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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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Coupling Between Electrons and Charge Density Wave Fluctuation and its Possible Role in Superconductivity

In most charge density wave (CDW) systems of different material classes, ranging from traditional correlated systems in low-dimension to recent topological systems with Kagome lattice, superconductivity emerges when the system is driven toward the quantum critical point (QCP) of CDW via external parameters of doping and pressure. Despite this rather universal trend, the essential hinge between CDW and superconductivity has not been established yet. Here, the evidence of coupling between electron and CDW fluctuation is reported, based on a temperature- and intercalation-dependent kink in the angle-resolved photoemission spectra of 2H-Pd x TaSe 2 . Kinks are observed only when the system is in the CDW phase, regardless of whether a long- or short-range order is established. Notably, the coupling strength is enhanced upon long-range CDW suppression, albeit the coupling energy scale is reduced. Interestingly, the estimation of the superconducting critical temperature by incorporating the observed coupling characteristics into McMillan's equation yields results closely resembling the known values of the superconducting dome. The results thus highlight a compelling possibility that this new coupling mediates Cooper pairs, which provides new insights into the competing relationship not only for CDW but also for other competing orders.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Investigation of magnetic fluctuations in L-H and H-L transition dynamics on DIII-D

The dynamics of the L-H transition is not fully understood, with many parameters changing the threshold power to enter H-mode and the self-regulation between zonal flows and turbulence in the plasma edge. This paper is primarily a presentation of experimental results for DIII-D L-H and H-L transitions and speculation on the observations made. Power threshold analysis and measurements of pedestal temperatures for these transitions are presented. A comparison is made between an L-H transition and H-L transition of comparable Psep exhibiting oscillatory behaviour, showing symmetry between forward and backward transition dynamics. This paper shows the first observations of magnetic fluctuations during L-H and H-L transitions on DIII-D, and shows that L-H and H-L transitions have similar magnetic fluctuation dynamics. Information geometry analysis has been performed on measurements of plasma density fluctuations, perpendicular plasma velocity fluctuations, and magnetic field fluctuations to investigate the self-regulation and evolution of these variables during the transitions. Perpendicular flow evolution is shown to dominate the transition dynamics in both directions, but self-regulation behaviour is observed between all three variables. A strong correlation between magnetic fluctuation information rate and density fluctuation information rate for these two shots shows the strong influence of magnetic behaviour on both the L-H and H-L transition, and that these transition dynamics necessarily include electromagnetic effects.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1980

Second moments of optical degradation due to a thin turbulent layer

The effect of thin turbulent layers, including boundary layers and shear layers, on light propagation is examined from a theoretical point of view. In particular, a mathematical model is developed to describe the interaction between the aerodynamic or, more precisely, the density fluctuations and the electromagnetic field. It is assumed that the turbulence induces a normally distributed phase aberration which is a homogeneous random function in the plane of the aperture. Hypotheses concerning the density fluctuations in the layer sufficient to guarantee such a phase aberration are exhibited. The optical degradation is described in terms of the optical transfer function (OTF) and the Strehl ratio (I/I O ) which are random. Expressions for the first and second moments of these two parameters are developed from the definitions. Asymptotic ("large" aperture) approximations to these expressions are derived and discussed. Finally, the exact and approximate results are compared for several "typical" values of the ratios of aperture diameter to scale of density fluctuations and rms phase aberration to wave length respectively.

W J Steinmetz↗
NASA NTRS1999

A Cutoff in the X-Ray Fluctuation Power Density Spectrum of the Seyfert 1 Galaxy NGC 3516

During 1997 March-July, RXTE observed the bright, strongly variable Seyfert 1 galaxy NGC 3516 once every approx. 12.8 hr for 4.5 months and nearly continuously (with interruptions due to SAA passage but not Earth occultation) for a 4.2 day period in the middle. These were followed by ongoing monitoring once every approx. 4.3 days. These data are used to construct the first well-determined X-ray fluctuation power density spectrum (PDS) of an active galaxy to span more than 4 decades of usable temporal frequency. The PDS shows no signs of any strict or quasi-periodicity, but does show a progressive flattening of the power-low slope from -1.74 at short time scales to -0.73 at longer time scales. This is the clearest observation to date of the long-predicted cutoff in the PDS. The characteristic variability time scale corresponding to this cutoff temporal frequency is approx. 1 month. Although it is unclear how this time scale may be interpreted in terms of a physical size or process, there are several promising candidate models. The PDS appears similar to those seen for Galactic black hole candidates such as Cyg X-1, suggesting that these two classes of objects with very different luminosities and putative black hole masses (differing by more than a factor of 10(exp 5)) may have similar X-ray generation processes and structures.

