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At least 1,207 records · Page 67
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Experimental investigation of PDI bifurcation of lower hybrid waves during electron density ramp-up in EAST

Abstract The effect of parametric decay instability (PDI) on the current drive efficiency of 4.6 GHz lower hybrid (LH) waves in EAST is investigated experimentally, showing the PDI channel bifurcation of LH waves for the first time in EAST. First, experiments with three platforms of LH power were performed, achieving the LH power required for the PDI occurrence. Second, PDI bifurcation experiments were further carried out by ramping up the plasma electron density. The loop voltage increases with an increase in density, implying a decrease in the current driven by the LH wave. PDI bifurcation during electron density ramp-up was studied by analyzing the parallel refractive index ( n ∥ ) and the frequency spectrum broadening, which is measured with a radio frequency magnetic probe array recently installed close to the LH antenna. It is observed for the first time that they both first increase with density, then there is not much variation and a clear sideband in the frequency spectrum is also observed when the density is up to 4 × 10 19 m − 3 , suggesting a change in the PDI channel. Calculation of the mode growth rate driven by PDI shows that when the edge electron density is up to 1.9 × 10 18 m − 3 , the growth rate of the ion cyclotron quasi-mode (ICQM) will exceed that of the ion sound quasi-mode (ISQM), quantitively explaining that with an increase in density, the PDI channel partly transits from the ISQM to the ICQM channel. Studies provide a possible way to reduce the power deposition in the edge region and improve drive capability by means of mitigating PDI behavior.

Physics↗
NASA NTRS1972

Development of a theory of the spectral reflectance of minerals, part 3

Significant refinements were made in the theory of the diffuse reflectance of particulate media. The theory predicts the opposite trends of reflectance with particle size in regions of the spectrum in which the particles are semi-transparent and those in which they are opaque. Enhanced absorption caused by wave-optical effects of small surface asperities and edges was used to improve the theory. The same mechanism remedies the theory to account for the data in spectral regions of anomalous dispersion.

Aronson, J. R.↗
NASA NTRS1973

A catalog of ionospheric F region irregularity behavior based on Ogo 6 retarding potential analyzer data

Review of in situ data obtained with the aid of the retarding potential analyzer on board Ogo 6 which reveal the nature of ionospheric irregularities in the total ion concentration above 400 km. Except for the high-latitude regions, the ionosphere is usually observed to be very smooth in the daytime, but considerable structure is observed at night, particularly near the equator and at Atlantic longitudes. Although most of the irregularities observed appear to be stochastic in nature, many nearly monochromatic waveforms are observed near the equator. The topics discussed include the midlatitude scintillation boundary, large-amplitude equatorial irregularities, the fluctuation spectrum of typical F-region irregularities, the lower edge of the equatorial F region, sinusoidal waveforms, ground glass irregularities, breaking wave irregularities, and regions of smooth and irregular ionization inside the polar cap.

Mcclure, J. P.↗
NASA NTRS1976

Radiation from a current filament driven by a traveling wave

Solutions are presented for the electromagnetic fields radiated by an arbitrarily oriented current filament located above a perfectly conducting ground plane and excited by a traveling current wave. Both an approximate solution, valid in the fraunhofer region of the filament and predicting the radiation terms in the fields, and an exact solution, which predicts both near and far field components of the electromagnetic fields, are presented. Both solutions apply to current waveforms which propagate along the channel but are valid regardless of the actual waveshape. The exact solution is valid only for waves which propagate at the speed of light, and the approximate solution is formulated for arbitrary velocity of propagation. The spectrum-magnitude of the fourier transform-of the radiated fields is computed by assuming a compound exponential model for the current waveform. The effects of channel orientation and length, as well as velocity of propagation of the current waveform and location of the observer, are discussed. It is shown that both velocity of propagation and an effective channel length are important in determining the shape of the spectrum.

Levine, D. M.↗
NASA NTRS1986

Dual-temperature acoustic levitation and sample transport apparatus

The properties of a dual-temperature resonant chamber to be used for acoustical levitation and positioning have been theoretically and experimentally studied. The predictions of a first-order dissipationless treatment of the generalized wave equation for an inhomogeneous medium are in close agreement with experimental results for the temperature dependence of the resonant mode spectrum and the acoustic pressure distribution, although the measured magnitude of the pressure variations does not correlate well with the calculated one. Ground-based levitation of low-density samples has been demonstrated at 800 C, where steady-state forces up to 700 dyn were generated.

