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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 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

A LIBS method for simultaneous monitoring of the impurities and the hydrogenic composition present in the wall of the TJ-II stellarator

The study of plasma-wall interactions and impurity transport in the plasma fusion devices is critical for the development of future fusion reactors. An experiment to perform laser induced breakdown spectroscopy, using minor modifications of our existing laser blow-off impurity injection system, has been set up thus making both experiments compatible. The radiation produced by the laser pulse focused at the TJ-II wall evaporates a surface layer of deposited impurities and the subsequent radiation produced by the laser-produced plasma is collected by two separate lens and fiber combinations into two spectrometers. The first spectrometer, with low spectral resolution, records a spectrum from 200 to 900 nm to give a survey of impurities present in the wall. The second one, with high resolution, is tuned to the wavelengths of the Hα and Dα lines in order to resolve them and quantify the hydrogen isotopic ratio present on the surface of the wall. The alignment, calibration, and spectral analysis method will be described in detail. First experimental results obtained with this setup will be shown and its relevance for the TJ-II experimental program discussed.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Pulsed flashover of solid dielectric materials in compressed gas environment

An experimental investigation of surface flashover characteristics of PMMA and POM is studied in compressed nitrogen gas environment with nitrogen as the background gas. The operating pressure range is from 1kg/cm{sup 2} to 4kg/cm{sup 2}. It is observed that the breakdown voltage of PMMA is higher than POM owing to a higher permittivity mismatch between POM- nitrogen interface as compared to the PMMA- nitrogen interface. The reduction in spacer efficiency with pressure for PMMA is 11% as compared to POM which shows a higher reduction of 18%. This paper further emphasizes on the role of energy level and density of charge carrier trapping centers for a reduced breakdown voltage in POM as compared to PMMA.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Multipactor threshold calculation of coaxial transmission lines in microwave applications with nonstationary statistical theory

This paper presents a statistical theory for the initial onset of multipactor breakdown in coaxial transmission lines, taking both the nonuniform electric field and random electron emission velocity into account. A general numerical method is first developed to construct the joint probability density function based on the approximate equation of the electron trajectory. The nonstationary dynamics of the multipactor process on both surfaces of coaxial lines are modelled based on the probability of various impacts and their corresponding secondary emission. The resonant assumption of the classical theory on the independent double-sided and single-sided impacts is replaced by the consideration of their interaction. As a result, the time evolutions of the electron population for exponential growth and absorption on both inner and outer conductor, in response to the applied voltage above and below the multipactor breakdown level, are obtained to investigate the exact mechanism of multipactor discharge in coaxial lines. Furthermore, the multipactor threshold predictions of the presented model are compared with experimental results using measured secondary emission yield of the tested samples which shows reasonable agreement. Finally, the detailed impact scenario reveals that single-surface multipactor is more likely to occur with a higher outer to inner conductor radius ratio.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS2016

Design and Calibration of a Raman Spectrometer for use in a Laser Spectroscopy Instrument Intended to Analyze Martian Surface and Atmospheric Characteristics for NASA

