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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 901 records · Page 50
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Design of a new charge exchange recombination spectroscopy diagnostic for impurity transport experiments at Wendelstein 7-X

In this study, we present the design of a new charge exchange recombination spectroscopy (CXRS) system for Wendelstein 7-X (W7-X), which aims at measuring line radiation from highly ionized iron impurities after laser blow off injections over transport timescales. New fiber bundles with a core diameter of 600 µm have been added to one of the existing optical systems at W7-X used for CXRS. The fibers direct collected light to five newly developed single channel high frame rate F/2.8 spectrometers. The new custom-made spectrometers are built using entirely off-the-shelf and 3D printed components, and employ a round-to-linear fiber bundle, instead of an entrance slit, to maximize light throughput. The detector is an ultra-low readout noise EM-CCD camera capable of frame rates of up to 10 000 fps for about 500 consecutive frames when operated using a single readout channel. Such high frame rates will enable the study of inward convection of injected impurities. An initial sensitivity study is performed using a newly developed Markov chain Monte Carlo approach based on the pySTRAHL impurity transport code. This study indicates that, with the addition of the new CX diagnostic, impurity convection velocities can be inferred.

Instruments & Instrumentation↗
DOE OSTI2025

Charge carrier extraction and recombination effects in GaInAs/GaAsP multi-quantum well solar cells

The carrier extraction and transport mechanisms as well as the relative contributions of radiative and non-radiative recombination processes are investigated in high-quality strain-balanced GaInAs/GaAsP multi-quantum well solar cells recently implemented in record efficiency multijunction solar cells. A comprehensive suite of complementary characterization techniques including temperature- and suns-dependent photoluminescence and photovoltaic measurements are employed to analyze thermal escape and tunneling rates, which demonstrate the need to move beyond simple drift-diffusion models of p–n junctions. This study examines the processes that best characterize the operation of these devices across varying temperatures using a simple two-diode model, incorporating multiple transport protocols, and provides insights into the performance-limiting processes and pathways for their optimization.

14 SOLAR ENERGY↗
DOE OSTI2017

Dissecting limiting factors of the Protein synthesis Using Recombinant Elements (PURE) system

Reconstituted cell-free protein synthesis systems such as the Protein synthesis Using Recombinant Elements (PURE) system give high-throughput and controlled access to in vitro protein synthesis. Here we show that compared with the commercial S30 crude extract based RTS 100 E. coli HY system, the PURE system has less mRNA degradation and produces up to ~6-fold full-length proteins. However the majority of polypeptides PURE produces are partially translated or inactive since the signal from firefly luciferase (Fluc) translated in PURE is only ~2/3 rd of that measured using the RTS 100 E. coli HY S30 system. Both of the 2 batch systems suffer from low ribosome recycling efficiency when translating proteins from 82 k D to 224 k D . A systematic fed-batch analysis of PURE shows replenishment of 6 small molecule substrates individually or in combination before energy depletion increased Fluc protein yield by ~1.5 to ~2-fold, while creatine phosphate and magnesium have synergistic effects when added to the PURE system. Additionally, while adding EF-P to PURE reduced full-length protein translated, it increased the fraction of functional protein and reduced partially translated protein probably by slowing down the translation process. Finally, ArfA, rather than YaeJ or PrfH, helped reduce ribosome stalling when translating Fluc and improved system productivity in a template-dependent fashion.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2017

Deuterium charge exchange recombination spectroscopy from the top of the pedestal to the scrape off layer in H-mode plasmas

Recent completion of the thirty two channel main-ion (deuterium) charge exchange recombination spectroscopy (CER) diagnostic on DIII-D enables detailed comparisons between impurity and main-ion temperature, density, and toroidal rotation. Sixteen sightlines cover the core of the plasma and another sixteen are densely packed towards the edge, providing high resolution measurements of the pedestal and steep gradient edge region of H-mode plasmas. The complexities of the D α spectrum require fitting with a comprehensive model, as well as using iterative collisional radiative modeling to determine the underlying thermal deuterium ion properties. Large differences in the structure and magnitude of impurity (C 6+ ) and main-ion (D + ) toroidal rotation profiles are seen in the H-mode pedestal. Additionally the D + temperature can be half the value of the C 6+ temperature at the separatrix and shows more of a pedestal structure. Typically only the impurity properties are measured and the main-ion properties are either assumed to be the same, or inferred using neoclassical models, which require validation in the steep gradient region. Furthermore, these measured differences have implications for transport model validation, intrinsic rotation studies, pedestal stability, and the boundary conditions for scrape off layer and plasma material interactions studies.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Thermophilic site-specific recombination system for rapid insertion of heterologous DNA into the Clostridium thermocellum chromosome

