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At least 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
NASA NTRS2015

Heat Capacity and Thermal Conductance Measurements of a Superconducting-Normal Mixed State by Detection of Single 3 eV Photons in a Magnetic Penetration Thermometer

We report on measurements of the detected signal pulses in a molybdenum-gold Magnetic Penetration Thermometer (MPT) in response to absorption of one or more 3 eV photons. We designed and used this MPT sensor for x-ray microcalorimetry. In this device, the diamagnetic response of a superconducting MoAu bilayer is used to sense temperature changes in response to absorbed photons, and responsivity is enhanced by a Meissner transition in which the magnetic flux penetrating the sensor changes rapidly to minimize free energy in a mixed superconducting normal state. We have previously reported on use of our MPT to study a thermal phonon energy loss to the substrate when absorbing x-rays. We now describe results of extracting heat capacity C and thermal conductance G values from pulse height and decay time of MPT pulses generated by 3 eV photons. The variation in C and G at temperatures near the Meissner transition temperature (set by an internal magnetic bias field) allow us to probe the behavior in superconducting normal mixed state of the condensation energy and the electron cooling power resulting from quasi-particle recombination and phonon emission. The information gained on electron cooling power is also relevant to the operation of other superconducting detectors, such as Microwave Kinetic Inductance Detectors.

superconductivity↗
NASA NTRS1974

Evaluation of atomic constants for optical radiation, volume 2

Various atomic constant for 23 elements from helium to mercury were computed and are presented in tables. The data given for each element start with the element name, its atomic number, its ionic state, and the designation and series limit for each parent configuration. This is followed by information on the energy level, parent configuration, and designation for each term available to the program. The matrix elements subtables are ordered by the sequence numbers, which represent the initial and final levels of the transitions. Each subtable gives the following: configuration of the core or parent, designation and energy level for the reference state, effective principal quantum number, energy of the series limit, value of the matrix element for the reference state interacting with itself, and sum of all of the dipole matrix elements listed in the subtable. Dipole and quadrupole interaction data are also given.

Kylstra, C. D.↗
NASA NTRS2012

Electron Beam Induced Luminescence of SiO2 Optical Coatings

Optical coatings of disordered thin film SiO,/SiO, samples undergoing electron beam bombardment exhibited a cathodoluminescence effect. Measurements were made of the absolute illuminance and emission spectra as functions of incident electron energy (0.2 keV to 30 keV). flux (approx. 0.1 nA/sq cm to > 100 nA/sq cm) and power (<50 nW/sq cm to > 3mW/sq cm) for coatings (approx.60 nm to approx.200 nm thick) on reflective metal substrates over a range of sample temperatures (approx.40 K to approx.400 K) and emission wavelengths (approx.260 nm to approx.5000 nm). Illuminance reached a plateau saturation intensity I(sub sat) approx. 10(exp -4)W/sq cm. scaling as [I(sub mc)/(I + I(sub mc)/I(sub sat)] as incident power I(sub mc) approached I(sub sat). Well below I(sub sat), illuminance scaled linearly with incident power for lower-energy non-penetrating radiation that deposited all of its energy in the dielectric film. At higher incident energies (> 1-5 keV) electrons penetrate the dielectric, depositing only a fraction of their energy in the dielectric film, where deposited energy scales as the ratio of film thickness to penetration range. Since electron range exhibits a peak below approx. 1 keV, the illuminance decreased with increasing energy for penetrating radiation. The temperature dependence of the illuminance. in general, increased with decreasing temperature. Changes in apparent color with temperature were also observed. Four bands in the UV/VIS/NIR range from 300 nm to 1000 nm were observed in spectral measurements; no significant luminescence was detected in a range from approx. 1000 nm to approx. 5000 nm. These different bands either increased or decreased in intensity and shifted slightly in energy with decreasing temperature. Our measurements are explained in terms of simple disordered band theory models based on atomic-scale stochastic transport of electrons in highly disordered insulating materials. The theoretical models provide a fundamental basis for understanding the dependence of cathodoluminescence on irradiation time, incident flux and energy. and sample thickness and temperature. The observed luminescence occurs when an incident high energy electron excites a valence band electron into the conduction band. Following the excitation. a complex series of transitions take place between the extended conduction states and the long-lived localized trapped states (or chromophores) associated with structural or compositional defects in the dielectric materials. Thc transport equations and models of thc spatial and energy distribution of the trapped states below the mobility edge, their occupancy, and transitions provide microscopic scale insights into this macroscopic phenomenon. Our observations and theoretical models are also compared with similar previous room temperature studies of similar SiO2 films.