This project's goal is the design of a Raman spectroscopy instrument to be utilized by NASA in an integrated spectroscopy strategy that will include Laser-Induced Breakdown Spectroscopy (LIBS) and Laser-Induced Florescence Spectroscopy (LIFS) for molecule and element identification on Mars Europa, and various asteroids. The instrument is to be down scaled from a dedicated rover mounted instrument into a compact unit with the same capabilities and accuracy as the larger instrument. The focus for this design is a spectrometer that utilizes Raman spectroscopy. The spectrometer has a calculated range of 218 nm wavelength spectrum with a resolution of 1.23 nm. To filter out the laser source wavelength of 532 nm the spectrometer design utilizes a 532 nm wavelength dichroic mirror and a 532 nm wavelength notch filter. The remaining scatter signal is concentrated by a 20 x microscopic objective through a 25-micron vertical slit into a 5mm diameter, 1cm focal length double concave focusing lens. The light is then diffracted by a 1600 Lines per Millimeter (L/mm) dual holographic transmission grating. This spectrum signal is captured by a 1-inch diameter double convex 3 cm focal length capture lens. An Intensified Charge Couple Device (ICCD) is placed within the initial focal cone of the capture lens and the Raman signal captured is to be analyzed through spectroscopy imaging software. This combination allows for accurate Raman spectroscopy to be achieved. The components for the spectrometer have been bench tested in a series of prototype developments based on theoretical calculations, alignment, and scaling strategies. The mounting platform is 2.5 cm wide by 8.8 cm long by 7 cm height. This platform has been tested and calibrated with various sources such as a neon light source and ruby crystal. This platform is intended to be enclosed in a ruggedized enclosure for mounting on a rover platform. The size and functionality of the Raman spectrometer allows for the rover to carry other mission critical devices. This project will be continued at NASA until the requirements are met for the expected initial 2020 launch date.

Lucas, John F.↗
DOE OSTI2019

Effect of an Optical Breakdown on the Stimulated Raman Scattering in Water in the Field of Picosecond Laser Pulses

Experiments on nonstationary stimulated Raman scattering (SRS) of laser pulses and an optical breakdown are performed to study water samples with gas nanobubbles (bubstons) and without the bubston phase. The SRS excitation threshold in water with the bubston phase turns out to be higher than that in bubston-free water, and the optical breakdown threshold in bubston-containing water is lower than that in bubston-free water. The spectral composition of the SRS signal from bubston-free water samples is found to differ from that of bubston-containing water samples. This effect is caused by the formation of long-lived high-molecular H{sub 2}O complexes near the bubston surface, which are stabilized by the dispersion forces acting from this surface. Since these complexes are located inside 1–2 monolayers adjacent to the surface, the volume density of these complexes is negligibly small as compared to the density of “free” (not bound to bubstons) molecules. To interpret the experimental results, we advance a hypothesis that the detected effect is related to the giant enhancement caused by the generation of a resonance plasmon inside a bubston during an optical breakdown. As follows from our estimates, the bubston-related plasma frequency falls in the range 2600–3200 cm{sup –1}; it is this range that contains the SRS spectral lines of water containing the bubston phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Temperature-induced Lifshitz transition in WTe 2

In this study, we use ultrahigh resolution, tunable, vacuum ultraviolet laser-based, angle-resolved photoemission spectroscopy (ARPES), temperature- and field-dependent resistivity, and thermoelectric power (TEP) measurements to study the electronic properties of WTe 2 , a compound that manifests exceptionally large, temperature-dependent magnetoresistance. The Fermi surface consists of two pairs of electron and two pairs of hole pockets along the X–Γ–X direction. Using detailed ARPES temperature scans, we find a rare example of a temperature-induced Lifshitz transition at T≃160 K, associated with the complete disappearance of the hole pockets. Our electronic structure calculations show a clear and substantial shift of the chemical potential μ(T) due to the semimetal nature of this material driven by modest changes in temperature. This change of Fermi surface topology is also corroborated by the temperature dependence of the TEP that shows a change of slope at T≈175 K and a breakdown of Kohler’s rule in the 70–140 K range. Our results and the mechanisms driving the Lifshitz transition and transport anomalies are relevant to other systems, such as pnictides, 3D Dirac semimetals, and Weyl semimetals.

36 MATERIALS SCIENCE↗
NASA NTRS1978

Insulator edge voltage gradient effects in spacecraft charging phenomena

Insulating surfaces on geosynchronous satellites have been charged by geomagnetic substorms to a point where discharges occur. The electromagnetic pulses from these discharges couple into satellite electronic systems disrupting operations. Laboratory tests conducted on insulator charging have indicated that discharges appear to be initiated at insulator edges where voltage gradients can exist. An experimental investigation has been conducted to measure edge voltage gradients on silvered Teflon samples as they are charged by monoenergetic electron beams. It has been found that the surface voltage at insulator edges can be approximated by an exponential expression based on an electron current density balance. Using this expression at known breakdown conditions results in a discharge voltage gradient down the insulator edge to ground of about 150,000 V/cm.

