinearity through acoustoelastic effects. Temperature changes also influence ZGV modes and need to be accounted for, particularly when measuring the small frequency shifts associated with acoustoelastic effects. We use a laser-based ultrasonic system to measure the third-order elastic constants of several materials. A temperature compensation scheme is devised to remove the effects of temperature fluctuations from the measurements. Furthermore, the measured third-order elastic constants are used to predict the temperature dependence of the ZGV resonances using thermo-acoustoelasticity theory, and these predictions are compared to experimental measurements. We find that thermo-acoustoelasticity theory was unable to reliably predict the shift in ZGV resonances with temperature. These results could have important implications in understanding the limitations of thermo-acoustoelasticity theory and in developing precision ZGV resonance-based systems to detect and quantify residual stress in parts, a particularly relevant problem in additive manufacturing techniques including powder bed fusion and directed energy deposition.

Engineering↗
DOE OSTI2023

Understanding the Inner-Workings of Language Models Through Representation Dissimilarity

We use model stitching to understand the internal representations of language models. Similar to vision models, we find that "more is better," and representations learned with more data and larger width can improve the performance of weaker models via stitching. We likewise find that certain architecture choices, using GeLU vs SoLU activation functions, influence the quality of learned representations. Finally, model stitching (as opposed to other model diagnostic methods, like mode connectivity) can localize the different generalization strategies of text classifiers under domain shift to certain hidden layers.

Brown, Davis R.↗