Dennison, J. R.↗
NASA NTRS2014

Transitional Disks Associated with Intermediate-Mass Stars: Results of the SEEDS YSO Survey

Protoplanetary disks are where planets form, grow, and migrate to produce the diversity of exoplanet systems we observe in mature systems. Disks where this process has advanced to the stage of gap opening, and in some cases central cavity formation, have been termed pre-transitional and transitional disks in the hope that they represent intermediate steps toward planetary system formation. Recent reviews have focussed on disks where the star is of solar or sub-solar mass. In contrast to the sub-millimeter where cleared central cavities predominate, at H-band some T Tauri star transitional disks resemble primordial disks in having no indication of clearing, some show a break in the radial surface brightness profile at the inner edge of the outer disk, while others have partially to fully cleared gaps or central cavities. Recently, the Meeus Group I Herbig stars, intermediate-mass PMS stars with IR spectral energy distributions often interpreted as flared disks, have been proposed to have transitional and pre-transitional disks similar to those associated with solar-mass PMS stars, based on thermal-IR imaging, and sub-millimeter interferometry. We have investigated their appearance in scattered light as part of the Strategic Exploration of Exoplanets and Disks with Subaru (SEEDS), obtaining H-band polarimetric imagery of 10 intermediate-mass stars with Meeus Group I disks. Augmented by other disks with imagery in the literature, the sample is now sufficiently large to explore how these disks are similar to and differ from T Tauri star disks. The disk morphologies seen in the Tauri disks are also found for the intermediate-mass star disks, but additional phenomena are found; a hallmark of these disks is remarkable individuality and diversity which does not simply correlate with disk mass or stellar properties, including age, including spiral arms in remnant envelopes, arms in the disk, asymmetrically and potentially variably shadowed outer disks, gaps, and one disk where only half of the disk is seen in scattered light at H. We will discuss our survey results in terms of spiral arm theory, dust trapping vortices, and systematic differences in the relative scale height of these disks compared to those around Solar-mass stars. For the disks with spiral arms we discuss the planet-hosting potential, and limits on where giant planets can be located. We also discuss the implications for imaging with extreme adaptive optics instruments. Grady is supported under NSF AST 1008440 and through the NASA Origins of Solar Systems program on NNG13PB64P. JPW is supported NSF AST 100314. 0) in marked contrast to protoplanetary disks, transitional disks exhibit wide range of structural features1) arm visibility correlated with relative scale height in disk2) asymmetric and possibly variable shadowing of outer portions some transitional disks3) confirm pre-transitional disk nature of Oph IRS 48, MWC 758, HD 169142, etc.

Protoplanetary disks↗
NASA NTRS1980

Subcritical transition to turbulence in plane channel flows

A linear three dimensional mechanism for the transition of plane Poiseuille flows to turbulence is presented which provides good agreement with experimental observations. The mechanism is based on the evolution of states within a band of quasi-equilibria which slowly approach the stable upper branch solutions for the evolution of flow energy but which are strongly unstable to infinitesimal three-dimensional disturbances. Numerical simulation has shown that if two-dimensional flow persists long enough for the three-dimensional perturbations to attain finite amplitude, the resulting three dimensional flow quickly develops a turbulent character with nonperiodic behavior, and thus transition can be predicted from knowledge of the initial two- and three-dimensional energies and time scales. The mechanism predicts transition to turbulence at Reynolds numbers greater than 1000, as observed in experiments, and implies higher threshold three-dimensional energies in plane Couette flow.