Stevens, N. J.↗
NASA NTRS1984

Improved Hollow Cathode

Improved hollow cathode for neutral-beam injector ion sources rapidly started by dielectric breakdown of ignitor plug located at one end of open ended tube. Used in low-current ion implantation, milling, sputtering, and bombardment for surface treatment.

Aston, G.↗
NASA NTRS1993

Application of a Reynolds stress model to separating boundary layers

Separating turbulent boundary layers occur in many practical engineering applications. Nonetheless, the physics of separation/reattachment of flows is poorly understood. During the past decade, various turbulence models were proposed and their ability to successfully predict some types of flows was shown. However. prediction of separating/reattaching flows is still a formidable task for model developers. The present study is concerned with the process of separation from a smooth surface. Features of turbulent separating boundary layers that are relevant to modeling include the following: the occurrence of zero wall shear stress, which causes breakdown of the boundary layer approximation; the law of the wall not being satisfied in the mean back flow region; high turbulence levels in the separated region; a significant low-frequency motion in the separation bubble; and the turbulence structure of the separated shear layer being quite different from that of either the mixing layers or the boundary layers. These special characteristics of separating boundary layers make it difficult for simple turbulence models to correctly predict their behavior.

Ko, Sung HO↗
NASA NTRS2007

On the Evolution of Localized Disturbances and their Spanwise Interactions Leading to Breakdown

Localized disturbances in a laminar boundary layer represent a more realistic model of transition than the extensively studies, two or quasi three-dimensional perturbations regardless of the fact if they evolve in a linear manner or not. Localized disturbances can originate by surface imperfections, insects or dust. The disturbances can be harmonic (i.e. containing a single frequency and a complete set of spanwise wave numbers) or Pulsed (i.e. containing a band of streamwise and spanwise wave numbers). At sufficiently low amplitudes localized disturbances behave according to a linear stability model. It is highly probably that in a natural transition process such localized disturbances will overslap and interact. These interactions could either delay transition because of a partial wave cancellation resulting in an attenuation of the disturbance, or adversely enhance it by promoting nonlinear interactions. The nonlinearity could be simply amplitude dependent or cause a triad resonance. Nonlinear processes in a wave packet lead to breakdown and to the formation of turbulent spots. When the amplitude of the harmonic disturbance saturates, nonlinear processes widen the band of the lower amplified frequencies adjacent to the frequency of excitation. Experimental results describing the spanwise interactions of harmonic and pulsed localized disturbances leading to breakdown will be presented and discussed. A comparison to the evolution and breakdown of a single localized disturbance will be provided.

Seifert, Avi↗
DOE OSTI2014

A relationship between statistical time to breakdown distributions and pre-breakdown negative differential resistance at nanometric scale

Using an ultra-high vacuum Conductive atomic force microscopy (C-AFM) current voltage, pre-breakdown negative differential resistance (NDR) characteristics are measured together with the time dependent dielectric breakdown (TDDB) distributions of Si/SiON (1.4 and 2.6 nm thick). Those experimental characteristics are systematically compared. The NDR effect is modelled by a conductive filament growth. It is showed that the Weibull TDDB statistic distribution scale factor is proportional to the growth rate of an individual filament and then has the same dependence on the electric field. The proportionality factor is a power law of the ratio between the surfaces of the CAFM tip and the filament's top. Moreover, it was found that, for the high fields used in those experiments, the TDDB acceleration factor as the growth rate characteristic is proportional to the Zener tunnelling probability. Those observations are discussed in the framework of possible breakdown or forming mechanism.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

C-H surface diamond field effect transistors for high temperature (400 °C) and high voltage (500 V) operation