Clostridium thermocellum is an anaerobic thermophile capable of producing ethanol and other commodity chemicals from lignocellulosic biomass. The insertion of heterologous DNA into the C. thermocellum chromosome is currently achieved via a time-consuming homologous recombination process, where a single stable insertion can take 2–4 weeks or more to construct. In this work, we developed a thermostable version of the Serine recombinase Assisted Genome Engineering (tSAGE) approach for gene insertion in C. thermocellum utilizing a site-specific recombinase from Geobacillus sp. Y412MC61, enabling quick and easy insertion of DNA into the chromosome for accelerated genetic tool screening and heterologous gene expression. Using tSAGE, chromosomal insertion of plasmid DNA occurred at a maximum transformation efficiency of 5 × 10 3 CFU/µg, which is comparable to the transformation efficiency of a replicating control plasmid in C. thermocellum. Using tSAGE, we chromosomally integrated and characterized 17 reporter genes, 15 homologous and 31 heterologous constitutive promoters of varying strengths, 4 inducible promoters, and 5 riboswitches in C. thermocellum. We also determined that a 6–7 nucleotide gap between the ribosome binding site (RBS) and the start codon is optimal for high expression by employing a library of superfolder green fluorescent protein expression constructs driven by our strongest tested promoter (P clo1313_1194 ) with different distances between the RBS and start codon. The tools developed here will aid in accelerating C. thermocellum strain engineering for producing sustainable fuels and chemicals directly from plant biomass.

Biofuels↗
DOE OSTI2013

Epigenome-wide inheritance of cytosine methylation variants in a recombinant inbred population

Cytosine DNA methylation is one avenue for passing information through cell divisions. Here, we present epigenomic analyses of soybean recombinant inbred lines (RILs) and their parents. Identification of differentially methylated regions (DMRs) revealed that DMRs mostly cosegregated with the genotype from which they were derived, but examples of the uncoupling of genotype and epigenotype were identified. Linkage mapping of methylation states assessed from whole-genome bisulfite sequencing of 83 RILs uncovered widespread evidence for local methylQTL. This epigenomics approach provides a comprehensive study of the patterns and heritability of methylation variants in a complex genetic population over multiple generations, paving the way for understanding how methylation variants contribute to phenotypic variation.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2025

Measurements and models of enhanced recombination following inner-shell vacancies in liquid xenon

Electron-capture decays of 125 Xe and 127 Xe , and double-electron-capture decays of 124 Xe , are backgrounds in searches for weakly interacting massive particles (WIMPs) conducted by dual-phase xenon time projection chambers such as LUX-ZEPLIN (LZ). These decays produce signals with more light and less charge than equivalent-energy 𝛽 decays and correspondingly overlap more with WIMP signals. We measure three electron-capture charge yields in LZ: the 1.1 keV M-shell, 5.2 keV L-shell, and 33.2 keV K-shell at drift fields of 193 and 96.5 V/cm. The LL double-electron-capture decay of 124 Xe exhibits even more pronounced shifts in charge and light. We provide a first model of double-electron-capture charge yields using the link between ionization density and electron-ion recombination, and identify a need for more accurate calculations. Finally, we discuss the implications of the reduced charge yield of these decays and other interactions creating inner-shell vacancies for future dark matter searches.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2025

𝐴𝑏 initio density-matrix approach to exciton coherence: Phonon scattering, Coulomb interactions, and radiative recombination

Relaxation processes following light excitation in semiconductors are key in materials-based quantum technology applications. These processes are broadly studied in atomically thin transition-metal dichalcogenides, quasi-two-dimensional excitonic semiconductors in which atomistic design allows for tunable excited-state properties, such as relaxation lifetimes and photoinduced coherence. In this work, we present a density-matrix-based approach to compute exciton relaxation within a many-body ab initio perspective. We expand our previously developed Lindblad density-matrix formalism to capture multichannel electron-hole pair relaxation processes, including phonon and Coulomb scattering as well as radiative recombination, and we study their effect on the time-resolved excited-state propagation. Using monolayer MoSe 2 as a prototypical example, we examine many-body effects on the time-dependent dynamics of photoactive excitations, exploring how the electron-hole pair interactions are reflected in variations of the excitation energy, spectral signature, and state coherence. In conclusion, our method supplies a detailed understanding of exciton relaxation mechanisms in realistic materials, offering a previously unexplored pathway to study excited-state dynamics in semiconductors from first principles.