Edelson, Rick↗
NASA NTRS1995

Observations of the solar wind speed near the sun

Two-antenna scintillation (IPS) observations can provide accurate measurements of the velocity with which electron density fluctuations drift past the line of sight. We will present recent IPS measurements made with the EISCAT and VLBA arrays. It is common, particularly during declining activity. for the line of sight to pass through plasma with a wide range of speed. Therefore it is important to account for the line of sight integration. It is clear from ULYSSES measurements that the speed is bimodal in nature, i.e., either 'fast' or 'slow.' Thus it is not necessary to model a continuous velocity distribution - one need only locate the 'fast-slow' interface. In addition one must consider the possibility that the density fluctuations are moving with respect to the flow of particles. Alfven waves propagating through field-aligned density fluctuations can mimic a sound wave in this respect, so the apparent IPS velocity can be the flow speed plus the Alfven speed. In modeling the IPS it is important that the scattering be 'weak,' because the weak scattering model requires only 1 spatial parameter instead of 3. Furthermore the effect of multiple velocities in much more distinct in weak scattering. EISCAT can only operate near 933 MHz, which limits the observations to outside of 17R(solar mass). The VLBA is the only facility with the combination of high frequency operation and long baselines required to observe inside of 15R(solar mass). A simple bimodal model has been successfully used to interpret our IPS observations near the sun. Farther out interaction regions have built up significantly and a two speed model is no longer valid. An apparent deceleration in the fast polar wind is sometimes evident when compared to the ULYSSES observation. The density variance delta N(exp 2)e in the fast wind appears to decrease from equator to pole.

Grall, R. R.↗
DOE OSTI2024

Millimeter-wave high-wavenumber scattering diagnostic developments on EAST and NSTX-U

A pioneering 4-channel, high-k poloidal, millimeter-wave collective scattering system has been successfully developed for the Experimental Advanced Superconducting Tokamak (EAST). Engineered to explore high-k electron density fluctuations, this innovative system deploys a 270 GHz mm-wave probe beam launched from Port K and directed toward Port P (both ports lie on the midplane and are 110° part), where large aperture optics capture radiation across four simultaneous scattering angles. Tailored to measure density fluctuations with a poloidal wavenumber of up to 20 cm -1 , this high-k scattering system underwent rigorous laboratory testing in 2023, and the installation is currently being carried out on EAST. Its primary purpose lies in scrutinizing ion and electron-scale instabilities, such as the electron temperature gradient (ETG) mode, by furnishing measurements of the k θ (poloidal wavenumber) spectrum. This advancement significantly bolsters the capacity to probe high-k electron density fluctuations within the framework of EAST. Finally, beam tracing and data interpretation modules developed for both EAST and NSTX-U high-k scattering diagnostics are described.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1979

The applicability of Brillouin scattering to flow field diagnostics

To fill the void between turbulence theory and experiment; particularly in the flow fields consisting of monatomic gases, for example in wind tunnels, means of measuring fluctuating quantities are needed. In the area of density fluctuation measurement, the optical method of Brillouin scattering was suggested. This was based on the theory, that the Brillouin scattered intensity is proportional to a function of density. In this investigation the potential of this method as a diagnostic tool was studied. Here the density fluctuations in gases were sought. Continuous wave lasers and interferometers were used as the primary illuminating source and scattered light filters respectively.

Laiosa, J.↗
DOE OSTI2025

Development and preliminary results of 270 GHz microwave forward scattering diagnostic system on the experimental advanced superconducting tokamak (EAST)