By forming a highly stable Al{sub 2}O{sub 3} gate oxide on a C-H bonded channel of diamond, high-temperature, and high-voltage metal-oxide-semiconductor field-effect transistor (MOSFET) has been realized. From room temperature to 400 °C (673 K), the variation of maximum drain-current is within 30% at a given gate bias. The maximum breakdown voltage (V{sub B}) of the MOSFET without a field plate is 600 V at a gate-drain distance (L{sub GD}) of 7 μm. We fabricated some MOSFETs for which V{sub B}/L{sub GD} > 100 V/μm. These values are comparable to those of lateral SiC or GaN FETs. The Al{sub 2}O{sub 3} was deposited on the C-H surface by atomic layer deposition (ALD) at 450 °C using H{sub 2}O as an oxidant. The ALD at relatively high temperature results in stable p-type conduction and FET operation at 400 °C in vacuum. The drain current density and transconductance normalized by the gate width are almost constant from room temperature to 400 °C in vacuum and are about 10 times higher than those of boron-doped diamond FETs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1991

Evolution of streamwise vortices and generation of small-scale motion in a plane mixing layer

The present study investigates the evolution of streamwise vortices in a plane mixing layer and their role in the generation of small-scale 3D motion in a closed-return water facility. Spanwise-periodic streamwise vortices are excited by a time-harmonic wavetrain with spanwise-periodic amplitude variations synthesized by a mosaic of 32 surface film heaters flush-mounted on the low partition. The onset of streamwise vortices is accompanied by significant distortion in the transverse distribution of the streamwise velocity component. The presence of inflexion points, absent in corresponding velocity distributions of the unforced flow, suggests the formation of locally unstable regions of large shear in which broadband perturbations already present in the base flow undergo rapid amplification, followed by breakdown to small-scale motion. The cores of the primary vortices are significantly altered as a result of spanwise nonuniform excitation. The 3D features of the streamwise vortices and their interaction with the base flow are inferred from surfaces or rms velocity fluctuations and an approximation to cross-stream vorticity using 3D single component velocity data.

Nygaard, K. J.↗
DOE OSTI2023

Deuterium supersaturated surface layer in tungsten: ion energy dependence

Abstract Properties of deuterium (D) supersaturated surface layers (DSSLs) formed in tungsten (W), such as thickness, internal microstructures, and D retention, are experimentally investigated as a function of the incident ion energy, E i . W samples were exposed to D plasmas in the PISCES-A linear plasma device in a range of E i ∼ 45–175 eV, while other plasma exposure parameters were fixed: sample temperature, T s , ∼423 K, ion flux, Γ i , ∼1.2 × 10 21 m −2 s −1 , and fluence, Φ i , ∼3.0 × 10 24 m −2 . High-resolution, cross-sectional, transmission electron microscopy observations confirm that (1) a DSSL forms even at the lowest E i ∼ 45 eV, (2) the DSSL thickness, Δ t DSSL , is found to decrease with decreasing E i from ∼11–12 nm at E i ∼ 175 eV to ∼5–6 nm at ∼45 eV, and to agree with approximately the maximum implantation depth calculated using SDTrimSP, and (3) high-density D nanobubbles with a diameter of ∼1 nm or less exist inside the DSSL, which is deemed to validate a theory-predicted vacancy stabilization process due to trapping of a solute D atom(s). Utilizing a D areal density of ∼4.2 × 10 19 m −2 in the first 14 nm from the surface at E i ∼ 75 eV from nuclear reaction analysis and the measured E i dependence of Δ t DSSL , our previous laser-induced breakdown spectroscopy data is updated: both dynamic and static D retention increase with decreasing E i , and the D/W atomic fraction during plasma exposure reaches ∼0.3 at E i ∼ 45 eV. A possible DSSL formation mechanism is proposed.

