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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 775 records · Page 43
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Effects of helium cavity size and morphology on the strength of pure titanium

In this study, high-purity α-titanium was implanted with helium to observe cavity morphology – size, number density, and form (i.e., bubbles vs. voids) – effects on materials strength. Increasing implantation temperatures lead to an Arrhenius-type (i.e., exponential) increase in cavity size, a transition from spherical bubbles to faceted voids, and a marked increase in strength (~20–50%). Implanted samples deformed uniformly in contrast to nano-compression of unimplanted or alloyed titanium, but pillars with the largest faceted voids developed a type of bulging localized deformation. Affected voids were repeatedly sheared, becoming rough and shrunken. Dislocation-cavity interactions and the role of helium content within cavities are considered.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Ultralow-temperature cryogenic transmission electron microscopy using a new helium flow cryostat stage

Advances in cryogenic electron microscopy have opened new avenues for probing quantum phenomena in correlated materials. This study reports the installation and performance of a new side-entry condenZero cryogenic cooling system for JEOL (Scanning) Transmission Electron Microscopes (S/TEM), utilizing compressed liquid helium (LHe) and designed for imaging and spectroscopy at ultra-low temperatures. The system includes an external dewar mounted on a vibration-damping stage and a pressurized, low-noise helium transfer line with a remotely controllable needle valve, ensuring stable and efficient LHe flow with minimal thermal and mechanical noise. Performance evaluation demonstrates a stable base temperature of 4.37 K measured using a Cernox bare chip sensor on the holder with temperature fluctuations within ±0.004 K. Complementary in-situ electron energy-loss spectroscopy (EELS) via aluminum bulk plasmon analysis was used to measure the local specimen temperature and validate cryogenic operation during experiments. The integration of cryogenic cooling with other microscopy techniques, including electron diffraction and Lorentz TEM, was demonstrated by resolving charge density wave (CDW) transitions in NbSe2 using electron diffraction, and imaging nanometric magnetic skyrmions in MnSi via Lorentz TEM. In conclusion, this platform provides reliable cryogenic operation below 7 K, establishing a low-drift route for direct visualization of electronic and magnetic phase transformations in quantum materials.

Charge density wave↗
DOE OSTI2020

Tribochemical changes of alloy 800HT under sliding contact at elevated temperature in impure helium environment

This work investigates the chemical changes of the surface of Alloy 800HT during sliding contact under helium atmosphere in the presence of impurities at 750 °C, which is of interest for the nuclear industry. The alloy samples are exposed to helium (He) atmosphere at high temperature for long duration of 500 h to identify the chemical and topographical changes of the alloy surface. The oxidation rate constant is reported based on the weight change data. For the tribo-experimental samples, the surfaces inside and outside the wear track are studied thoroughly using techniques such as secondary electron microscopy, energy dispersive spectroscopy, x-ray diffraction, Raman spectroscopy and secondary ion mass spectroscopy. The formation and evolution of surface oxides are discussed in He atmosphere, compared with air. The presence of Fe-rich thin layer of oxide on top of a thicker Cr-rich oxide and a mixed spinel-based inner layer on the bulk is found. The persistent presence of thicker Fe3O4 in the wear track in high temperature air proved to be the reason of lower friction property. Pre-oxidation of the samples in air or He does not improve but worsen the tribological behavior in He atmosphere.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Atomistic Positioning of Defects in Helium Ion Treated Single-Layer MoS 2

Structuring materials with atomic precision is the ultimate goal of nanotechnology and is becoming increasingly relevant as an enabling technology for quantum electronics/spintronics and quantum photonics. Here, we create atomic defects in monolayer MoS 2 by helium ion (He-ion) beam lithography with a spatial fidelity approaching the single-atom limit in all three dimensions. Using low-temperature scanning tunneling microscopy (STM), we confirm the formation of individual point defects in MoS 2 upon He-ion bombardment and show that defects are generated within 9 nm of the incident helium ions. Atom-specific sputtering yields are determined by analyzing the type and occurrence of defects observed in high-resolution STM images and compared with Monte Carlo simulations. Both theory and experiment indicate that the He-ion bombardment predominantly generates sulfur vacancies.