Band structure methods↗
DOE OSTI2018

Validity of the Born-Oppenheimer approximation in the indirect-dissociative-recombination process

An alternative method is introduced to solve a simple two-dimensional model describing vibrational excitation and dissociation processes during the electron-molecule collisions. The model works with one electronic and one nuclear degree of freedom. The two-dimensional R matrix can be constructed simultaneously on the electronic and nuclear surfaces using all three forms developed previously for electron-atom and electron-molecule collisions. These are the eigenchannel R-matrix form, inversion technique of Nesbet and Robicheaux, and the Wigner-Eisenbud-type form using expansion over the poles of the symmetrized Hamiltonian. The 2D R-matrix method is employed to solve a simple model tailored to describe the dissociative recombination and the vibrational excitation of H 2 + cation in the singlet ungerade symmetry 1 Σ u . These results then serve as a (near-exact) benchmark for the following calculation in which the R-matrix states are replaced by their Born-Oppenheimer approximations. Furthermore, the accuracy of this approach and its correction with the first-order nonadiabatic couplings are discussed.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2019

Dynamic defect as nonradiative recombination center in semiconductors

We present a theory of nonradiative recombination (NRR) with an emphasis on the so far little-explored dynamic effect in the process. Here, we show that it can significantly enhance the NRR rate over that of a static midgap level as suggested by the Shockley-Read-Hall theory, whereby offering an alternative explanation to the long-lasting discrepancy between theory and experiment for semiconductors. As an illustration, we show that dynamic NRR can take place at the DX center in Si-doped GaAs which, combined with a modified ABC model at high carrier-density limit, makes it possible to verify the theory directly by experiment.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Intervalley electron-hole exchange interaction and impurity-assisted recombination of indirect excitons in WS 2 and WSe 2 monolayers

The variety of excitonic states in tungsten-based dichalcogenide monolayers stems from unique interplay between the spin and valley degrees of freedom. One of the exciton species is the indirect exciton (momentum or valley dark), which is responsible for a series of resonances when the monolayer is charge neutral. We investigate the short-range electron-hole exchange interaction of the indirect exciton, as well as its recombination mechanism mediated by impurities. The analysis provides thorough understanding of the energy and polarization of the zero-phonon indirect exciton resonance in the emission spectrum.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Localization landscape theory of disorder in semiconductors. III. Application to carrier transport and recombination in light emitting diodes

In this paper we introduce a novel method to account for quantum disorder effects into the classical drift-diffusion model of semiconductor transport through the localization landscape theory. Quantum confinement and quantum tunneling in the disordered system change dramatically the energy barriers acting on the perpendicular transport of heterostructures. In addition, they lead to percolative transport through paths of minimal energy in the two-dimensional (2D) landscape of disordered energies of multiple 2D quantum wells. This model solves the carrier dynamics with quantum effects self-consistently and provides a computationally much faster solver when compared with the Schrödinger equation resolution. The theory also provides a good approximation to the density of states for the disordered system over the full range of energies required to account for transport at room temperature. The current-voltage characteristics modeled by three-dimensional simulation of a full nitride-based light emitting diode (LED) structure with compositional material fluctuations closely match the experimental behavior of high-quality blue LEDs. The model allows also a fine analysis of the quantum effects involved in carrier transport through such complex heterostructures. Finally, details of carrier population and recombination in the different quantum wells are given.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Recombination Era Magnetic Fields from Axion Dark Matter

We introduce a new mechanism for generating magnetic fields in the recombination era. This Harrison-like mechanism utilizes vorticity in baryons that is sourced through the Bose-Einstein condensate of axions via gravitational interactions. The magnetic fields generated are on galactic scales ∼10 kpc and have a magnitude of the order of B∼10−23G today. The field has a greater magnitude than those generated from other mechanisms relying on second-order perturbation theory, and is sufficient to provide a seed for battery mechanisms.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

New Phases and Dissociation-Recombination of Hydrogen Deuteride to 3.4 Mbar

Here, we present infrared absorption studies of solid hydrogen deuteride to pressures as high as 340 GPa (100 GPa = 1 Mbar) in a diamond anvil cell and temperatures in the range 5–295 K. Above 198 GPa the HD sample transforms to a mixture of HD, H 2 , and D 2 , interpreted as a process of dissociation and recombination. Three new phase lines are observed, two of which differ remarkably from those of the highpressure homonuclear species, but none are metallic. The time-dependent spectral changes are analyzed to determine the molecular concentrations as a function of time; the nucleon exchange achieves steady state concentrations in ~20 h at ~200 GPa.

