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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

Temperature Dependence of the Microwave Dielectric Behavior of Selected Materials

A very sensitive thermal bath is designed to study the effect of temperature on the microwave dielectric response of a sample of nickel oxide and cobalt oxide. The sample under study is placed in a microwave resonant cavity in the TE(sub 011) mode. The perturbations of the electric field are recorded in terms of the frequency shifts and the width changes of the microwave resonant signal as seen on the oscilloscope. The real and imaginary parts of the dielectric constant are calculated by using the values of the frequency shifts and width changes in the Slater's perturbation equations. The dielectric behavior of nickel oxide and cobalt oxide and also their mixture is studied at a microwave frequency of 9.2 GHz as a function of temperature. A computer program is written to analyze the dielectric constant values at different temperatures. The resonant cavity seems to be very sensitive in studying the dielectric relaxation mechanism in these materials. The dielectric behavior is also analyzed using Debye's equations, and relaxation times for these materials are calculated at microwave frequencies.

Dahiya, Jai N.↗
NASA NTRS2020

Plasma-environment effects on K lines of astrophysical interest III. IPs, K thresholds, radiative rates, and Auger widths in Fe ix – Fe xvi

Aims. In the context of black-hole accretion disks, we aim to compute the plasma-environment e ects on the atomic parameters used to model the decay of K-vacancy states in moderately charged iron ions, namely Fe ix – Fe xvi. Methods. We used the fully relativistic multiconfiguration Dirac–Fock method approximating the plasma electron–nucleus and electron–electron screenings with a time-averaged Debye–Hückel potential. Results. We report modified ionization potentials, K-threshold energies, wavelengths, radiative emission rates, and Auger widths for plasmas characterized by electron temperatures and densities in the ranges 10^(5)-10^(7) K and 10^(18)-10^(22)/cu. cm. Conclusions. This study confirms that the high-resolution X-ray spectrometers onboard the future XRISM and Athena space missions will be capable of detecting the lowering of the K edges of these ions due to the extreme plasma conditions occurring in accretion disks around compact objects.

J. Deprince↗
NASA NTRS2020

Plasma environment effects on K lines of astrophysical interest IV. IPs, K thresholds, radiative rates, and Auger widths in Fe ii–Fe viii

Aims. Within the framework of compact-object accretion disks, we calculate plasma environment e ects on the atomic structure and decay parameters used in the modeling of K lines in lowly charged iron ions, namely Fe ii–Fe viii. Methods. For this study, we used the fully relativistic multiconfiguration Dirac–Fock method approximating the plasma electron–nucleus and electron-electron screenings with a time-averaged Debye-Hückel potential. Results. We report modified ionization potentials, K-threshold energies, wavelengths, radiative emission rates, and Auger widths for plasmas characterized by electron temperatures and densities in the ranges 10(exp 5)-10(exp 7) K and 10(exp 18)-10(exp 22) per cu.cm. In addition, we propose two universal fitting formulae to predict the IP and K-threshold lowerings in any elemental ion. Conclusions. We conclude that the high-resolution X-ray spectrometers onboard the future XRISM and ATHENA space missions will be able to detect the lowering of the K edges of these Fe ions due to the extreme plasma conditions occurring in the accretion disks around compact objects.

J. Deprince↗
NASA NTRS1981

Ion extraction from a plasma

The physical processes governing ion extraction from a plasma have been examined experimentally. The screen hole plasma sheath (the transition region wherein significant ion acceleration and complete electron retardation occurs) has been defined by equipotential plots for a variety of ion accelerator system geometries and operating conditions. It was found that the screen hole plasma sheath extends over a large distance, and influences ion and electron trajectories at least 15 Debye lengths within the discharge chamber. The electron density variation within the screen hole plasma sheath satisfied a Maxwell-Boltzmann density distribution at an effective electron temperature dependent on the discharge plasma primary-to-Maxwellian electron density ratio. Plasma ion flow up to and through the sheath was predominantly one-dimensional, and the ions entered the sheath region with a modified Bohm velocity. Low values of the screen grid thickness to screen hole diameter ratio were found to give good ion focusing and high extracted ion currents because of the effect of screen webbing on ion focusing.

