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At least 1,045 records · Page 58

All sources NASA NTRS DOE OSTI

NASA NTRS1972

Electrostatic thrusters.

The current status of research and development programs on electrostatic thrusters is reviewed. Current programs that utilize mercury electron-bombardment thrusters range from 5- to 30-cm in diameter. Recent progress on the 5-cm thruster has emphasized durability, with accelerator time exceeding 6300 hours and total time on the rest of the thruster exceeding 8300 hours. Recent progress on the 30-cm thruster has been outstanding in dished-grid accelerator systems. Ion beams up to 5 amperes have been obtained for short periods with 1000 volts net accelerating potential difference. The cesium electron-bombardment and cesium contact programs are also described.

Kaufman, H. R.↗

NASA NTRS1978

Analytical and numerical investigation of structural response of compliant wall materials, part 2

Theoretical analysis of an electrostatically driven wall system for a compliant wall drag reduction program is reported. The electrostatic wall system is capable of producing deflections of many orders greater than the wall thicknesses and at small wavelengths. An intermediate large response theory is used for structural analysis. The theoretical predictions are compared to bench test results, and good agreement between the two is obtained. The effects of aerodynamic loads and perturbation electric fields on the theoretical solutions are considered. It is shown that for very small wavelengths (lambda almost equals 2 mm) the aerodynamic effects can be estimated using potential theory without loss of accuracy, and the perturbation electric fields do not effect solutions as long as the deflections are less than one percent of the wavelength. Resonance effects for this type of structure are shown to be fairly small.

Balasubramanian, R.↗

NASA NTRS1986

A magnetospheric critical velocity experiment - Particle results

In March of 1983, a barium injection sounding rocket experiment (The Star of Lima) was conducted to investigate Alfven's critical ionization velocity (CIV) hypothesis in space. Included in the instrumented payload was a particle detection experiment consisting of five retarding potential analyzers. Despite conditions that appeared to be optimal for the critical velocity effect, the particle data, in agreement with optical observations, indicates that a fractional ionization of only approximately .0005 was observed, indicating that the conditions required for the effect to occur are still not well understood. However many of the required phenomena associated with the CIV effect were observed; in particular a superthermal electron population was formed at the expense of ion drift kinetic energy in the presence of intense electrostatic waves near the lower hybrid frequency. The amount of ionization produced is plausibly consistent with the observed electron flux, but could also be accounted for by residual solar UV at the injection point. It is shown based on the data set that one obvious explanation for the low ionization efficiency, namely that the ionizing superthermal electrons may rapidly escape along field lines, can be ruled out.

Torbert, R. B.↗

DOE OSTI2021

Imaging Quantum Interference in Stadium-Shaped Monolayer and Bilayer Graphene Quantum Dots

Experimental realizations of graphene-based stadium-shaped quantum dots (QDs) have been few and have been incompatible with scanned probe microscopy. Yet, the direct visualization of electronic states within these QDs is crucial for determining the existence of quantum chaos in these systems. Here we report the fabrication and characterization of electrostatically defined stadium-shaped QDs in heterostructure devices composed of monolayer graphene (MLG) and bilayer graphene (BLG). To realize a stadium-shaped QD, we utilized the tip of a scanning tunneling microscope to charge defects in a supporting hexagonal boron nitride flake. The stadium states visualized are consistent with tight-binding-based simulations but lack clear quantum chaos signatures. The absence of quantum chaos features in MLG-based stadium QDs is attributed to the leaky nature of the confinement potential due to Klein tunneling. In contrast, for BLG-based stadium QDs (which have stronger confinement) quantum chaos is precluded by the smooth confinement potential which reduces interference and mixing between states.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Electrostatic gating and intercalation in 2D materials

The doping or the alteration of crystals with guest species to obtain desired properties has long been a research frontier in materials science. However, the closely packed lattice structure in many crystals has limited the applicability of this strategy. The advent of 2D layered materials has led to revitalized interest in utilizing this approach through two important strategies, gating and intercalation, offering reversible modulation of the properties of the host material without breaking chemical bonds. In addition, these dynamically tunable techniques have enabled the synthesis of new hybrid materials. Here, we review how interactions between guest species and host 2D materials can tune the physics and chemistry of materials and discuss their remarkable potential for creating artificial materials and architectures beyond the reach of conventional methods.