Two-dimensional materials↗
DOE OSTI2017

Optical nanoscopy of high T c cuprate nanoconstriction devices patterned by helium ion beams

Helium ion beams (HIB) focused to subnanometer scales have emerged as powerful tools for high-resolution imaging as well as nanoscale lithography, ion milling, or deposition. Quantifying irradiation effects is an essential step toward reliable device fabrication, but most of the depth profiling information is provided by computer simulations rather than the experiment. Here, we demonstrate the use of atomic force microscopy (AFM) combined with scanning near-field optical microscopy (SNOM) to provide three-dimensional (3D) dielectric characterization of high-temperature superconductor devices fabricated by HIB. By imaging the infrared dielectric response obtained from light demodulation at multiple harmonics of the AFM tapping frequency, we find that amorphization caused by the nominally 0.5 nm HIB extends throughout the entire 26.5 nm thickness of the cuprate film and by ~500 nm laterally. This unexpectedly widespread damage in morphology and electronic structure can be attributed to a helium depth distribution substantially modified by the internal device interfaces. Lastly, our study introduces AFM-SNOM as a quantitative tomographic technique for noninvasive 3D characterization of irradiation damage in a wide variety of nanoscale devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Effect of packing solid material on characteristics of helium dielectric barrier discharge at atmospheric pressure

The influence of a solid material (e.g., Al{sub 2}O{sub 3}) on the physical characteristics of helium dielectric barrier discharge (DBD) has been investigated in a cylindrical plasma reactor. The helium DBD with the solid packing shows a multiple current-peak effect, which can be ascribed to the enhancement of charge density due to charge deposition on the Al{sub 2}O{sub 3} pellet surface. The combination of plasma and the solid increases both the peak-to-peak voltage and breakdown voltage of the discharge at a fixed input power but decreases the discharge temperature. It is also found that the presence of the solid has a weak effect on the electron temperature and density in the discharge.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Relations between light emission and electron density and temperature fluctuations in a helium plasma

The relations between three atomic lines, He I 667.8 nm (3{sup 1}D{yields} 2{sup 1}P), 706.5 nm (3{sup 3}S{yields} 2{sup 3}P), and 728.1 nm (3{sup 1}S{yields} 2{sup 1}P), and the underlying fluctuations in a helium plasma are investigated for the quantitative interpretation of optical observations in plasma fluctuation measurements. Frequency dependent fluctuation amplitude ratios and phase delays between the line emission fluctuation and the electron density and temperature fluctuations are calculated based on a quasi-static collisional-radiative model and a linear approximation technique. For frequencies up to the upper limit of practical interest (<1 MHz), the fluctuation amplitude ratios and phase delays are similar to those directly evaluated by the quasi-static model. It is found that the difference between the results from the linear approximation technique and from the quasi-static model is due to the absence of metastable fluctuations. Contributions of the 2{sup 1}S and 2{sup 3}S metastable fluctuations to the three helium line emission fluctuations are analyzed. The linearity between fluctuations in the line emission and in the electron density and temperature is valid for fluctuation levels higher than 10%.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Cs atoms on helium nanodroplets and the immersion of Cs{sup +} into the nanodroplet