Aston, G.↗
DOE OSTI2023

Superconductivity in a strange metal

In a gas of charged particles with the density n, mass m, and charge e, the electrical conductivity σ is given by the Drude formula: σ = ne 2 /mΓ, where Γ is the scattering rate. Standard metals are well described by Landau's Fermi Liquid (FL) theory, in which the electric current is carried by quasi-particles, low-energy excitations of the FL that resemble electrons with some effective mass m*. The scattering rate Γ= 1/τ= v F /l, where t is the (momentum) relaxation) time, l is the mean-free path, and v F is the Fermi velocity, can be expressed ("Matthiessen's Rule") as a sum of contributions from various scattering channels: Γ = Γ 0 + Γ el-el + Γ el-ph + …, where Γ 0 describes scattering on lattice imperfections, Γ el-el the electron-electron scattering, Γ el-ph the electron-phonon scattering, etc. Of these, Γ 0 = v F /l 0 , where l 0 is the average distance between the defects, is temperature-independent. Γ el-el should scale as T 2 because of Fermi statistics; for two electrons to scatter on one another, both must come from the "Debye shell" of the width k B T/E F , where k B is the Boltzmann constant and E F is the Fermi energy. Γ el-ph typically grows as T 5 , so we expect this to overwhelm the other channels at a high enough T. However, since l cannot be shorter than the distance between the atoms, the total Γ saturates at Mott-Ioffe-Regel (MIR) limit, roughly v F /a 0 , where v F is the Fermi velocity and a 0 is the lattice constant. The resistivity should also saturate at low T, at ρ 0 = m*v F /ne 2 l 0 T. FL theory also describes other electronic properties; e.g., it predicts that in the magnetic field B, the resistivity of the metal should increase with B 2 , because σ(B) = σ(B=0)/(1 + (ω c /Γ) 2 ), where ω c = eB/m*.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Bohm Criterion of Plasma Sheaths away from Asymptotic Limits

The plasma exit flow speed at the sheath entrance is constrained by the Bohm criterion. The so-called Bohm speed regulates the plasma particle and power exhaust fluxes to the wall, and it is commonly deployed as a boundary condition to exclude the sheath region in quasineutral plasma modeling. In this work, the Bohm criterion analysis is performed in the intermediate plasma regime away from the previously known limiting cases of adiabatic laws and the asymptotic limit of infinitesimal Debye length in a finite-size system, using the transport equations of an anisotropic plasma. The resulting Bohm speed has explicit dependence on local plasma heat flux, temperature isotropization, and thermal force. Comparison with kinetic simulations demonstrates its accuracy over the plasma-sheath transition region in which quasineutrality is weakly perturbed and the Bohm criterion applies.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1985

HCl vibrational fundamental band - Line intensities and temperature dependence of self-broadening coefficients

Self-broadening in the vibrational fundamental of HCl is inversely proportional to the temperature for transitions which lie near the Boltzmann rotational maximum and becomes monotonically less temperature-dependent as the rotational quantum number increases. The rotationless transition moment was found to have the value of 5.57 + or - 0.13 x 10 to the -3rd (Debye)-squared and the first Herman-Wallis factor, C = -2.543 + or - 0.019 x 10 to the -2nd.

Chackerian, C., Jr.↗
NASA NTRS2013

CheMin Instrument Performance and Calibration on Mars

The CheMin (Chemistry and Mineralogy) instrument on the Mars Science Laboratory rover Curiosity uses a CCD detector and a Co-anode X-ray tube source to acquire both mineralogy (from the pattern of Co diffraction) and chemical information (from energies of fluoresced X-rays). A key component of the CheMin instrument is the ability to move grains within sample cells during analysis, providing multiple, random grain orientations that disperse diffracted X-ray photons along Debye rings rather than producing discrete Laue spots. This movement is accomplished by piezoelectric vibration of the sample cells. A cryocooler is used to maintain the CCD at a temperature at about -50 C in order to obtain energy resolution better than 250 eV, allowing discrimination of diffracted Co K X-rays from Fe K and other fluorescent X-rays. A detailed description of CheMin is provided in [1]. The CheMin flight model (FM) is mounted within the body of Curiosity and has been operating on Mars since August 6, 2012. An essentially identical sister instrument, the CheMin demonstration model (DM), is operated in a Mars environment chamber at JPL.

Vaniman, D. T.↗
DOE OSTI2021

Room-temperature superconductivity in boron- and nitrogen-doped lanthanum superhydride

Recent theoretical and experimental studies of hydrogen-rich materials at megabar pressures (i.e., >100 GPa) have led to the discovery of very high-temperature superconductivity in these materials. Lanthanum superhydride LaH10 has been of particular focus as the first material to exhibit a superconducting critical temperature (T c ) near room temperature. Experiments indicate that the use of ammonia borane as the hydrogen source can increase the conductivity onset temperatures of lanthanum superhydride to as high as 290 K. In this work, we examine the doping effects of B and N atoms on the superconductivity of LaH 10 in its fcc ($Fm\bar{3}m$) clathrate structure at megabar pressures. Doping at H atomic positions strengthens the H 32 cages of the structure to give higher phonon frequencies that enhance the Debye frequency and thus the calculated T c . The predicted T c can reach 288 K in LaH 9.985 N 0.015 within the average high-symmetry structure at 240 GPa. The remarkably large shift in T c to higher values produced by small degrees of chemical doping opens the prospect for the creation of still higher-temperature superconductivity in superhydrides potentially at even lower-pressure conditions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS2000