36 MATERIALS SCIENCE↗

NASA NTRS1978

Economics of ion propulsion for large space systems

This study of advanced electrostatic ion thrusters for space propulsion was initiated to determine the suitability of the baseline 30-cm thruster for future missions and to identify other thruster concepts that would better satisfy mission requirements. The general scope of the study was to review mission requirements, select thruster designs to meet these requirements, assess the associated thruster technology requirements, and recommend short- and long-term technology directions that would support future thruster needs. Preliminary design concepts for several advanced thrusters were developed to assess the potential practical difficulties of a new design. This study produced useful general methodologies for assessing both planetary and earth orbit missions. For planetary missions, the assessment is in terms of payload performance as a function of propulsion system technology level. For earth orbit missions, the assessment is made on the basis of cost (cost sensitivity to propulsion system technology level).

Masek, T. D.↗

NASA NTRS1980

Analysis of differential and active charging phenomena on ATS-5 and ATS-6

Spacecraft charging on the differential charging and artificial particle emission experiments on ATS 5 and ATS 6 were studied. Differential charging of spacecraft surfaces generated large electrostatic barriers to spacecraft generated electrons, from photoemission, secondary emission, and thermal emitters. The electron emitter could partially or totally discharge the satellite, but the mainframe recharged negatively in a few 10's of seconds. The time dependence of the charging behavior was explained by the relatively large capacitance for differential charging in comparison to the small spacecraft to space capacitance. A daylight charging event on ATS 6 was shown to have a charging behavior suggesting the dominance of differential charging on the absolute potential of the mainframe. Ion engine operations and plasma emission experiments on ATS 6 were shown to be an effective means of controlling the spacecraft potential in eclipse and sunlight. Elimination of barrier effects around the detectors and improving the quality of the particle data are discussed.

Olsen, R. C.↗

NASA NTRS1995

(abstract) Production and Levitation of Free Drops of Liquid Helium

We are interested in the nucleation and behavior of quantized vorticies and surface excitations in free drops of superfluid helium. We have constructed an apparatus to maintain liquid helium drops isolated from any material container in the Earth's gravitational field, and have investigated two techniques for generating and introducing liquid drops into the region of confinement. The levitation apparatus utilizes the electrostatic force acting upon a charged liquid drop to counteract the gravitational force, with drop position stability provided by a static magnetic field acting upon the helium diamagnetic moment. Electrically neutral superfluid drops have been produced with a miniature thermomechanical pump; for a given configuration the liquid initial velocity has been varied up to several centimeters per second. Liquid drops carrying either net positive or negative charge are produced by an electrode which generates a flow of ionized liquid from the bulk liquid surface. Potentials of less than one thousand volts to several thousand volts are required. The mass flow is controlled by varying duration of the ionizing voltage pulse; drops as small as 30 micrometers diameter, charged to near the Rayleigh limit, have been observed.