Physics↗
DOE OSTI2022

In-situ Laser Induced Breakdown Spectroscopy Measurements in PISCES-A

The goal of the project was to develop a Laser Induced Breakdown Spectroscopy (LIBS) diagnostic that could be employed during the plasma exposure of targets in the PISCES-A linear plasma device to interrogate the surface as the plasma exposure progressed. The objectives of the measurements were twofold; first, to measure in real-time the composition of elements in the near surface ablated zone to enable a tool that could be used to validate material migration models of eroded material, and second, to measure the plasma fuel contained in the ablated surface to determine the static and dynamic inventory of gas in a target during the plasma exposure. The LIBS diagnostic was successfully deployed on PISCES-A and measured both elemental composition and gas accumulation in the near-surface ablated layer during plasma operations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1971

Postulation of the arc restrike mechanisms for exploding wires and tubes.

The many facets of exploding wire restrikes are studied. They are related to the cylindrical shock wave which pumps down the density of the center region of a line energy source or around the surface of the heated wire continuously to a lower density. When the density of any region is pumped down to lower than Paschen's electric breakdown criteria, restrike occurs. The cylindrical shocks occurring in exploding wires are, in general, divided into two types. In the first type the shock is generated by the wire explosion and in the second type the shock is generated by the heat conduction of the wire to the surrounding gas. Application of this mechanism to exploding tubes will enable us to create arc channels in arbitrary gaseous atmospheres with controllable plasma density with the contamination excluded. Neutral density shielded plasma experiments, therefore, can be conducted in a controllable environment.

Cheng, D. Y.↗
DOE OSTI2015

PRD3000: A novel Personnel Radiation Detector with Radiation Exposure Monitoring

PRD3000{sup TM} is a novel Personal Radiation Detector (PRD) with personnel radiation dose exposure monitoring. It is intended for First Responders, Law Enforcement, Customs Inspectors protecting critical infrastructures for detecting unexpected radioactive sources, who also need real time Hp(10) dose equivalent information. Traditional PRD devices use scintillator materials instrumented through either a photomultiplier tube or a photodiode photodetector. While the former is bulky and sensitive to magnetic fields, the latter has to compromise radiation sensitivity and energy threshold given its current noise per unit of photo-detection surface. Recently, solid state photodetectors (SiPM), based on arrays of Geiger operated diodes, have emerged as a scalable digital photodetector for photon counting. Their strong breakdown voltage temperature dependence (on the order of tens of milli-volts per K) has however limited their use for portable instruments where strong temperature gradients can be experienced, and limited power is available to temperature stabilize. The PRD3000 is based on the industry standard DMC3000 active dosimeter that complies with IEC 61526 Ed. 3 and ANSI 42.20 for direct reading personal dose equivalent meters and active personnel radiation monitors. An extension module is based on a CsI(Tl) scintillator readout by a temperature compensated SiPM. Preliminary nuclear tests combined with a measured continuous operation in excess of 240 hours from a single AAA battery cell indicate that the PRD3000 complies with the IEC 62401 Ed.2 and ANSI 42.32 without sacrificing battery life time. We present a summary of the device test results, starting with performance stability over a temperature range of - 20 deg. C to 50 deg. C, false alarm rates and dynamic response time. (authors)

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2010

Ultrafast thin-film laser-induced breakdown spectroscopy of doped oxides

Single-shot femtosecond laser induced breakdown spectroscopy (LIBS) has been shown to be an effective means of detecting heavy metal dopants in porous thin films. Traditional LIBS analysis of trace dopants in modern painted surfaces or TiO2 films is difficult due to the broad noisy spectra of the titanium constituent and interference due to the substrate material. Femtosecond laser pulses provide excellent ablation of the target material with little damage to the underlying substrate. In this study a Ti:sapphire femtosecond laser pulse operated at 800 and 266 nm wavelengths and an Nd:YAG nanosecond laser operated at 266 nm were used to ablate 0.15-15 {mu}m films of TiO2 doped with varying amounts of MgO. This application shows excellent detection of Mg down to 60 ppm with little interference by the substrate material.

36 MATERIALS SCIENCE↗