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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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All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Electron optics for 4 MeV ultrafast electron microscope - Final Report

Calabazas Creek Research, Inc. (CCR), in collaboration with Northern Illinois University (NIU), proposed to leverage CCR’s experience in electron beam generation and focusing and NIU’s experience in electron microscopy to develop a 4 MeV UEM with sub-ns temporal resolution and 0.3 nm spatial resolution with a relative energy spread of 10-5. Important design tools for the program were CCR’s 3D, finite element code Beam Optics Analyzer (BOA) and the photocathode design code PHOTOQE. However, early in the program it became clear that information necessary to use PHOTOQE was not available, and the program focused on the modifying to BOA to provide the required functionality.

beam optics simulation↗
DOE OSTI2016

ELECTRON CYCLOTRON MASER EMISSIONS FROM EVOLVING FAST ELECTRON BEAMS

Fast electron beams (FEBs) are common products of solar active phenomena. Solar radio bursts are an important diagnostic tool for understanding FEBs and the solar plasma environment in which they propagate along solar magnetic fields. In particular, the evolution of the energy spectrum and velocity distribution of FEBs due to the interaction with the ambient plasma and field during propagation can significantly influence the efficiency and properties of their emissions. In this paper, we discuss the possible evolution of the energy spectrum and velocity distribution of FEBs due to energy loss processes and the pitch-angle effect caused by magnetic field inhomogeneity, and we analyze the effects of the evolution on electron-cyclotron maser (ECM) emission, which is one of the most important mechanisms for producing solar radio bursts by FEBs. Our results show that the growth rates all decrease with the energy loss factor Q , but increase with the magnetic mirror ratio σ as well as with the steepness index δ . Moreover, the evolution of FEBs can also significantly influence the fastest growing mode and the fastest growing phase angle. This leads to the change of the polarization sense of the ECM emission. In particular, our results also reveal that an FEB that undergoes different evolution processes will generate different types of ECM emission. We believe the present results to be very helpful for a more comprehensive understanding of the dynamic spectra of solar radio bursts.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Constraint on a cosmological variation in the proton-to-electron mass ratio from electronic co absorption

Carbon monoxide (CO) absorption in the sub-damped Lyα absorber at redshift z{sub abs}≃2.69 toward the background quasar SDSS J123714.60+064759.5 (J1237+0647) was investigated for the first time in order to search for a possible variation of the proton-to-electron mass ratio, μ, over a cosmological timescale. The observations were performed with the Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph with a signal-to-noise ratio of 40 per 2.5 km s{sup −1} per pixel at ∼5000 Å. Thirteen CO vibrational bands in this absorber are detected: the A{sup 1}Π − X{sup 1}Σ{sup +} (ν{sup ′}, 0) for ν{sup ′}=0--8, B{sup 1}Σ{sup +} − X{sup 1}Σ{sup +} (0, 0), C{sup 1}Σ{sup +} − X{sup 1}Σ{sup +} (0, 0), and E{sup 1}Π − X{sup 1}Σ{sup +} (0, 0) singlet–singlet bands and the d{sup 3}Δ − X{sup 1}Σ{sup +} (5, 0) singlet–triplet band. An updated database including the most precise molecular inputs needed for a μ-variation analysis is presented for rotational levels J = 0–5, consisting of transition wavelengths, oscillator strengths, natural lifetime damping parameters, and sensitivity coefficients to a variation of the proton-to-electron mass ratio. A comprehensive fitting method was used to fit all the CO bands at once and an independent constraint of Δμ/μ=(0.7±1.6{sub stat}±0.5{sub syst})×10{sup −5} was derived from CO only. A combined analysis using both molecular hydrogen and CO in the same J1237+0647 absorber returned a final constraint on the relative variation of Δμ/μ=(−5.6±5.6{sub stat}±3.1{sub syst})×10{sup −6}, which is consistent with no variation over a look-back time of ∼11.4 Gyr.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

AlGaN High Electron Mobility Transistor for High-Temperature Logic

Here we report on AlGaN high electron mobility transistor (HEMT)-based logic development, using combined enhancement- and depletion-mode transistors to fabricate inverters with operation from room temperature up to 500°C. Our development approach included: (a) characterizing temperature-dependent carrier transport for different AlGaN HEMT heterostructures, (b) developing a suitable gate metal scheme for use in high temperatures, and (c) over-temperature testing of discrete devices and inverters. Hall mobility data (from 30°C to 500°C) revealed the reference GaN-channel HEMT experienced a 6.9x reduction in mobility, whereas the AlGaN channel HEMTs experienced about a 3.1x reduction. Furthermore, a greater aluminum contrast between the barrier and channel enabled higher carrier densities in the two-dimensional electron gas for all temperatures. The combination of reduced variation in mobility with temperature and high sheet carrier concentration showed that an Al-rich AlGaN-channel HEMT with a high barrier-to-channel aluminum contrast is the best option for an extreme temperature HEMT design. Three gate metal stacks were selected for low resistivity, high melting point, low thermal expansion coefficient, and high expected barrier height. The impact of thermal cycling was examined through electrical characterization of samples measured before and after rapid thermal anneal. The 200-nm tungsten gate metallization was the top performer with minimal reduction in drain current, a slightly positive threshold voltage shift, and about an order of magnitude advantage over the other gates in on-to-off current ratio. After incorporating the tungsten gate metal stack in device fabrication, characterization of transistors and inverters from room temperature up to 500°C was performed. The enhancement-mode (e-mode) devices’ resistance started increasing at about 200°C, resulting in drain current degradation. This phenomenon was not observed in depletion-mode (d-mode) devices but highlights a challenge for inverters in an e-mode driver and d-mode load configuration.

42 ENGINEERING↗
DOE OSTI2018

Crossover-Induced Spin Fluctuation and Electron Pairing in Strongly Correlated Electrons

We investigate the two-dimensional (2D) Hubbard model and d-p model as a model for cuprate high-temperature superconductors, by employing an optimized variational Monte Carlo method. We improved the many-body wave function starting from the simple Gutzwiller function. On the basis of optimized variational wave functions, we show that there is a superconducting phase in the ground state of the 2D Hubbard model. The on-site repulsive Coulomb interaction U can bring about high-temperature superconductivity because its characteristic energy is of the order of eV. The attractive interaction between electrons is induced by the crossover from weakly to strongly correlated regions. High-temperature superconductivity is possible in the strongly correlated region, where U is greater than the bandwidth.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