To measure localized (intermediate, high) poloidal wavenumber electron density fluctuations, a tangential millimeter-wave collective scattering system has been designed and successfully developed on the experimental advanced superconducting tokamak (EAST). This innovative system employs a 270 GHz mm-wave probe beam, emitted from the mid-plane of Port K and directed towards Port P after passing through a combination of two in-vessel mirrors. Here these two ports are located on the EAST device’s midplane and are 110° apart. The downward scattered signals pass through two in-vessel mirrors, and exit through the P window. The received multi-channel separated scattering signals are arranged along the poloidal direction. This optical arrangement makes the monitored turbulent wave number is poloidal dominant and up to 40 cm −1 . The tangential microwave scattering scheme enables this diagnostic to monitor local density fluctuations, with typically a radial spatial resolution △ R = 5 cm, and a wavenumber resolution of △ k = 0.4 cm −1 . The scattering system underwent comprehensive laboratory testing in 2023, with installation finalized in 2024. The system features steerable launch and receiver optics, allowing remote control to position the scattering volume from normalized radius ρ –0 to the pedestal region on a shot-by-shot basis. Effective experimental data, characterized by distinct off-center spectral peaks, have been successfully obtained in neutral beam-heated plasmas on EAST. The beam tracing forward modeling has been employed for experimental data interpretation. The advanced millimeter-wave scattering system provides fluctuation measurement capability from ion temperature gradient to electron temperature gradient scale on low field side. Preliminary experimental density fluctuation data have been successfully obtained in neutral beam-heated plasmas on EAST. The localized measurement will be used for kinetic turbulence transport numerical simulation validation.

high-k collective scattering↗
NASA NTRS2018

ULF Wave-Associated Density Irregularities and Scintillation at the Equator

This paper presents independent multi-instrument observations that address the physical mechanisms of how ultralow-frequency (ULF) wave-associated electric fields initiate ionospheric density fluctuation and scintillation at the equator. Since the magnetic field at the equator is entirely embedded in a relatively high-collision and high-conductivity medium, the condition may not be possible for the geomagnetic field to fluctuate due to ULF wave activity. This implies that the fluctuating electric field at the equator may not be produced through equatorial dynamo action due to fluctuating magnetic fields. Instead, the electric field penetrates from high latitudes and produces fluctuating magnetic field as well as modulates the vertical drift and hence causes the density to fluctuate at the equatorial region. We demonstrate this by estimating the ULF associated fluctuating electric field at high latitudes and at the equatorial region by applying the appropriate attenuation factor as it penetrates to lower latitudes. The periodicity of both electric field and density fluctuations appears to be between 6 and 9 min, which is a typical period of ULF waves in the Pc5 range. Because of its large amplitude and long periods compared to other ULF wave frequency bands, the Pc5 wave-associated electric field, which can even be estimated using magnetograms with low sensitivity and low sampling rate (e.g., 1 min), can easily penetrate to the lower latitude region and produce significant ionospheric density fluctuations that can be strong enough to create scintillation at the equatorial region.

Yizengaw, E.↗
DOE OSTI2025

Atacama Cosmology Telescope: Multiprobe cosmology with unWISE galaxies and ACT DR6 CMB lensing

We present a joint analysis of the cosmic microwave background (CMB) lensing power spectra measured from the Data Release 6 of the Atacama Cosmology Telescope (ACT) and Planck PR4, cross-correlations between the ACT and Planck lensing reconstruction and galaxy clustering from unWISE, and the unWISE clustering auto-spectrum. We obtain 1.5% constraints on the matter density fluctuations at late times parametrized by the best constrained parameter combination 𝑆$^{3⁢x⁢2⁢pt}_{8}$ ≡ 𝜎 8 ⁢(Ω 𝑚 /0.3) 0.4 = 0.815 ± 0.012. The commonly used 𝑆 8 ≡ 𝜎 8⁢ (Ω 𝑚 /0.3) 0.5 parameter is constrained to 𝑆 8 = 0.816 ± 0.015. In combination with baryon acoustic oscillation (BAO) measurements we find 𝜎 8 = 0.815 ± 0.012. We also present sound-horizon-independent estimates of the present day Hubble rate of 𝐻 0 = 66.4$^{+3.2}_{−3.7}$ km s −1 Mpc −1 from our large scale structure data alone and 𝐻 0 = 64.3$^{+2.1}_{−2.4}$ km s −1 Mpc −1 in combination with uncalibrated supernovae from Pantheon+. Using parametric estimates of the evolution of matter density fluctuations, we place constraints on cosmic structure in a range of high redshifts typically inaccessible with cross-correlation analyses. Combining lensing cross- and autocorrelations, we derive a 3.3% constraint on the integrated matter density fluctuations above 𝑧 = 2.4, one of the tightest constraints in this redshift range and fully consistent with a Λ cold dark matter (Λ⁢CDM) model fit to the primary CMB from Planck. Finally, combining with primary CMB observations and using the extended low redshift coverage of these combined datasets we derive constraints on a variety of extensions to the Λ⁢CDM model including massive neutrinos, spatial curvature, and dark energy. We find in flat Λ⁢CDM⁢ ∑𝑚 𝜈 < 0.12 eV at 95% confidence using the large scale structure data, BAO measurements from Sloan Digital Sky Survey, and primary CMB observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

