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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 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Low-frequency shear Alfvén waves at DIII-D: Theoretical interpretation of experimental observations

The linear properties of the low-frequency shear Alfvén waves such as those associated with the beta-induced Alfvén eigenmodes (BAEs) and the low-frequency modes observed in reversed-magnetic-shear DIII-D discharges [W. Heidbrink et al., Nucl. Fusion 61, 066031 (2021)] are theoretically investigated and delineated based on the theoretical framework of the general fishbone-like dispersion relation (GFLDR). By adopting representative experimental equilibrium profiles, it is found that, even though both modes are predominantly of Alfvénic polarization, the low-frequency mode is a reactive unstable mode with weak coupling to the energetic particles, while the BAE involves a dissipative instability due to resonant excitation by the energetic ions. Thus, the low-frequency mode is more appropriately called a low-frequency Alfvén mode (LFAM). Moreover, the ascending frequency spectrum patterns of the experimentally observed BAEs and LFAMs can be theoretically reproduced by varying qmin and also be well interpreted based on the GFLDR. The present analysis illustrates the solid predictive capability of the GFLDR and its practical usefulness in enhancing the interpretative capability of both experimental and numerical simulation results.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Analytical harmonic vibrational frequencies with VV10-containing density functionals: Theory, efficient implementation, and benchmark assessments

VV10 is a powerful nonlocal density functional for long-range correlation that is used to include dispersion effects in many modern density functionals, such as the meta-generalized gradient approximation (mGGA), B97M-V, the hybrid GGA, ωB97X-V, and the hybrid mGGA, ωB97M-V. While energies and analytical gradients for VV10 are already widely available, this study reports the first derivation and efficient implementation of the analytical second derivatives of the VV10 energy. The additional compute cost of the VV10 contributions to analytical frequencies is shown to be small in all but the smallest basis sets for recommended grid sizes. Here, this study also reports the assessment of VV10-containing functionals for predicting harmonic frequencies using the analytical second derivative code. The contribution of VV10 to simulating harmonic frequencies is shown to be small for small molecules but important for systems where weak interactions are important, such as water clusters. In the latter cases, B97M-V, ωB97M-V, and ωB97X-V perform very well. The convergence of frequencies with respect to the grid size and atomic orbital basis set size is studied, and recommendations are reported. Finally, scaling factors to allow comparison of scaled harmonic frequencies with experimental fundamental frequencies and to predict zero-point vibrational energy are presented for some recently developed functionals (including r2SCAN, B97M-V, ωB97X-V, M06-SX, and ωB97M-V).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Stabilisation of a fibre frequency synthesiser using acousto-optical and electro-optical modulators

A fibre-optic frequency synthesiser is developed that is stabilised to the optical frequency standard based on molecular iodine (Nd : YAG/I{sub 2}). The possibility of transferring stability of the optical frequency standard to other optical frequencies in the IR range 1−2 μm and to the RF range by using synthesiser phase-locked loops (PLLs) with acousto-optical and electro-optical modulators is experimentally demonstrated. The additive instability introduced into the optical frequency comb of the synthesiser (which arises due to PLL residual random errors) is several orders less than the intrinsic instability of the reference optical frequency standard employed (i.e., is noticeably less than 1×10{sup −13} for 1 s and 5×10{sup −15} for 1000 s). (paper)

42 ENGINEERING↗
DOE OSTI2020

Two-frequency sub-Doppler spectroscopy of the caesium D{sub 1} line in various configurations of counterpropagating laser beams

Sub-Doppler resonances in caesium vapours are studied in a laser field produced by counterpropagating two-frequency light beams with mutually orthogonal linear polarisations. The beams are in resonance with optical transitions in the D{sub 1} line, the frequency difference of the field spectral components being equal to the hyperfine ground-state splitting in the Cs atom (∼9.2 GHz). It has already been shown that in this configuration, the hypercontrast effect can be observed for sub-Doppler resonances, which makes this configuration promising for the employment in new-generation miniature optical frequency standards. In the present work, two different two-frequency configurations are compared with each other and with the single-frequency configuration widely used in practice for observing saturated absorption resonances. The parameters of nonlinear resonances are measured at various temperatures of caesium vapours and at different optical field intensities. The results of the investigations performed make it possible to find an optimal two-frequency scheme for exciting nonlinear resonances and to estimate a potential of the scheme for its applications in quantum metrology. (laser spectroscopy of atoms)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