Trinh, E.↗
NASA NTRS1987

The interaction of solar p-modes with a sunspot. II - Simple theoretical models

The interaction of solar p-modes with a sunspot magnetic flux tube is investigated theoretically by means of two simple models. An increase in horizontal wavelength between the nonmagnetic and magnetic regions, due to the different characteristic wave speeds in the two regions, explains the corresponding observed wavelength shift of powe in the umbral k-omega power spectrum. The variation of the transmission coefficient with wavenumber along the p-mode diagnostic curves, due to resonant transmission, is responsible for the observed selective filtering of the p-modes by the sunspot.

Abdelatif, Toufik E.↗
NASA NTRS1996

Stratospheric Horizontal Wavenumber Spectra of Winds, Potential Temperature, and Atmospheric Tracers Observed by High-Altitude Aircraft

Horizontal wavenumber power spectra of vertical and horizontal wind velocities, potential temperatures, and ozone and N(2)O mixing ratios, as measured in the mid-stratosphere during 73 ER-2 flights (altitude approx. 20km) are presented. The velocity and potential temperature spectra in the 100 to 1-km wavelength range deviate significantly from the uniform -5/3 power law expected for the inverse energy-cascade regime of two-dimensional turbulence and also for inertial-range, three-dimensional turbulence. Instead, steeper spectra approximately consistent with a -3 power law are observed at horizontal scales smaller than 3 km for all velocity components as well as potential temperature. Shallower spectra are observed at scales longer than 6 km. For horizontal velocity and potential temperature the spectral indices at longer scales are between -1.5 and -2.0. For vertical velocity the spectrum at longer scales become flat. It is argued that the observed velocity and potential temperature spectra are consistent with gravity waves. At smaller scales, the shapes are also superficially consistent with a Lumley-Shur-Weinstock buoyant subrange of turbulence and/or nonlinear gravity waves. Contemporaneous spectra of ozone and N(sub 2)O mixing ratio in the 100 to 1-km wavelength range do conform to an approximately uniform -5/3 power law. It is argued that this may reflect interactions between gravity wave air-parcel displacements and laminar or filamentary structures in the trace gas mixing ratio field produced by enstropy-cascading two-dimensional turbulence.

TRACE GAS MIXING RATIO FIELDS↗
DOE OSTI2022

Matrix-Model Simulations Using Quantum Computing, Deep Learning, and Lattice Monte Carlo

Matrix quantum mechanics plays various important roles in theoretical physics, such as a holographic description of quantum black holes, and it underpins the only practical numerical approach to the study of complex high-dimensional supergravity theories. Understanding quantum black holes and the role of entanglement in a holographic setup is of paramount importance for the realization of a quantum theory of gravity. Moreover, a complete numerical understanding of the holographic duality and the emergence of geometric space-time features from microscopic degrees of freedom could pave the way for new discoveries in quantum information science. Euclidean lattice Monte Carlo simulations are the de facto numerical tool for understanding the spectrum of large matrix models and have been used to test the holographic duality. However, they are not tailored to extract dynamical properties or even the quantum wave function of the ground state of matrix models. Quantum computing and deep learning provide potentially useful approaches to study the dynamics of matrix quantum mechanics. If successful in the context of matrix models, these rapidly improving numerical techniques could become the new Swiss army knife of quantum gravity practitioners. In this paper, we perform the first systematic survey for quantum computing and deep-learning approaches to matrix quantum mechanics, comparing them to lattice Monte Carlo simulations. These provide baseline benchmarks before addressing more complicated problems. In particular, we test the performance of each method by calculating the low-energy spectrum.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1991

Simulation of the inertial-conductive subrange

The inertial-conductive subrange spectrum of a passive temperature field of a fluid of small Prandtl number is determined by large-eddy simulation. Results for simulations of both freely decaying and forced turbulence are presented. In the simulations of freely decaying turbulence, a subgrid model is used to simulate a decaying turbulent velocity field with a k exp -5/3 inertial subrange energy spectrum convecting eight different decaying temperature fields with well-resolved conductive subranges. In the simulations of forced turbulence, a subgrid model is again used to simulate an inertial subrange velocity field; however, the velocity field is now forced at the lowest wave numbers of the simulation, and an external uniform mean temperature gradient is imposed. Statistically stationary velocity and temperature fluctuations are generated. The results of the decaying and forced simulations are in excellent agreement with the Batchelor et al. (1959) k exp -17/3 spectrum in the far inertial-conductive subrange, whereas significant departures from the Batchelor spectrum are observed in the near inertial-conductive subrange.