We report the non-desorption of cesium (Cs) atoms on the surface of helium nanodroplets (He{sub N}) in their 6{sup 2}P{sub 1/2} ({sup 2}{Pi}{sub 1/2}) state upon photo-excitation as well as the immersion of Cs{sup +} into the He{sub N} upon photo-ionization via the 6{sup 2}P{sub 1/2} ({sup 2}{Pi}{sub 1/2}) state. Cesium atoms on the surface of helium nanodroplets are excited with a laser to the 6{sup 2}P states. We compare laser-induced fluorescence (LIF) spectra with a desorption-sensitive method (Langmuir-Taylor detection) for different excitation energies. Dispersed fluorescence spectra show a broadening of the emission spectrum only when Cs-He{sub N} is excited with photon energies close to the atomic D{sub 1}-line, which implies an attractive character of the excited state system (Cs*-He{sub N}) potential energy curve. The experimental data are compared with a calculation of the potential energy curves of the Cs atom as a function of its distance R from the center of the He{sub N} in a pseudo-diatomic model. Calculated Franck-Condon factors for emission from the 6{sup 2}P{sub 1/2} ({sup 2}{Pi}{sub 1/2}) to the 6{sup 2}S{sub 1/2} ({sup 2}{Sigma}{sub 1/2}) state help to explain the experimental data. The stability of the Cs*-He{sub N} system allows to form Cs{sup +} snowballs in the He{sub N}, where we use the non-desorbing 6{sup 2}P{sub 1/2} ({sup 2}{Pi}{sub 1/2}) state as a springboard for ionization in a two-step ionization scheme. Subsequent immersion of positively charged Cs ions is observed in time-of-flight mass spectra, where masses up to several thousand amu were monitored. Only ionization via the 6{sup 2}P{sub 1/2} ({sup 2}{Pi}{sub 1/2}) state gives rise to a very high yield of immersed Cs{sup +} in contrast to an ionization scheme via the 6{sup 2}P{sub 3/2} ({sup 2}{Pi}{sub 3/2}) state. When resonant two-photon ionization is applied to cesium dimers on He droplets, Cs{sub 2}{sup +}-He{sub N} aggregates are observed in time-of-flight mass spectra.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Theoretical investigation of thermophysical properties in two-temperature argon-helium thermal plasma

The thermophysical properties of argon-helium thermal plasma have been studied in the temperature range from 5000 to 40 000 K at atmospheric pressure in local thermodynamic equilibrium and non-local thermodynamic equilibrium conditions. Two cases of thermal plasma considered are (i) ground state plasma in which all the atoms and ions are assumed to be in the ground state and (ii) excited state plasma in which atoms and ions are distributed over various possible excited states. The influence of electronic excitation and non-equilibrium parameter {theta} = T{sub e}/T{sub h} on thermodynamic properties (composition, degree of ionization, Debye length, enthalpy, and total specific heat) and transport properties (electrical conductivity, electron thermal conductivity, and thermal diffusion ratio) have been studied. Within the framework of Chapman-Enskog method, the higher-order contributions to transport coefficient and their convergence are studied. The influence of different molar compositions of argon-helium plasma mixture on convergence of higher-orders is investigated. Furthermore, the effect of different definitions of Debye length has also been examined for electrical conductivity and it is observed that electrical conductivity with the definition of Debye length (in which only electrons participate in screening) is less than that of the another definition (in which both the electrons and ions participate in screening) and this deviation increases with electron temperature. Finally, the effect of lowering of ionization energy is examined on electron number density, Debye length, and higher-order contribution to electrical conductivity. It is observed that the lowering of the ionization energy affects the electron transport-properties and consequently their higher-order contributions depending upon the value of the non-equilibrium parameter {theta}.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Process Options for Nominal 2-K Helium Refrigeration System Designs

Nominal 2-K helium refrigeration systems are frequently used for superconducting radio frequency and magnet string technologies used in accelerators. This paper examines the trade-offs and approximate performance of four basic types of processes used for the refrigeration of these technologies; direct vacuum pumping on a helium bath, direct vacuum pumping using full or partial refrigeration recovery, cold compression, and hybrid compression (i.e., a blend of cold and warm sub-atmospheric compression).