HIGH-FREQUENCY GRAVITATIONAL WAVES FROM SUPERMASSIVE BLACK HOLES: PROSPECTS FOR LIGO-VIRGO DETECTIONS

It is commonly assumed that ground-based gravitational wave (GW) instruments will not be sensitive to supermassive black holes (SMBHs) because the characteristic GW frequencies are far below the {approx}10-1000 Hz sensitivity bands of terrestrial detectors. Here, however, we explore the possibility of SMBH GWs to leak to higher frequencies. In particular, if the high-frequency spectral tail asymptotes to h-tilde (f){proportional_to}f{sup -{alpha}}, where {alpha} {<=} 2, then the spectral amplitude is a constant or increasing function of the mass M at a fixed frequency f >> c {sup 3}/GM. This will happen if the time-domain waveform or its derivative exhibits a discontinuity. Ground-based instruments could search for these universal spectral tails to detect or rule out such features irrespective of their origin. We identify the following processes which may generate high-frequency signals: (1) gravitational bremsstrahlung of ultrarelativistic objects in the vicinity of an SMBH, (2) ringdown modes excited by an external process that has a high-frequency component or terminates abruptly, and (3) gravitational lensing echoes and diffraction. We estimate the order of magnitude of the detection signal-to-noise ratio for each mechanism (1, 2, and 3) as a function of the waveform parameters. In particular for (3), SMBHs produce GW echoes of inspiraling stellar mass binaries in galactic nuclei with a delay of a few minutes to hours. The lensed primary signal and GW echo are both amplified if the binary is within a {approx}10 deg (r/100M){sup -1/2} cone behind the SMBH relative to the line of sight at a distance r from the SMBH. For the rest of the binaries near SMBHs, the amplitude of the GW echo is {approx}0.1(r/100M){sup -1} of the primary signal on average.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

PULSATION FREQUENCIES AND MODES OF GIANT EXOPLANETS

We calculate the eigenfrequencies and eigenfunctions of the acoustic oscillations of giant exoplanets and explore the dependence of the characteristic frequency {nu}{sub 0} and the eigenfrequencies on several parameters: the planet mass, the planet radius, the core mass, and the heavy element mass fraction in the envelope. We provide the eigenvalues for degree l up to 8 and radial order n up to 12. For the selected values of l and n, we find that the pulsation eigenfrequencies depend strongly on the planet mass and radius, especially at high frequency. We quantify this dependence through the calculation of the characteristic frequency {nu}{sub 0} which gives us an estimate of the scale of the eigenvalue spectrum at high frequency. For the mass range 0.5 M{sub J} {<=} M{sub P} {<=} 15 M{sub J} , and fixing the planet radius to the Jovian value, we find that {nu}{sub 0} {approx} 164.0 Multiplication-Sign (M{sub P} /M{sub J} ){sup 0.48}{mu}Hz, where M{sub P} is the planet mass and M{sub J} is Jupiter's mass. For the radius range from 0.9 to 2.0 R{sub J} , and fixing the planet's mass to the Jovian value, we find that {nu}{sub 0} {approx} 164.0 Multiplication-Sign (R{sub P} /R{sub J} ){sup -2.09}{mu}Hz, where R{sub P} is the planet radius and R{sub J} is Jupiter's radius. We explore the influence of the presence of a dense core on the pulsation frequencies and on the characteristic frequency of giant exoplanets. We find that the presence of heavy elements in the envelope affects the eigenvalue distribution in ways similar to the presence of a dense core. Additionally, we apply our formalism to Jupiter and Saturn and find results consistent with both the observational data of Gaulme et al. and previous theoretical work.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Consequences of dynamical disruption and mass segregation for the binary frequencies of star clusters