quantized vorticity levitation liquid helium↗

NASA NTRS2007

Arrays of Bundles of Carbon Nanotubes as Field Emitters

Experiments have shown that with suitable choices of critical dimensions, planar arrays of bundles of carbon nanotubes (see figure) can serve as high-current-density field emitter (cold-cathode) electron sources. Whereas some hot-cathode electron sources must be operated at supply potentials of thousands of volts, these cold-cathode sources generate comparable current densities when operated at tens of volts. Consequently, arrays of bundles of carbon nanotubes might prove useful as cold-cathode sources in miniature, lightweight electron-beam devices (e.g., nanoklystrons) soon to be developed. Prior to the experiments, all reported efforts to develop carbon-nanotube-based field-emission sources had yielded low current densities from a few hundred microamperes to a few hundred milliamperes per square centimeter. An electrostatic screening effect, in which taller nanotubes screen the shorter ones from participating in field emission, was conjectured to be what restricts the emission of electrons to such low levels. It was further conjectured that the screening effect could be reduced and thus emission levels increased by increasing the spacing between nanotubes to at least by a factor of one to two times the height of the nanotubes. While this change might increase the emission from individual nanotubes, it would decrease the number of nanotubes per unit area and thereby reduce the total possible emission current. Therefore, to maximize the area-averaged current density, it would be necessary to find an optimum combination of nanotube spacing and nanotube height. The present concept of using an array of bundles of nanotubes arises partly from the concept of optimizing the spacing and height of field emitters. It also arises partly from the idea that single nanotubes may have short lifetimes as field emitters, whereas bundles of nanotubes could afford redundancy so that the loss of a single nanotube would not significantly reduce the overall field emission.

Manohara, Harish↗

DOE OSTI2026

Depth-Dependent Emission from Silver Dopants in Single CdSe Nanoplatelets

Dopants in semiconductor nanostructures offer tremendous control over electronic, optical, and magnetic properties beyond what is achievable in bulk materials. We demonstrate that the broad dopant emission in semiconductor nanoplatelets effectively maps the electron wave function across the nanoplatelet thickness. Both the emission energy and lifetime of the dopant transition depend strongly on the depth of the dopant within the nanoplatelet. This dependence arises from the electrostatic self-interaction of the charged dopant, which varies with proximity to the dielectric discontinuity at the nanoplatelet surface. Through comprehensive single-particle spectroscopy of silver-doped CdSe nanoplatelets, we verify that acceptors near the center emit at higher energies with shorter lifetimes, while those near the surface emit at lower energies with longer lifetimes. This spatial mapping also reveals unusual two-color emission from individual nanoplatelets, with enhanced Auger recombination yielding exceptional photon antibunching (>90% purity) at room temperature, suggesting potential applications in quantum information technologies.

cadmium selenide↗

NASA NTRS2011

Solar Wind Access to Lunar Polar Craters: Feedback Between Surface Charging and Plasma Expansion

Determining the plasma environment within permanently shadowed lunar craters is critical to understanding local processes such as surface charging, electrostatic dust transport, volatile sequestration, and space weathering. In order to investigate the nature of this plasma environment, the first two-dimensional kinetic simulations of solar wind expansion into a lunar crater with a self-consistent plasma-surface interaction have been undertaken. The present results reveal how the plasma expansion into a crater couples with the electrically-charged lunar surface to produce a quasi-steady wake structure. In particular, there is a negative feedback between surface charging and ambipolar wake potential that allows an equilibrium to be achieved, with secondary electron emission strongly moderating the process. A range of secondary electron yields is explored, and two distinct limits are highlighted in which either surface charging or ambipoiar expansion is responsible for determining the overall wake structure.

Zimmerman, M. I.↗

DOE OSTI2021

Hydrogen Wide Area Monitoring of LH2 Releases at HSE for the PRESHLY Project

The characterization of liquid hydrogen (LH2) releases has been identified as an international research priority to expand the safe use of hydrogen as an energy carrier. Empirical field measurements such as those afforded by Hydrogen Wide Area Monitoring (HyWAM) can elucidate the behavior of LH2 releases, which can then be used to support and validate dispersion models. HyWAM can be defined as the quantitative three-dimensional spatial and temporal profiling of planned or unintentional hydrogen releases. The NREL Sensor Laboratory developed a HyWAM based upon a distributed array of hydrogen Sensors. The NREL Sensor Laboratory and Health and Safety Executive (HSE) formally committed to collaborate on profiling GH2 and LH2 releases, which allowed for the integration of the NREL HyWAM into the HSE LH2 release behavior investigation supported by the FCH JU Prenormative Research for the Safe Use of Liquid Hydrogen (PRESLHY) program. A HyWAM system consisting of 32 hydrogen measurement points and co-located temperature sensors were distributed downstream of the LH2 release apparatus developed by HSE. In addition, there were proximal wind and weather monitors. In a separate presentation at this conference "HSE SD Experimental Summary for the Characterisation, Dispersion and Electrostatic Hazards of LH2 for the PRESLHY Project" HSE researchers summarize the experimental apparatus and experimental protocols utilized in the HSE LH2 releases that were performed under the auspices of PRESLHY. To supplement the HSE presentation, this presentation will focus on the spatial and temporal behavior LH2 releases as measured by the NREL HyWAM. Correlations to ambient conditions, such as wind speed and direction, plume temperature and hydrogen concentrations will be discussed in addition to the design and performance of the NREL HyWAM, and its potential for improving hydrogen facility safety.