08 HYDROGEN↗
DOE OSTI2017

Precise Access to the Molecular-Frame Complex Recombination Dipole through High-Harmonic Spectroscopy

In this paper, we report on spectral intensity and group delay measurements of the highest-occupied molecular-orbital (HOMO) recombination dipole moment of ${\mathrm{N}}_{2}$ in the molecular-frame using high harmonic spectroscopy. We take advantage of the long-wavelength $1.3\text{ }\text{ }{\mu}\mathrm{m}$ driving laser to isolate the HOMO in the near threshold region, 19–67 eV. Finally, the precision of our group delay measurements reveals previously unseen angle-resolved spectral features associated with autoionizing resonances, and allows quantitative comparison with cutting-edge correlated 8-channel photoionization dipole moment calculations.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2023

Quasiparticle Generation-Recombination Noise in the Limit of Low Detector Volume

We have measured the quasiparticle generation-recombination (GR) noise in aluminium lumped element kinetic inductors with a wide range of detector volumes at various temperatures. The basic detector consists of meandering inductor and interdigitated capacitor fingers. The inductor volume is varied from 2 to 153 mu m(3) by changing the inductor width and length to maintain a constant inductance. We started with measuring the power spectrum density (PSD) of the detectors frequency noise which is a function of GR noise and we clearly observed the spectrum roll off at 10 kHz which corresponds to the quasiparticle lifetime. Using data from a temperature sweep of the resonator frequency we convert the frequency fluctuation to quasiparticle fluctuation and observe its strong dependence on detector volume: detectors with smaller volume display less quasiparticle noise amplitude. Meanwhile we observe a saturated quasiparticle density at low temperature from all detectors as the quasiparticle life time tqp approaches a constant value at low temperature.

Li, Juliang↗
DOE OSTI2017

Comparative plastome genomics and phylogenomics of Brachypodium : flowering time signatures, introgression and recombination in recently diverged ecotypes

Few pan-genomic studies have been conducted in plants, and none of them have focused on the intraspecific diversity and evolution of their plastid genomes. We address this issue in Brachypodium distachyon and its close relatives B. stacei and B. hybridum, for which a large genomic data set has been compiled. We analyze inter- and intraspecific plastid comparative genomics and phylogenomic relationships within a family-wide framework. Major indel differences were detected between Brachypodium plastomes. Within B. distachyon, we detected two main lineages, a mostly Extremely Delayed Flowering (EDF+) clade and a mostly Spanish (S+) – Turkish (T+) clade, plus nine chloroplast capture and two plastid DNA (ptDNA) introgression and micro-recombination events. Early Oligocene (30.9 million yr ago (Ma)) and Late Miocene (10.1 Ma) divergence times were inferred for the respective stem and crown nodes of Brachypodium and a very recent Mid-Pleistocene(0.9 Ma) time for the B. distachyon split. Flowering time variation is a main factor driving rapid intraspecific divergence in B. distachyon, although it is counterbalanced by repeated introgression between previously isolated lineages. Swapping of plastomes between the three different genomic groups, EDF+,T+, S+, probably resulted from random backcrossing followed by stabilization through selection pressure.

Sancho, Rubén↗
DOE OSTI2019

Electrical suppression of all nonradiative recombination pathways in monolayer semiconductors

Defects in conventional semiconductors substantially lower the photoluminescence (PL) quantum yield (QY), a key metric of optoelectronic performance that directly dictates the maximum device efficiency. Here, two-dimensional transition-metal dichalcogenides (TMDCs), such as monolayer MoS 2 , often exhibit low PL QY for as-processed samples, which has typically been attributed to a large native defect density. We show that the PL QY of as-processed MoS 2 and WS 2 monolayers reaches near-unity when they are made intrinsic through electrostatic doping, without any chemical passivation. Surprisingly, neutral exciton recombination is entirely radiative even in the presence of a high native defect density. This finding enables TMDC monolayers for optoelectronic device applications as the stringent requirement of low defect density is eased.

36 MATERIALS SCIENCE↗