The massive (13,000-26,000 M {sub ☉}) and young (15-30 Myr) Large Magellanic Cloud star cluster NGC 1818 reveals an unexpected increasing binary frequency with radius for F-type stars (1.3-2.2 M {sub ☉}). This is in contrast to many older star clusters that show a decreasing binary frequency with radius. We study this phenomenon with sophisticated N-body modeling, exploring a range of initial conditions, from smooth virialized density distributions to highly substructured and collapsing configurations. We find that many of these models can reproduce the cluster's observed properties, although with a modest preference for substructured initial conditions. Our models produce the observed radial trend in binary frequency through disruption of soft binaries (with semi-major axes, a ≳ 3000 AU), on approximately a crossing time (∼5.4 Myr), preferentially in the cluster core. Mass segregation subsequently causes the binaries to sink toward the core. After roughly one initial half-mass relaxation time (t {sub rh}(0) ∼ 340 Myr) the radial binary frequency distribution becomes bimodal, the innermost binaries having already segregated toward the core, leaving a minimum in the radial binary frequency distribution that marches outward with time. After 4-6 t {sub rh}(0), the rising distribution in the halo disappears, leaving a radial distribution that rises only toward the core. Thus, both a radial binary frequency distribution that falls toward the core (as observed for NGC 1818) and one that rises toward the core (as for older star clusters) can arise naturally from the same evolutionary sequence owing to binary disruption and mass segregation in rich star clusters.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Frequency and damping rate of fast sausage waves

We investigate the frequency and damping rate of fast axisymmetric waves that are subject to wave leakage for a one-dimensional magnetic cylindrical structure in the solar corona. We consider the ideal magnetohydrodynamic (MHD) dispersion relation for axisymmetric MHD waves superimposed on a straight magnetic cylinder in the zero β limit, similar to a jet or loop in the solar corona. An analytic study accompanied by numerical calculations has been carried out to model the frequency, damping rate, and phase speed of the sausage wave around the cut-off frequency and in the long wavelength limit. Analytic expressions have been obtained based on equations around the points of interest. They are linear approximations of the dependence of the sausage frequency on the wave number around the cut-off wavelength for both leaky and non-leaky regimes and in the long wavelength limit. Moreover, an expression for the damping rate of the leaky sausage wave has been obtained both around the cut-off frequency and in the long wavelength limit. These analytic results are compared with numerical computations. The expressions show that the complex frequencies are mainly dominated by the density ratio. In addition, it is shown that the damping eventually becomes independent of the wave number in the long wavelength limit. We conclude that the sausage mode damping directly depends on the density ratios of the internal and external media where the damping declines in higher density contrasts. Even in the long wavelength limit, the sausage mode is weakly damped for high-density contrasts. As such, sausage modes could be observed for a significant number of periods in high-density contrast loops or jets.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Synthesizing exoplanet demographics from radial velocity and microlensing surveys. II. The frequency of planets orbiting M dwarfs

In contrast to radial velocity (RV) surveys, results from microlensing surveys indicate that giant planets with masses greater than the critical mass for core accretion (∼0.1 M {sub Jup}) are relatively common around low-mass stars. Using the methodology developed in the first paper, we predict the sensitivity of M-dwarf RV surveys to analogs of the population of planets inferred by microlensing. We find that RV surveys should detect a handful of super-Jovian (>M {sub Jup}) planets at the longest periods being probed. These planets are indeed found by RV surveys, implying that the demographic constraints inferred from these two methods are consistent. Finally, we combine the results from both methods to estimate planet frequencies spanning wide regions of parameter space. We find that the frequency of Jupiters and super-Jupiters (1 ≲ m{sub p} sin i/M {sub Jup} ≲ 13) with periods 1 ≤ P/days ≤ 10{sup 4} is f{sub J}=0.029{sub −0.015}{sup +0.013}, a median factor of 4.3 (1.5-14 at 95% confidence) smaller than the inferred frequency of such planets around FGK stars of 0.11 ± 0.02. However, we find the frequency of all giant planets with 30 ≲ m{sub p} sin i/M {sub ⊕} ≲ 10{sup 4} and 1 ≤ P/days ≤ 10{sup 4} to be f{sub G}=0.15{sub −0.07}{sup +0.06}, only a median factor of 2.2 (0.73-5.9 at 95% confidence) smaller than the inferred frequency of such planets orbiting FGK stars of 0.31 ± 0.07. For a more conservative definition of giant planets (50 ≲ m{sub p} sin i/M {sub ⊕} ≲ 10{sup 4}), we find f{sub G{sup ′}}=0.11±0.05, a median factor of 2.2 (0.73-6.7 at 95% confidence) smaller than that inferred for FGK stars of 0.25 ± 0.05. Finally, we find the frequency of all planets with 1 ≤ m{sub p} sin i/M {sub ⊕} ≤ 10{sup 4} and 1 ≤ P/days ≤ 10{sup 4} to be f{sub p} = 1.9 ± 0.5.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Simulation of Alfv n frequency cascade modes in reversed shear discharges using a Landau-closure model