HYDROGEN↗

NASA NTRS1986

Electrostatic charging of electrically active spacecraft

A model is presented which predicts the temporal behavior of the ionospheric charging of spacecraft which eject energetic positive ions. The dynamic interaction of the spacecraft with the space-plasma environment is modelled as an equivalent electrical circuit consisting of a variable capacitor in parallel with two current sources. The spacecraft/space-plasma sheath is modelled as a variable capacitor containing positive-ion space charge. The ejection of energetic positive ions from the spacecraft represents a current source which charges the capacitor. Return current collected from the space-plasma sheath to the spacecraft is modelled as a variable current source which discharges the capacitor. The model can predict the temporal behavior of sheath movement, spacecraft potential, and space-plasma return current for a wide range of altitude, duty cycle, and ejected positive-ion current. Experimentally measured charging results, obtained for the Satellite Positive Ion Beam System (SPIBS) rocket flight, are accurately predicted by the model. Anticipated vehicle charging levels are presented for neutral-particle-beam platforms operating with a net, unneutralized, positive-ion-current component present in the emitted neutral-particle beam.

Matossian, J. N.↗

DOE OSTI2020

Current–Induced Ion Concentration Polarization at a Perfect Ion–Exchange Patch in an Infinite Insulating Wall

This research examines theoretically the ion concentration polarization, ion fluxes, and electrostatic fields near an ion exchange patch in the wall of an electrified fluidic channel. These phenomena are important in related microfluidic ion-preconcentration systems. Under an electric field, counterions enter the ion-exchange patch at one side and leave at the other, with salt depletion occurring near the entrance and accumulation near the exit. Furthermore, the high patch conductivity and the concentration profiles lead to local electric field perturbations that may facilitate preconcentration. This study includes analytical expressions of ion concentrations and electrochemical potentials at small to moderate electric fields, as well as numerical simulations. Additionally, a simple matrix of polynomial coefficients (obtained via fitting of numerical data) enables analytical calculation of the 2-dimensional concentration profiles at all electric fields within the range investigated in the numerical simulations. This is possible because a single dimensionless parameter controls this problem.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

Reining in Radium for Nuclear Medicine: Extra-Large Chelator Development for an Extra-Large Ion

Targeted α therapy (TAT) of soft-tissue cancers using the α particle-emitting radionuclide 223 Ra holds great potential because of its favorable nuclear properties, adequate availability, and established clinical use for treating metastatic prostate cancer of the bone. Despite these advantages, the use of 223 Ra has been largely overshadowed by other α emitters due to its challenging chelation chemistry. A key criterion that needs to be met for a radionuclide to be used in TAT is its stable attachment to a targeting vector via a bifunctional chelator. The low charge density of Ra 2+ arising from its large ionic radius weakens its electrostatic binding interactions with chelators, leading to insufficient complex stability in vivo. In this study, we synthesized and evaluated macropa-XL as a novel chelator for 223 Ra. It bears a large 21-crown-7 macrocyclic core and two picolinate pendent groups, which we hypothesized would effectively saturate the large coordination sphere of the Ra 2+ ion. The structural chemistry of macropa-XL was first established with the nonradioactive Ba 2+ ion using X-ray diffraction and X-ray absorption spectroscopy, which revealed the formation of an 11-coordinate complex in a rare anti pendent-arm configuration. Subsequently, the stability constant of the [Ra(macropa-XL)] complex was determined via competitive cation exchange with 223 Ra and 224 Ra radiotracers and compared with that of macropa, the current state-of-the-art chelator for Ra 2+ . A moderate log K ML value of 8.12 was measured for [Ra(macropa-XL)], which is approximately 1.5 log K units lower than the stability constant of [Ra(macropa)]. This relative decrease in Ra 2+ complex stability for macropa-XL versus macropa was further probed using density functional theory calculations. Additionally, macropa-XL was radiolabeled with 223 Ra, and the kinetic stability of the resulting complex was evaluated in human serum. Furthermore, although macropa-XL could effectively bind 223 Ra under mild conditions, the complex appeared to be unstable to transchelation. Collectively, this study sheds additional light on the chelation chemistry of the exotic Ra 2+ ion and contributes to the small, but growing, number of chelator development efforts for 223 Ra-based TAT.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