Real-time steerable frequency-stepped Doppler backscattering (DBS) system for local helicon wave electric field measurements on the DIII-D tokamak

A new frequency-stepped Doppler backscattering (DBS) system has been integrated into a real-time steerable electron cyclotron heating launcher system to simultaneously probe local background turbulence (f < 10 MHz) and high-frequency (20–550 MHz) density fluctuations in the DIII-D tokamak. The launcher allows for 2D steering (horizontally and vertically) over wide angular ranges to optimize probe location and wavenumber response. The vertical steering can be optimized during a discharge in real time. The new DBS system employs a programmable frequency synthesizer with adjustable dwell time as a source to launch either O or X-mode polarized millimeter waves. This system can step in real-time over the entire E-band frequency range (60–90 GHz). This combination of capabilities allows for the diagnosis of the complex internal spatial structure of high power (>200 kW) helicon waves (476 MHz) injected from an external antenna during helicon current drive experiments in DIII-D. Broadband density fluctuations around the helicon frequency are observed during real-time scans of measurement location and wavenumber during these experiments. Analysis indicates that these broadband high-frequency fluctuations are a result of backscattering of the DBS millimeter-wave probe beam from plasma turbulence modulated by the helicon wave. It is observed that background turbulence is effectively locally “tagged” with the helicon wave electric field, forming images of the turbulent spectrum in the overall density fluctuation spectrum that appear as high-frequency sidebands of the turbulence. These observations of background turbulence and high-frequency fluctuations open up the possibility of monitoring local helicon wave amplitude by comparing the high-frequency signal amplitude to the simultaneously measured background turbulence. In combination with the real-time measurement location and wavenumber scanning capabilities (offered by real-time frequency-stepping and steering), this allows rapid determination of the spatial distribution of the helicon wave power during steady-state plasma operation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1986

Plasma fluctuations and large-scale mixing near Comet Giacobini-Zinner

The relationship among plasma densities, flow speeds and directions, and temperatures, at distances within about 100,000 km of Comet Giacobini-Zinner's nucleus, are presently examined in view of lowpass-filtered data from the International Cometary Explorer's electron spectrometer. While the largest amplitude density spikes often have more significant flow changes associated with them, a consistent pattern is not found. Power spectral analyses in and near the sheath/transition regions show that density fluctuation levels are enhanced at all detectable frequencies, consistent with strong density fluctuations on all time scales. Such mechanisms as the amplification of convected ion pickup waves and cometary rays for producing the large plasma variations are discussed. It is suggested that the Rayleigh-Taylor effect-driven mixing mechanism at a mass-loading boundary abut 100,000 km from the nucleus may be operative.

Baker, D. N.↗
NASA NTRS1989

Ultrasonic imaging of porosity variations produced during sintering

A silicon carbide disk was sintered from 2090 to 2190 C in 25-C steps. After each sintering step, the disk was examined using a precision acoustic scanning system to determine acoustic attenuation and velocity. The bulk density was found to vary nonmonotonically with sintering temperature. The density varied as much as 10 percent from its value at 2090 C during the sintering process. Local density fluctuations occurred in an organized and history-dependent way. These local density fluctuations varied up to + or - 7 percent of the bulk density and were made visible by acoustic attenuation and velocity mapping.

Generazio, Edward R.↗
NASA NTRS1981

Anisotropy of the cosmic microwave background radiation

Theoretical predictions of the angular anisotropy in the cosmic microwave background radiation on both small and large angular scales are presented, and the effect of massive neutrinos on both the background radiation anisotropy and on the galaxy correlation function over very large scales is reviewed. Current observations show that the quadrupole anisotropy provides the greatest constraint on theory, and the values for the gravitational potential fluctuations indicate that small amplitude but sufficiently large-scale density fluctuations, both at the present epoch and on the surface of last scattering, can produce significant large angular scale variations in the radiation temperature. Most importantly, it is proposed that the quadrupole moment is most simply and elegantly interpreted in terms of the density fluctuations on very large scales whose presence is inferred from the requirement that an initial fluctuation spectrum is required in order for structure to develop.