The dynamics of energetic particle destabilized Alfve n frequency sweeping modes in tokamak reversed-shear safety factor discharges are modelled using a new Landau-closure model that includes coupling to geodesic acoustic wave dynamics and closure relations optimized for energetic particle Alfve n mode resonances. Profiles and equilibria are based upon reconstructions of a DIII-D discharge (#142111) in which a long sequence of frequency sweeping modes were observed. This model (TAEFL) has recently been included in a verification and validation study of n = 3 frequency sweeping modes for this case along with two gyrokinetic codes, GTC and GYRO. This paper provides a more detailed documentation of the equations and methods used in the TAEFL model and extends the earlier calculation to a range of toroidal mode numbers: n = 2 to 6. By considering a range of toroidal mode numbers and scanning over a range of safety factor profiles with varying qmin, both up-sweeping frequency (reversed-shear Alfve n eigenmode) and down-sweeping frequency (toriodal Alfve n eigenmode) modes are present in the results and show qualitative similarity with the frequency variations observed in the experimental spectrograms.

shear-Alfven instabiltiy tokamak Landau-closure Al↗
DOE OSTI2016

High frequency pacing of edge localized modes by injection of lithium granules in DIII-D H-mode discharges

A newly installed Lithium Granule Injector (LGI) was used to pace edge localized modes (ELM) in DIII-D. ELM pacing efficiency was studied injecting lithium granules of nominal diameter 0.3–0.9mm, speed of 50–120 m s -1 and average injection rates up to 100 Hz for 0.9mm granules and up to 700 Hz for 0.3mm granules. The efficiency of ELM triggering was found to depend strongly on size of the injected granules, with triggering efficiency close to 100% obtained with 0.9mm diameter granules, lower with smaller sizes, and weakly depending on granule velocity. Robust ELM pacing was demonstrated in ITER-like plasmas for the entire shot length, at ELM frequencies 3–5 times larger than the ‘natural’ ELM frequency observed in reference discharges. Within the range of ELM frequencies obtained, the peak ELM heat flux at the outer strike point was reduced with increasing pacing frequency. The peak heat flux reduction at the inner strike point appears to saturate at high pacing frequency. Lithium was found in the plasma core, with a concurrent reduction of metallic impurities and carbon. Altogether, high frequency ELM pacing using the lithium granule injection appears to be compatible with both H-mode energy confinement and attractive H-mode pedestal characteristics, but further assessment is need