Micromechanical and Electrical Properties of Monolithic Aluminum Nitride at High Temperatures

Micromechanical spectroscopy of aluminum nitride reveals it to possess extremely low background internal friction at less than 1x10(exp-4) logarithmic decrement (log dec) from 20 to 1200 T. Two mechanical loss peaks were observed, the first at 350 C approximating a single Debye peak with a peak height of 60x10(exp-4) log dec. The second peak was seen at 950 'C with a peak height of 20x 10' log dec and extended from 200 to over 1200 C. These micromechanical observations manifested themselves in the electrical behavior of these materials. Electrical conduction processes were predominately intrinsic. Both mechanical and electrical relaxations appear to be thermally activated processes, with activation energies of 0.78 and 1.32 eV respectively.

Goldsby, Jon C.↗
DOE OSTI2023

The critical role of ultra-low-energy vibrations in the relaxation dynamics of molecular qubits

Improving the performance of molecular qubits is a fundamental milestone towards unleashing the power of molecular magnetism in the second quantum revolution. Taming spin relaxation and decoherence due to vibrations is crucial to reach this milestone, but this is hindered by our lack of understanding on the nature of vibrations and their coupling to spins. Here we propose a synergistic approach to study a prototypical molecular qubit. It combines inelastic X-ray scattering to measure phonon dispersions along the main symmetry directions of the crystal and spin dynamics simulations based on DFT. We show that the canonical Debye picture of lattice dynamics breaks down and that intra-molecular vibrations with very-low energies of 1-2 meV are largely responsible for spin relaxation up to ambient temperature. We identify the origin of these modes, thus providing a rationale for improving spin coherence. The power and flexibility of our approach open new avenues for the investigation of magnetic molecules with the potential of removing roadblocks toward their use in quantum devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1997

The Diagnostics of the External Plasma for the Plasma Rocket

Three regions of plasma temperature/energy are being investigated to understand fully the behavior of the plasma created by the propulsion device and the operation of the RPA. Each type of plasma has a RPA associated with it; i.e. a thermal RPA, a collimated RPA, and a high temperature RPA. Through the process of developing the thermal and collimated RPAs, the proper knowledge and experience has been gained to not only design a high temperature RPA for the plasma rocket, but to understand its operation, results, and uncertainty. After completing a literature search for, reading published papers on, and discussing the operation of the RPA with electric propulsion researchers, I applied the knowledge gained to the development of a RPA for thermal plasma. A design of a thermal RPA was made which compensates for a large Debye length and low ionized plasma. From this design a thermal RPA was constructed. It consists of an outer stainless steel casing, a phenolic insulator (outgases slightly), and stainless steel mesh for the voltage screens. From the experience and knowledge gained in the development of the thermal RPA, a RPA for collimated plasma was developed. A collimated RPA has been designed and constructed. It compensate for a smaller Debye length and much higher ionization than that existing in the thermal plasma. It is 17% of the size of the thermal RPA. A stainless steel casing shields the detector from impinging electrons and ions. An insulating material, epoxy resin, was utilized which has a negligible outgassing. This material can be molded in styrofoam and machined quite nicely. It is capable of withstanding moderately high temperatures. Attached to this resin insulator are inconel screens attached by silver plated copper wire to a voltage supply. All the work on the RPAs and thermal ion source, I performed in the University of Alabama in Huntsville's (UAH) engineering machine shop.

Karr, Gerald R.↗
DOE OSTI2020

Conductivity of superconductors in the flux flow regime

Here, we develop a theory of conductivity of type-II superconductors in the flux flow regime taking into account random spatial fluctuations of the system parameters, such as the gap magnitude Δ(r) and the diffusion coefficient $\textit{D}$(r). We find a contribution to the conductivity that is proportional to the inelastic relaxation time $τ_{\text{in}}$, which is much longer than the elastic relaxation time. This contribution is due to Debye-type relaxation, and it can be much larger than the conventional flux flow conductivity due to Bardeen and Stephen. The new contribution is expected to dominate in clean superconductors at low temperatures and in magnetic fields much smaller than $H_{\text{c2}}$.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1979

The near-wake structure of the Atmosphere Explorer C /AE-C/ satellite - A parametric investigation