Chasnov, Jeffrey R.↗
NASA NTRS2006

High-Resolution Inspection of the Space Shuttle External Tank Spray-on-Foam Insulation (SOFI) using Focused Millimeter Waves at D-Band (150 GHz)

Space Shuttle Columbia's catastrophic failure has been attributed to a piece of spray-on-foam insulation (SOFI) that was dislodged from the external tank and struck the leading edge of the left wing. A piece of SOFI was also dislodged in the recent Space Shuttle Discovery's flight. Clearly, there is a great and urgent need to inspect the external tank SOFI and other similar insulating structures (including the acreage heat tile) in a reliable and robust fashion. In the past two years, millimeter wave nondestructive testing methods, using both real and synthetic focusing techniques, have shown great potential for this purpose. Recently obtained real-focused images from several different and complex SOFI panels have demonstrated the utility of these methods as being viable, robust, repeatable, simple, portable and effective. D-band frequency range which covers a frequency spectrum of 110- 170 GHz is well-suited for this purpose given the nature of the foam which causes significant scattering at much higher frequencies. This paper presents the results of using continuous-wave (CW) reflectometry conducted on several typical and complex SOFI panes at 150 GHz.

Kharkovsky, S.↗
NASA NTRS1984

Solar energy converter using surface plasma waves

Sunlight is dispersed over a diffraction grating formed on the surface of a conducting film on a substrate. The angular dispersion controls the effective grating period so that a matching spectrum of surface plasmons is excited for parallel processing on the conducting film. The resulting surface plasmons carry energy to an array of inelastic tunnel diodes. This solar energy converter does not require different materials for each frequency band, and sunlight is directly converted to electricity in an efficient manner by extracting more energy from the more energetic photons.

Anderson, L. M.↗
NASA NTRSDate not supplied

Modelling Sonic Boom Propagation through Planetary Boundary Layer Turbulence near the Lateral Extent of the Carpet

Nonlinear acoustic propagation of sonic booms in the atmospheric boundary layer is considered in the context of a one-way solution to a third order wave equation. A split-step approach is utilized to compute different physical effects efficiently. Unlike previous approaches, the diffraction effects are computed exactly in the forward direction with no restriction on the angle of propagation. This results in more accurate modelling of sonic boom near the carpet edge. Heterogenous flow effects are incorporated with a wide-angle parabolic approximation. Nonlinear propagation is computed with a Burgers-Hayes method. Turbulence in the medium is constructed by the method of random Fourier modes. The turbulence spectra are constructed using an altitude dependent 3-dimensional von Karman spectrum. The turbulence field is considered frozen, as the eddy turnover time in the atmospheric boundary layer is generally much larger than the propagation time of the acoustic wave. Comparisons with benchmark predictions from the PCBoom software are conducted. The benchmark cases provide insight into the accuracy of the current prediction code for non-turbulent atmospheres. Future work is discussed regarding the prediction of shaped booms, boundary conditions, and predictions near the lateral extent of the boom carpet where previous approaches fail to capture the diffraction effects.

sonic boom↗
NASA NTRS1994

Limits on coronal reflection using high-frequency solar oscillations

Acoustic waves in the Sun with frequencies above about 5.3 mHz can propagate in the chromosphere. We examine imaged solar intensity data for evidence of reflection of these waves in the upper chromosphere, where the temperature increases by a large factor over a short distance. Our method is to compare the observed and theoretically derived frequency spacings between peaks in the power spectrum. We find that our theoretical frequencies provide the best fit to the data when the reflection in the upper atmosphere is eliminated. In particular, the model of Kumar (1993b), which includes the source depth, and radiative damping, in the calculation of power spectra but ignores chromospheric reflection, gives peak frequencies that are in good agreement with the observations. For acoustic waves of frequency greater than 6 mHz we put an upper limit to the reflectivity of chromosphere and corona, using our method, of about 10%. At a given spherical harmonic degree, the frequency spacing between peaks in the data generally decreases with increasing frequency, because the lower turning point of the waves is moving inward. However, between 5 and 5.5 mHz the frequency spacing increases slightly. This feature is probably associated with the acoustic cutoff frequency in the solar atmosphere, i.e., it indicates a transition from trapped waves to propagating waves. We are able to reproduce the observed behavior by a crude modeling of the solar atmosphere. Further study of these peaks should provide an independent way of exploring the mean structure of the solar atmosphere, particularly around the temperature minimum region.