42 ENGINEERING↗
DOE OSTI2024

Study of synchronous RF pulsing in dual frequency capacitively coupled plasma

Low-pressure multi-frequency capacitively coupled plasmas (CCPs) are used for numerous etch and deposition applications in the semiconductor industry. Pulsing of the radio-frequency (RF) sources enables control of neutral and charged species in the plasma on a millisecond timescale. The synchronous (i.e. simultaneous, in-phase) pulsing of both power sources in a dual frequency CCP is examined in this article. Due to the low gas pressure, modeling has been done using the electrostatic particle-in-cell/Monte Carlo collision method. Here, the objective of this work is to investigate the sensitivity of the plasma properties to small changes in timing during synchronous pulsing of the two RF sources. It is demonstrated that small deviations in the on and off times of the two RF sources can lead to major changes in the plasma characteristics. This high sensitivity is of concern for process repeatability but can be utilized to enable better control of the dynamics of plasma-surface interaction. In the simulations, the pulsing parameters (on and off times and ramp rates) are varied and the temporal evolution of plasma characteristics such as electron density (n e ), species current at the electrode, and electron temperature are examined. It is demonstrated that if the low-frequency (LF) source is turned off a few μs before (or after) the high-frequency source, n e during the off-state is significantly higher (or lower) due to the frequency coupling effect. Similarly, turning on the LF source with a small delay results in a sharp increase in the plasma density when the RF sources are turned on.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

An underestimated role of precipitation frequency in regulating summer soil moisture

Soil moisture induced droughts are expected to become more frequent under future global climate change. Precipitation has been previously assumed to be mainly responsible for variability in summer soil moisture. However, little is known about the impacts of precipitation frequency on summer soil moisture, either interannually or spatially. To better understand the temporal and spatial drivers of summer drought, 415 site yr measurements observed at 75 flux sites world wide were used to analyze the temporal and spatial relationships between summer soil water content (SWC) and the precipitation frequencies at various temporal scales, i.e., from half-hourly, 3, 6, 12 and 24 h measurements. Summer precipitation was found to be an indicator of interannual SWC variability with r of 0.49 (p < 0.001) for the overall dataset. However, interannual variability in summer SWC was also significantly correlated with the five precipitation frequencies and the sub-daily precipitation frequencies seemed to explain the interannual SWC variability better than the total of precipitation. Spatially, all these precipitation frequencies were better indicators of summer SWC than precipitation totals, but these better performances were only observed in non-forest ecosystems. Our results demonstrate that precipitation frequency may play an important role in regulating both interannual and spatial variations of summer SWC, which has probably been overlooked or underestimated. However, the spatial interpretation should carefully consider other factors, such as the plant functional types and soil characteristics of diverse ecoregions.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2010

INDEPENDENT SIGNALS FROM THE INFLUENCE OF INTERNAL MAGNETIC LAYERS ON THE FREQUENCIES OF SOLAR p-MODES

The discovery that p-mode frequencies of low degree do not follow changes of solar surface activity during the recent solar minimum offers the possibility of a new diagnostic signature of the responsible pressure perturbation in the wave guiding medium, potentially rich of information regarding the structure of the Sun and the cause of the unusually long solar minimum. Magnetic fields, as well as temperature changes, introduce equilibrium pressure deviations that modify the resonant frequencies of p-mode oscillations. Assuming the perturbation to be caused by a horizontal layer of magnetic field located in a plane-stratified model of the Sun, we compile analytical frequency shifts and process them to allow direct comparison with observations. The effect of magnetism itself on the central p-mode frequencies can be neglected in comparison with the thermal effect of a perturbative layer buried in the solar interior. A parametric study shows that a layer as thin as 2100 km at subsurface depths is able to reproduce reported mean anomalous frequency shifts (not correlated with the surface activity), while a layer of size around 4200 km increasing by a small amount at depths near 0.08 R {sub sun} can explain individual low-degree shifts. It is also possible to obtain the mean shifts via the upward motion through depths near 0.03 R {sub sun} of a rising perturbative layer of thickness around 7000 km. Hence, the anomalous frequency shifts are best explained by thermal effects in the upper regions of the convection zone. The effects of latitudinal distribution are not treated here.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