Orszag, S. A.↗
NASA NTRS1976

The acceleration of energetic particles in the interplanetary medium by transit-time damping

Transit time damping is examined as a possible means for accelerating low energy particles in co-rotating streams and interstellar ions. Data show that: the protons in co-rotating streams may be accelerated by transient-time damping the small-scale variations in the field magnitude that are observed at a low level in the inner solar system. The interstellar ions may be accelerated by transit time damping large-scale field variations in the outer solar system.

Fisk, L. A.↗
NASA NTRS1987

C-type transitions in methyl formate

Based on previously determined spectral constants for methyl formate in its ground torsional degenerate substate, the frequencies and intensities of forbidden c-type transitions in this molecule, which is represented by a large number of lines in OMC-1, are calculated along with other 'forbidden' transitions labeled x-type. The stronger c-type transitions below 300 GHz with angular momentum quantum number of 30 or less and with upper state rotational energy of 350/cm or less are included in a list of spectral frequencies presented in this paper. Because the c-type transitions borrow intensity from the b-type transitions, the intensities of strongly affected b-type spectra are recalculated and presented.

Plummer, Grant M.↗
NASA NTRS1994

Time-dependent evolution of cosmic-ray-modified shock structure: Transition to steady state

Steady state solutions to the two-fluid equations of cosmic-ray-modified shock structure were investigated first by Drury and Volk (1981). Their analysis revealed, among other properties, that there exist regions of upstream parameter space where the equations possess three different downstream solutions for a given upstream state. In this paper we investigate whether or not all these solutions can occur as time-asymptotic states in a physically realistic evolution. To do this, we investigate the time-dependent evolution of the two-fluid cosmic-ray equations in going from a specified initial condition to a steady state. Our results indicate that the time-asymptotic solution is strictly single-valued, and it undergoes a transition from weakly to strongly cosmic-ray-modified at a critical value of the upstream cosmic ray energy density. The expansion of supernova remnant shocks is considered as an example, and it is shown that the strong to weak transition is in fact more likely. The third intermediate solution is shown to influence the time-dependent evolution of the shock, but it is not found to be a stable time-asymptotic state. Timescales for convergence to these states and their implications for the efficiency of shock acceleration are considered. We also investigate the effects of a recently introduced model for the injection of seed particles into the shock accelerated cosmic-ray population. The injection is found to result in a more strongly cosmic-ray-dominated shock, which supports our conclusion that for most classes of intermediate and strong cosmic-ray-modified shocks, the downstream cosmic-ray pressure component is at least as large as the thermal gas pressure, independent of the upstream state. As a result, cosmic rays almost always play a significant role in determining the shock structure and dissipation and they cannot be regarded as test particles.

Donohue, D. J.↗
NASA NTRS1986

Accurate determination of the fine-structure intervals in the 3P ground states of C-13 and C-12 by far-infrared laser magnetic resonance

Accurate values are presented for the fine-structure intervals in the 3P ground state of neutral atomic C-12 and C-13 as obtained from laser magnetic resonance spectroscopy. The rigorous analysis of C-13 hyperfine structure, the measurement of resonant fields for C-12 transitions at several additional far-infrared laser frequencies, and the increased precision of the C-12 measurements, permit significant improvement in the evaluation of these energies relative to earlier work. These results will expedite the direct and precise measurement of these transitions in interstellar sources and should assist in the determination of the interstellar C-12/C-13 abundance ratio.

Cooksy, A. L.↗
NASA NTRS1991

An ab initio study of the C3(+) cation using multireference methods

The energy difference between the linear 2 sigma(sup +, sub u) and cyclic 2B(sub 2) structures of C3(+) has been investigated using large (5s3p2d1f) basis sets and multireference electron correlation treatments, including complete active space self consistent fields (CASSCF), multireference configuration interaction (MRCI), and averaged coupled-pair functional (ACPF) methods, as well as the single-reference quadratic configuration interaction (QCISD(T)) method. Our best estimate, including a correction for basis set incompleteness, is that the linear form lies above the cyclic from by 5.2(+1.5 to -1.0) kcal/mol. The 2 sigma(sup +, sub u) state is probably not a transition state, but a local minimum. Reliable computation of the cyclic/linear energy difference in C3(+) is extremely demanding of the electron correlation treatment used: of the single-reference methods previously considered, CCSD(T) and QCISD(T) perform best. The MRCI + Q(0.01)/(4s2p1d) energy separation of 1.68 kcal/mol should provide a comparison standard for other electron correlation methods applied to this system.