Silk, J.↗
NASA NTRS1983

Transport by weak electrostatic density drift fluctuations

Expressions are derived for transport by weak electrostatic microfluctuations driven unstable by a density gradient perpendicular to a uniform magnetic field in a Vlasov plasma. These expressions, which are evaluated for the universal and lower-hybrid density drift instabilities, provide improved physical interpretations for various wave-particle exchange frequencies previously reported. It is demonstrated that wave-particle effects generally reduce the free energy driving these instabilities, and that some forms of steady-state distribution functions are inappropriate for nonlinear theory.

Sanderson, J. J.↗
NASA NTRS1992

N-point correlation functions in the CfA and SSRS redshift distribution of galaxies

Using counts in cells, we estimate the volume-average N-point galaxy correlation functions for N = 2, 3, and 4, in redshift samples of the CfA and SSRS catalogs. Volume-limited samples of different sizes are used to study the uncertainties at different scales, the shot noise, and the problem with the boundaries. The hierarchical constants S3 and S4 agree well in all samples in CfA and SSRS, with average S3 = 194 +/- 0.07 and S4 = 4.56 +/- 0.53. We compare these results with estimates obtained from angular catalogs and recent analysis over IRAS samples. The amplitudes SJ seem larger in real space than in redshift space, although the values from the angular analysis correspond to smaller scales, where we might expect larger nonperturbative effects. It is also found that S3 and S4 are smaller for IRAS than for optical galaxies. This, together with the fact that IRAS galaxies have smaller amplitude for the above correlation functions, indicates that the density fluctuations of IRAS galaxies cannot be simply proportional to the density fluctuations of optical galaxies, i.e., biasing has to be nonlinear between them.

Gaztanaga, Enrique↗
NASA NTRS1993

Small-scale density irregularities in the nightside Venus ionosphere - Comparison of theory and observations

The theory of the lower-hybrid-drift instability is compared with observations of plasma density irregularities. Marginal stability boundaries (gamma = 0) for the lower-hybrid-drift instability and the occurrence of small-scale density fluctuations are presented as a function of magnetic field B and density n. For plasma density gradient scale lengths in the range 2-10 km, 80-85 percent of the density fluctuations lie in the unstable B/n parameter regime (gamma greater than 0). Stability boundaries for the onset of instability at a wavelength for which the Pioneer Venus Orbiter could measure a Doppler-shifted frequency of 100 Hz are presented. The conditions on B and n for instability in this situation are more stringent than those for marginal stability, especially at low densities (n less than or equal to 5000/cu cm). In general, the instability is most likely to be active in regions of low beta: high magnetic field strength and low density, as found in ionospheric holes or troughs.

Huba, J. D.↗
NASA NTRS2020

Multi-Point Line Focused Laser Differential Interferometer for High-Speed Flow Fluctuation Measurements

A multi-point focused laser differential interferometer (FLDI) has been developed to measure density fluctuations at 16 points along a line. A pair of cylindrical lenses on the transmitter side of a conventional single-point FLDI instrument form two closely spaced (≤200 μm), orthogonally polarized, parallel laser lines at the instrument’s focus. On the receiver side of the instrument, the interference of the beams on a 16-element photodiode array results in a single line of measurements. The further addition of a Nomarski prism creates two separate measurement lines, and the addition of a second photodiode array to the instrument enables simultaneous measurements of density fluctuations along the two lines separated by several millimeters. These two lines of measurement can be conveniently oriented at any azimuthal angle relative to the instrument’s optical axis on the measurement plane, coinciding with the instrument’s focus. Two experiments were performed to demonstrate the capabilities of the instrument. In the first experiment, a laser-induced breakdown spark generated a traveling spherical shock wave, and measurements of the resulting density disturbance and wave velocity were obtained. These results were compared to high-speed schlieren images of the shock wave acquired at 400 kHz. In the second experiment, the multi-point FLDI instrument was used to measure density disturbances in the boundary layer of a flat plate in a Mach 6 freestream flow. The measurements were made along two lines, both approximately 6 mm in length, extending from the surface of the plate through the boundary layer. High-speed schlieren images were acquired at 100 kHz during separate wind tunnel runs at matching unit Reynolds numbers to visualize the unsteady boundary layer flow and compare to the FLDI measurements.

Laser beam shaping↗