Measurement of the conduction band spin-orbit splitting in $\mathrm{WSe_2}$ and $\mathrm{WS_2}$ monolayers

We have investigated charge tunable devices based on $\mathrm{WSe_2}$ and $\mathrm{WS_2}$monolayers encapsulated in hexagonal boron nitride. In addition to the well-known radiative recombination of neutral and charged excitons, stationary photoluminescence measurements highlight a weaker-intensity optical transition that appears when the monolayer is electrostatically doped with electrons. We have carried out a detailed characterization of this photoluminescence line based on its doping dependence, its variation as a function of the optical excitation power, its polarization characteristics, and its g factor measured under longitudinal magnetic field. We interpret this luminescence peak as an impurity-assisted radiative recombination of the intervalley negatively charged exciton (triplet trion). In addition to the standard direct recombination, the triplet trion has a second recombination channel triggered by elastic scattering off the short-range impurity potential. The energy difference between the emitted photons from these two possible recombination processes of the same triplet trion is simply the single-particle conduction band spin-orbit splitting Δ c . We measure Δ c = 12 ± 0.5 meV for the $\mathrm{WSe_2}$ monolayer and Δ c = 12 ± 1 meV for the $\mathrm{WS_2}$ monolayer. Here, these results demonstrate the importance of second-order exciton recombination processes in transition-metal dichalcogenide structures.

2-dimensional systems↗

NASA NTRS1981

Electrostatic antenna space environment interaction study

The interactions of the electrostatic antenna with the space environment in both low Earth orbit and geosynchronous orbit are investigated. It is concluded that the electrostatically controlled membrane mirror is a viable concept for space applications. However, great care must be taken to enclose the high voltage electrodes in a Faraday cage structure to separate the high voltage region from the ambient plasma. For this reason, metallized cloth is not acceptable as a membrane material. Conventional spacecraft charging at geosynchronous orbit should not be a problem provided ancillary structures (such as booms) are given nonnegligible conductivity and adequate grounding. Power loss due to plasma electrons entering the high field region is a potentially serious problem. In low earth orbit any opening whatever in the Faraday cage is likely to produce an unacceptable power drain.

Katz, I.↗

NASA NTRS1981

The aerodynamics of bodies in a rarefied ionized gas with applications to spacecraft environmental dynamics

The objectives are to provide a parametric description of the electrostatic interaction of a mesosonic, collisionless plasma with conducting bodies on the order of 1 to 10 Debye lengths in size, and to extend this description to the satellite-ionospheric interaction, where possible. Experimental findings include: the wake of the geometrically complex body appears to be a linear superposition of the wakes of its simple geometric components; and vector ion flux measurements show converging ion streams at the wake axis and direct evidence of ion streams deflected from the wake axis by the positive space charge potential associated with the axial ion peak. The extension to the satellite-ionospheric interaction utilizes qualitative scaling and indicates that similar, but smaller amplitude, wake structures may be expected for small or highly charged bodies. However, for large bodies at small potentials, the structure may be diffused by the thermal ion motion and the dispersion resulting for space charge potentials.

Stone, N. H.↗