Measurements of ion current, electron temperature, and values of space potential obtained from the cylindrical electrostatic probe on board the Atmosphere Explorer C (AE-C) satellite were used to examine, in a parametric manner, the angular distribution of charge around the satellite. Interest is focused on nighttime equatorial data in the altitude range 275-620 km, which yields a wide range for the parameter R sub D (the radius of the satellite divided by the ambient value of the Debye lengths), including R sub D greater than 100, which is of practical significance to large space platforms. The variations of normalized ion current in the wake zone of the AE-C satellite appear to display an exponential dependence on R sub D for 'constant' values of other relevant parameters. The angular variations of electron temperature and space potential in the close vicinity of the satellite's surface were examined and compared with results from the Explorer 31 satellite. The variation of the ratio of measured to computed space potential with electron temperature was examined using data from both the AE-C and Explorer 31 satellites. It was found that the ratio is greater than unity. Possible causes for the above inequality are discussed.

Samir, U.↗
NASA NTRS2001

Micromechanical and Electrical Properties of Monolithic Aluminum Nitride at High Temperatures

Micromechanical spectroscopy of aluminum nitride reveals it to possess extremely low background internal friction at less than 1 x 10 (exp -4) logarithmic decrement (log dec.) from 20 to 1200 C. Two mechanical loss peaks were observed, the first at 350 C approximating a single Debye peak with a peak height of 60 x 10 (exp -4) log dec. The second peak was seen at 950 C with a peak height of 20 x 10 (exp -4) log dec. and extended from 200 to over 1200 C. These micromechanical observations manifested themselves in the electrical behavior of these materials. Electrical conduction processes were predominately intrinsic. Both mechanical and electrical relaxations appear to be thermally activated processes, with activation energies of 0.78 and 1.32 eV respectively.

Goldsby, Jon C.↗
DOE OSTI2026

Expansion-driven Weibel instability in magnetohydrodynamics: Linear theory for static systems

The Weibel dispersion relation is obtained in the magnetohydrodynamic (MHD) approximation by including a tensor expression for temperature. MHD gives an upper cutoff wavenumber for Weibel growth identical to kinetic theory if the electron drift velocity is included in the evolution of the electron temperature. MHD overestimates growth rates compared to kinetic theory and gives maximum growth at larger wavenumbers, but it only leads to divergent results as temperature anisotropy tends to infinity. Thermal conduction in MHD lowers the growth rates and shifts maximum growth to smaller wavenumbers, but flux-limited or nonlocal thermal conduction is found to significantly limit these effects. The results are used as a first step in evaluating the potential of MHD to simulate expansion-driven Weibel instability, which occurs due to the temperature tensor only cooling in the direction of expansion. Finally, the small temperature anisotropy found in expansion-driven Weibel instability means that MHD could be an adequate model because growth rates are much less than the electron plasma frequency, and wavelengths are much greater than the Debye length.

Davies, J. R. [Univ. of Rochester, NY (United Stat↗
DOE OSTI2020

High-temperature ion-thermal behavior from average-atom calculations

Atom-in-jellium calculations of the Einstein frequency were used to calculate the mean displacement of an ion over a wide range of compression and temperature. Expressed as a fraction of the Wigner-Seitz radius, the displacement is a measure of the asymptotic freedom of the ion at high temperature, and thus of the change in heat capacity from six to three quadratic degrees of freedom per atom. A functional form for free energy was proposed based on the Maxwell-Boltzmann distribution as a correction to the Debye free energy, with a single free parameter representing the effective density of potential modes to be saturated. Finally, this parameter was investigated using molecular dynamics simulations, and found to be ~0.2 per atom. In this way, the ion-thermal contribution can be calculated for a wide-range equation of state (EOS) without requiring a large number of molecular dynamics simulations. Example calculations were performed for carbon, including the sensitivity of key EOS loci to ionic freedom.

74 ATOMIC AND MOLECULAR PHYSICS↗
NASA NTRS2019

K-line X-ray fluorescence from highly charged iron ions under dense astrophysical plasma conditions

In the present work, we report an investigation of plasma environment effects on the atomic parameters associated with the K-vacancy states in highly charged iron ions within the astrophysical context of accretion disks around black holes. More particularly, the sensitivity of K-line X-ray fluorescence parameters (wavelengths, radiative transition probabilities, and Auger rates) in Fe XVII–Fe XXV ions has been estimated for plasma conditions characterized by an electron temperature ranging from 105 to 107K and an electron density ranging from 1018 to1022 cm−3. In order to do this, relativistic multiconfiguration Dirac-Fock atomic structure calculations have been carried out by considering a time averaged Debye-Hückel potential for both the electron–nucleus and electron–electron interactions.

Jérôme Deprince↗