Kumar, P.↗
DOE OSTI2022

On numerical energy conservation for an implicit particle-in-cell method coupled with a binary Monte-Carlo algorithm for Coulomb collisions

We report conventional particle-in-cell (PIC) methods suffer from enhanced numerical heating (explicit PIC) or cooling (semi-implicit PIC) when coupled with a binary Monte-Carlo algorithm for Coulomb collisions. In this work, a fully-implicit θ-PIC scheme (with adjustable time-biasing parameter 1/2 ≤ θ ≤) is considered. The discrete change in energy of a closed system after a time step for this scheme scales with (1/2 - θ)C θ , where C θ is a positive definite quantity that depends on the frequency spectrum of the energy in the fields. Collisions lead to additional energy in the field fluctuations associated with high-frequency light waves produced by a numerical Bremsstrahlung process, which can result in a large increase in the numerical cooling rate for θ > 1/2. However, for θ = 1/2, energy is exactly conserved. The energy in the field fluctuations on long time scales agrees with that calculated using the equipartition theorem for a classical system in thermodynamic equilibrium.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Novel fractal-textured solar absorber surfaces for concentrated solar power

The solar absorptance of a thermal receiver surface significantly affects the photothermal conversion efficiency of concentrated solar power (CSP) plants. The development of low-cost Gen3 CSP systems calls for increasing the solar absorptance of the thermal receivers at operating temperatures above 750 °C. This study presents an innovative approach to the fractal, multiscale texturing of absorber surfaces to significantly enhance solar energy absorption for Gen3 CSP applications. The absorber surface is described in terms of its fractal parameters that are uniquely determined from surface profile measurements. The interaction of solar radiation with the fractal surface is numerically simulated by solving the governing Maxwell’s equations for electromagnetic wave propagation to investigate the effect of texturing on the absorptance of the surfaces. It is shown that absorption of the solar spectrum increases with an increase in the fractal dimension and the multiscale asperity height of the surface texturing. Here, the theoretical model is demonstrated to be in close agreement with experimental measurements of spectral absorptance of electrodeposited copper (Cu), copper mono-oxide (CuO), and copper-manganese oxide (CuMnO) surfaces that are textured to produce a range of fractal parameters by tailoring the deposition parameters. Fractal surface texturing is shown to reduce reflectance by over one order of magnitude, yielding an absorptance of greater than 0.985 for CuMnO. For the first time, the study presents a significant effective means of increasing solar absorptance and a fundamental theoretical description of the underlying physics.

14 SOLAR ENERGY↗
DOE OSTI2022

Two methods to study inelastic neutron-scattering measurements based on ω n (q) versus S(q, ω) applied to the magnetic open honeycomb lattice Tb 2 Ir 3 Ga 9

This work describes two methods to fit the inelastic neutron-scattering spectrum S(q, ω) with wavevector q and frequency ω. The common and well-established method extracts the experimental spin-wave branches ω n (q) from the measured spectra S(q, ω) and then minimizes the difference between the observed and predicted frequencies. When n branches of frequencies are predicted but the measured frequencies overlap to produce only m < n branches, the weighted average of the predicted frequencies must be compared to the observed frequencies. A penalty is then exacted when the width of the predicted frequencies exceeds the width of the observed frequencies. The second method directly compares the measured and predicted intensities S(q, ω) over a grid {q i , ω j } in wavevector and frequency space. After subtracting background noise from the observed intensities, the theoretical intensities are scaled by a simple wavevector-dependent function that reflects the instrumental resolution. Furthermore, the advantages and disadvantages of each approach are demonstrated by studying the open honeycomb material Tb 2 Ir 3 Ga 9 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

A forecast for large-scale structure constraints on Horndeski gravity with CO line intensity mapping

Here we consider the potential for line intensity mapping (LIM) of the rotational CO(1-0), CO(2-1), and CO(3-2) transitions to detect deviations from General Relativity from 0 < z < 3 within the framework of a very general class of modified gravity models, called Horndeski’s theories. Our forecast assumes a multitracer analysis separately obtaining information from the matter power spectrum and the first two multipoles of the redshift space distortion power spectrum. To achieve ±0.1 level constraints on the slope of the kinetic gravity braiding and Planck mass evolution parameters, a mm-wave LIM experiment would need to accumulate ≈10 8 –10 9 spectrometre-hours, feasible with instruments that could be deployed in the 2030s. Such a measurement would constrain the parameters of Horndeski’s theory at a level at worst competitive to and at best an order of magnitude tighter than existing constraints from the CMB and LSS. Our modelling code is publicly available.

79 ASTRONOMY AND ASTROPHYSICS↗