THE LOW-FREQUENCY CHARACTERISTICS OF PSR J0437–4715 OBSERVED WITH THE MURCHISON WIDE-FIELD ARRAY

We report on the detection of the millisecond pulsar PSR J0437–4715 with the Murchison Wide-field Array (MWA) at a frequency of 192 MHz. Our observations show rapid modulations of pulse intensity in time and frequency that arise from diffractive scintillation effects in the interstellar medium (ISM), as well as prominent drifts of intensity maxima in the time-frequency plane that arise from refractive effects. Our analysis suggests that the scattering screen is located at a distance of ∼80-120 pc from the Sun, in disagreement with a recent claim that the screen is closer (∼10 pc). Comparisons with higher frequency data from Parkes reveal a dramatic evolution of the pulse profile with frequency, with the outer conal emission becoming comparable in strength to that from the core and inner conal regions. As well as demonstrating the high time resolution science capabilities currently possible with the MWA, our observations underscore the potential to conduct low-frequency investigations of timing-array millisecond pulsars, which may lead to increased sensitivity in the detection of nanoHertz gravitational waves via the accurate characterization of ISM effects.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2026

Machine learning for seismic low-frequency extrapolation

The cycle-skipping problem that plagues full waveform inversion (FWI) can be at least partially mitigated if low frequencies (which encode the kinematics of wave propagation in seismic data) are recorded. However, seismic sources and receivers are band-limited, so seismic data does not generally include signals down to 0 Hz. To improve our ability to solve the seismic inverse problem, one can synthesize this missing low-frequency (LF) content from the recorded high-frequency (HF) data using machine learning (ML) models. Deep learning models such as convolutional neural networks (CNNs) demonstrate impressive ability to perform low frequency extrapolation. However, such models require powerful hardware (GPU machines) and careful training. We assess the extrapolation capabilities of three different ML models that do not require GPU machines, namely, random forest, Gaussian process regression and gradient boosting, on both synthetic and real data. Experimental results on two synthetic data sets (generated from a low velocity lens embedded in a homogeneous medium, and the Marmousi model) demonstrate that FWI applied to the extrapolated data consistently improves inversion accuracy relative to FWI applied to the original data sets that do not contain low frequencies. Application of low-frequency extrapolation to real data from the Northwest Shelf of Australia demonstrates that tree-based ML models such as gradient boosting can outperform CNNs in terms of both accuracy and computational cost on non-GPU architectures.

58 GEOSCIENCES↗
DOE OSTI2011

Elimination of the Stark shift from the vibrational transition frequency of optically trapped {sup 174}Yb{sup 6}Li molecules

Transition frequencies of cold molecules must be accurately evaluated to test the variance in the proton-to-electron mass ratio. Measuring the X {sup 2}{Sigma}(v,N)=(0,0){yields}(1,0) transition frequency of optically trapped {sup 174}Yb{sup 6}Li molecules is a promising method for achieving this goal. The Stark shifts induced by trap and probe (for the Raman transition) lasers are eliminated by choosing appropriate frequencies (magic frequencies) during the construction of the optical lattice. In the far-off resonance region, the Stark shift is found to be less than 10{sup -16} even when the laser frequencies are detuned from the magic frequencies by {approx}1 MHz.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2024

Extraordinary frequency stabilization by resonant nonlinear mode coupling

Here, we show that a self-sustained oscillator with a frequency-selective element operating with two nonlinearly coupled modes can achieve a level of frequency stability well beyond that available using single-mode operation. The system of interest consists of a self-sustained oscillator based on a nonlinear primary mode that is coupled via an internal resonance to a passive secondary mode. Analysis of a generic model for this resonance with both additive and multiplicative noises reveals that the stability improvements accrue from two sources: (i) nonlinear frequency veering in the primary mode, a classical analogue to quantum-level repulsion, that eliminates amplitude-to-frequency noise conversion; and (ii) phase cleaning of the oscillator through an intrinsic phase constraint arising from synchronization of the modes. This latter effect can significantly reduce the effects of intrinsic frequency fluctuations of the primary mode, which are not accessible by any known strategy using single-mode operation. The theoretical predictions are supported by experimental measurements of a microelectromechanical systems-based oscillator that demonstrate a reduction in oscillator line width of several orders of magnitude. This approach offers a means of optimizing frequency stability in self-sustained oscillators, which has direct implications for applications in timekeeping and sensing.

36 MATERIALS SCIENCE↗