43 PARTICLE ACCELERATORS↗
DOE OSTI2012

Supersonic helium beam diagnostic for fluctuation measurements of electron temperature and density at the Tokamak TEXTOR

A supersonic helium beam diagnostic, based on the line-ratio technique for high resolution electron density and temperature measurements in the plasma edge (r/a > 0.9) was designed, built, and optimised at TEXTOR (Torus Experiment for Technology Oriented Research). The supersonic injection system, based on the Campargue skimmer-nozzle concept, was developed and optimised in order to provide both a high neutral helium beam density of n{sub 0}= 1.5 Multiplication-Sign 10{sup 18} m{sup -3} and a low beam divergence of {+-}1 Degree-Sign simultaneously, achieving a poloidal resolution of {Delta}{sub poloidal}= 9 mm. The setup utilises a newly developed dead volume free piezo valve for operation in a high magnetic field environment of up to 2 T with a maximum repetition rate of 80 Hz. Gas injections are realised for a duration of 120 ms at a repetition rate of 2 Hz (duty cycle 1/3). In combination with a high sensitivity detection system, consisting of three 32 multi-channel photomultipliers (PMTs), measurements of edge electron temperature and density with a radial resolution of {Delta}{sub radial}= 2 mm and a maximum temporal resolution of {Delta}t Asymptotically-Equal-To 2 {mu}s (470 kHz) are possible for the first time. The diagnostic setup at TEXTOR is presented. The newly developed injection system and its theoretical bases are discussed. The applicability of the stationary collisional-radiative model as basis of the line-ratio technique is shown. Finally, an example of a fluctuation analysis demonstrating the unique high temporal and spatial resolution capabilities of this new diagnostic is presented.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2012

Nanometer-scale tunnel formation in metallic glass by helium ion irradiation

We have shown that upon high fluence helium ion irradiation, metallic glass Cu{sub 50}Zr{sub 45}Ti{sub 5} becomes highly porous at the depth of the helium projected range. The resulting porous region is characterized by the formation of a tunnel like structure and self-linkage of nanometer size gas bubbles. Furthermore, the irradiation leads to the formation of nanometer size Cu{sub x}Zr{sub y} crystals that are randomly distributed. The results of this study indicate that the He-filled bubbles have attractive interactions and experience considerable mobility. Movement of the bubbles is believed to be assisted by ballistic collisions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Diagnosis of gas temperature, electron temperature, and electron density in helium atmospheric pressure plasma jet

The optical emission spectra of helium atmospheric pressure plasma jet (APPJ) are captured with a three grating spectrometer. The grating primary spectrum covers the whole wavelength range from 200 nm to 900 nm, with the overlapped grating secondary spectrum appearing from 500 nm to 900 nm, which has a higher resolution than that of the grating primary spectrum. So the grating secondary spectrum of OH (A{sup 2}{Sigma} {sup +}({upsilon} Prime = 0) {yields} X{sup 2}{Pi}({upsilon} Double-Prime = 0)) is employed to calculate the gas temperature (T{sub g}) of helium APPJ. Moreover, the electron temperature (T{sub e}) is deduced from the Maxwellian electron energy distribution combining with T{sub g}, and the electron density (n{sub e}) is extracted from the plasma absorbed power. The results are helpful for understanding the physical property of APPJs.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Electron scattering in helium for Monte Carlo simulations

An analytical approximation for differential cross-section of electron scattering on helium atoms is introduced. It is intended for Monte Carlo simulations, which, instead of angular distributions based on experimental data (or on first-principle calculations), usually rely on approximations that are accurate yet numerically efficient. The approximation is based on the screened-Coulomb differential cross-section with energy-dependent screening. For helium, a two-pole approximation of the screening parameter is found to be highly accurate over a wide range of energies.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Excitation of helium Rydberg states and doubly excited resonances in strong extreme ultraviolet fields: Full-dimensional quantum dynamics using exponentially tempered Gaussian basis sets