Taylor, Peter R.↗
NASA NTRS2023

Evidence of Abrupt Transitions Between Sea Ice Dynamical Regimes in the East Greenland Marginal Ice Zone

Sea ice modulates the energy exchange between the atmosphere and the ocean through its kinematics. Marginal ice zone (MIZ) dynamics are complex and are not well resolved in routine observations. Here, we investigate sea ice dynamics in the Greenland Sea MIZ using two Lagrangian drift datasets. We find evidence of tidal currents strongly affecting sub-daily sea ice motion. Velocity anomalies show abrupt transitions aligned with gradients in seafloor topography, indicating changes in ocean currents. Remote-sensed ice floe trajectories derived from moderate resolution satellite imagery provide a view of small-scale variability across the Greenland continental shelf. Ice floe trajectories reveal an west-east increasing velocity gradient imposed by the East Greenland Current, with maximum velocities aligned along the continental shelf edge. These results highlight the importance of small scale ocean variability for ice dynamics in the MIZ.

Daniel M. Watkins↗
NASA NTRS1983

The pulse profile of the Crab pulsar in the energy range 45 keV-1.2 MeV

The Crab Nebula pulsar (PSR 0531+21) is the best studied and most intense of the nontransient X-ray pulsars. However, since its spectrum drops rapidly with energy, a well-resolved pulse profile has not previously been obtained above 200 keV. In the hard X-ray and low-energy gamma-ray region, an accurate pulse profile can be obtained with a balloon-borne detector of sufficient area during a single transit of the source. A new measurement of the pulse profile of PSR 0531+21 in the energy range above 45 keV obtained with a large-area scintillation detector array is reported. The detector array was flown on a balloon launched from Palestine, Texas on 1980 October 6, reaching a float altitude 4.5 g/sq cm at 0230 UTC October 7. The primary objective of the experiment was to detect and study weak gamma-ray bursts.

Wilson, R. B.↗
NASA NTRS1973

Laser addressed holographic memory system

Holographic recall and storage system uses red-lipid microcrystalline wax as storage medium. When laser beam strikes wax, its energy heats point of incidence enough to pass wax through transition temperature. Holograph image can then be written or erased in softened wax.

Gange, R. A.↗
NASA NTRS1976

Injection boundary dynamics during a geomagnetic storm

A series of proton and electron injections were observed by Explorer 45 associated with several substorms during the main phase of the Feb. 24, 1972 geomagnetic storm. The 1- to 290-keV protons and 1- to 560-keV electrons were observed in the evening quadrant up to L of about 5.2. A model distorted dipole magnetic field and McIlwain's E3 convection electric field were used to backtrack the energy-dispersed electron and proton fluxes to their source at the time of injection. The source turns out to be a region extending over several earth radii outside an injection boundary. In the night magnetosphere, the inferred injection boundary is displaced inward with each successive substorm. The energy dispersion plot of the particles injected during orbit 314 indicates that as the energy of the observed particles decreases there is a smooth transition to the position of the plasmapause. This suggests that for that substorm the injection boundary and the plasmapause were one and the same. The proton 'noses' reported by Smith and Hoffman (1974) are discussed.

Konradi, A.↗
NASA NTRS1981

Chromospheres and coronae in binary systems

Recent observations and spectral analysis of chromospheres and coronae are discussed for various types of binary systems containing cool stars. These include the W UMa-type systems, the RS CVn variables, and selected widely separated binary systems such as VV Cephei and the giant stars in the Hyades cluster. In many cases evidence for extreme examples of solar activity are found. Radiative losses in ultraviolet resonance lines can be at least one or two orders of magnitude larger than those found in the quiet sun. In lambda And, a RS CVn type system, there is evidence for inhomogeneous structure in the chromosphere and transition region. Rotation appears to be a determinant in the energy losses of these atmospheres although not the only one.

Dupree, A. K.↗