Recently optimized exponentially tempered Gaussian basis sets [P. R. Kapralova-Zdanska and J. Smydke, J. Chem. Phys. 138, 024105 (2013)] are employed in quantitative simulations of helium absorption cross-sections and two-photon excitation yields of doubly excited resonances. Linearly polarized half-infinite and Gaussian laser pulses at wavelengths 38–58 nm and large intensities up to 100 TW/cm{sup 2} are considered. The emphasis is laid on convergence of the results with respect to the quality of the Gaussian basis sets (typically limited by a number of partial waves, density, and spatial extent of the basis functions) as well as to the quality of the basis set of field-free states (typically limited by the maximum rotational quantum number and maximum excitation of the lower electron). Particular attention is paid to stability of the results with respect to varying complex scaling parameter. Moreover, the study of the dynamics is preceded by a thorough check of helium energies and oscillator strengths as they are obtained with the exponentially tempered Gaussian basis sets, being also compared with yet unpublished emission wavelengths measured in electric discharge experiments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

A liquid-helium-free superconducting coil system forming a flat minimum-magnetic-field distribution of an electron cyclotron resonance ion source

A flat distribution of the minimum magnetic field (flat-B{sub min}) of an electron cyclotron resonance ion source (ECRIS) is expected to perform better in highly charged ion production than classical B{sub min}. To form a flat-B{sub min} structure with a liquid helium-free superconducting device, a coil system of seven coils with four current leads has been designed. The lead number was reduced by connecting the plural coils in series to maintain the flat-B{sub min} structure even when the coil currents are changed for adjustment. This coil system can be operated with a helium-free cryostat, since the estimation of heat from the leads to the coils is nearly equivalent to the existing superconducting ECRIS of a similar type.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

Ignition and extinction phenomena in helium micro hollow cathode discharges

Micro hollow cathode discharges (MHCD) were produced using 250 μm thick dielectric layer of alumina sandwiched between two nickel electrodes of 8 μm thickness. A through cavity at the center of the chip was formed by laser drilling technique. MHCD with a diameter of few hundreds of micrometers allowed us to generate direct current discharges in helium at up to atmospheric pressure. A slowly varying ramped voltage generator was used to study the ignition and the extinction periods of the microdischarges. The analysis was performed by using electrical characterisation of the V-I behaviour and the measurement of He*({sup 3}S{sub 1}) metastable atoms density by tunable diode laser spectroscopy. At the ignition of the microdischarges, 2 μs long current peak as high as 24 mA was observed, sometimes followed by low amplitude damped oscillations. At helium pressure above 400 Torr, an oscillatory behaviour of the discharge current was observed just before the extinction of the microdischarges. The same type of instability in the extinction period at high pressure also appeared on the density of He*({sup 3}S{sub 1}) metastable atoms, but delayed by a few μs relative to the current oscillations. Metastable atoms thus cannot be at the origin of the generation of the observed instabilities.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Process model and capacity upgrades of the CTI-4000 liquid helium coldbox

Fermi National Accelerator Laboratory (FNAL) is in the process of re-commissioning a vintage CTI-4000 liquid helium coldbox, initially supplied by CTI-Cryogenics/Sulzer to Los Alamos in 1979. The coldbox was originally designed as a liquid helium refrigerator with capacity of ∼1200 W at nominal 4-K. The process utilized LN{sub 2} precooling, in-series operation of two centrifugal gas bearing turboexpanders and final Joule-Thomson (J-T) expansion. At FNAL, the coldbox will be utilized as a liquefier to support 2-K operations. A process model was developed to aid in the upgrade decisions and used to determine the nominal capacity of the liquefier. Capacity upgrades are achieved by safely utilizing the internal LN2 precooler, the addition of a 3-inch reciprocating wet expansion engine and increasing the overall process pressure by recertifying two limiting pressure vessels to a higher MAWP.

43 PARTICLE ACCELERATORS↗