Neutral Gas Environment Research Articles

Synthesis of porous Si–C composite powder from activated raw materials

A porous silicon-carbon composite was synthesized using a mixture of sucrose, ammonium chloride, and silicon in a neutral gas environment at 1000 °C. Effect of raw materials activation were studied (0, ‌ 1, 2, and 4 h) on the characteristics (microstructure, morphology, phase structure, carbon structure, and electrical resistance) of prepared composite powders using SEM (Scanning Electron Microscope), XRD (X-Ray Diffraction), RAMAN spectroscopy methods, and electrical resistivity measurements. Results showed that mechanical activation promoted the decomposition reaction of raw materials during heat treatment, helped to form spherical carbon particles, reduced carbon particle size (less than 200 nm), and facilitated the formation of the porous carbon structure. Raman spectra also showed the appearance of some carbon in the milled samples before the heat treatment process. Carbon peaks were not observed in the XRD patterns of composite powders due to the nanocrystalline structure of the carbon, while Raman spectra proved the presence of nanocrystalline and amorphous carbon in the structure of the prepared powders. Moreover, mechanical activation increased the content of nanocrystalline and amorphous carbon in the composite powders. Increasing the mechanical activation time up to 4 h, reduced the electrical resistivity of Si–C composite powders to 1.44 Ω-cm.

Comparative study of Shot creep of single-phase titanium alloys in air and neutral gas environment on the test temperature in range from 673 to 1323 K

The paper presents the results of comparative studies of short-term creep in tensile tests of the VT1-0 commercially pure titanium (analogue Grade 2 according to ASTM A485) and the VT5-1 alloy (analogue Grade 6 according to ASTM B265) in air and argon. Tests by the Dorn’s method were carried out in a gas-tight chamber under constant pressure. The specimens were heated by passing alternating current. Empirical coefficients are determined in power dependences of the creep rate and the conventional creep strength on test temperature in the range from 673 to 1323 K for argon and air environment and nominal stress ranging between 4.45 and 9.36 MPa. It is found that heating in argon environment, as compared to heating in air, decreases the rate of short-term creep and increases creep strength at identical nominal stresses in the entire range of test temperatures. Calculations demonstrate that the energy of the activation of short-term creep does not depend on test conditions. This indicates that the deformation mechanism is unchanged.

Graphene–CNT hybrids by thermal and plasma‐enhanced chemical vapor deposition process: Numerical modeling of growth and energy kinetics

AbstractThe numerical computations for the species density and energy kinetics are used to elaborate the growth mechanism and differences between the graphene–carbon nanotube (CNT) hybrids grown by the chemical vapor deposition (CVD) and plasma‐enhanced CVD (PECVD) process. The modeling results show that the CNT, graphene, and graphene–CNT hybrids have enhanced growth rates in a plasma medium than in a neutral gas environment of a CVD method. Plasma enables low‐temperature growth of graphene–CNT hybrids and offers a broad temperature range for the growth than the CVD technique that operates only at elevated temperatures. The underlying reasons and mechanisms behind the better growth rates and temperature range offered by the PECVD process (~400–1,600 K) are explained through the results.

Study of silicon films deposited by EB-PVD and the effect of applying annealing conditions on them

This paper investigates structural, electronic, and optical silicon poly-crystalline coatings created in high vacuum condition (10−6Torr) by Electron Beam Physical Vapor Deposition (EB-PVD). Samples were annealed after deposition in neutral gas environment of 800°C temperature. Results of microscopic images, micro Raman analysis and Grazing Incidence X-ray Diffraction (GIXRD) showed that phase change happened from amorphous structure to crystalline structure in initial amorphous context which could be significantly enhanced by increasing annealing temperature. Aggregation and accumulation of silicon nano-crystals were also observed. Moreover, applying temperature of 800°C in one-hour resulted in complete crystallization and homogeneity of films. The situation caused the increase of SP2 SP3 bonds formed with more free electrons producing higher conductivity of electric film in the layers. Finally, because of increased density of film structure, light reflectance decreased from %12 to %3 due to poly-crystalline structure created in initial amorphous context.

Four‐fluid MHD simulations of the plasma and neutral gas environment of comet 67P/Churyumov‐Gerasimenko near perihelion

AbstractThe neutral and plasma environment is critical in understanding the interaction of the solar wind and comet 67P/Churyumov‐Gerasimenko (CG), the target of the European Space Agency's Rosetta mission. To serve this need and support the Rosetta mission, we have developed a 3‐D four‐fluid model, which is based on BATS‐R‐US (Block‐Adaptive Tree Solarwind Roe‐type Upwind Scheme) within SWMF (Space Weather Modeling Framework) that solves the governing multifluid MHD equations and the Euler equations for the neutral gas fluid. These equations describe the behavior and interactions of the cometary heavy ions, the solar wind protons, the electrons, and the neutrals. This model incorporates different mass loading processes, including photoionization and electron impact ionization, charge exchange, dissociative ion‐electron recombination, and collisional interactions between different fluids. We simulated the plasma and neutral gas environment near perihelion in three different cases: an idealized comet with a spherical body and uniform neutral gas outflow, an idealized comet with a spherical body and illumination‐driven neutral gas outflow, and comet CG with a realistic shape model and illumination‐driven neutral gas outflow. We compared the results of the three cases and showed that the simulations with illumination‐driven neutral gas outflow have magnetic reconnection, a magnetic pileup region and nucleus directed plasma flow inside the nightside reconnection region, which have not been reported in the literature.

ANISOTROPIC LYMAN-ALPHA EMISSION

As a result of resonant scatterings off hydrogen atoms, Lyman-alpha (Lya) emission from star-forming galaxies provides a probe of the (hardly isotropic) neutral gas environment around them. We study the effect of the environmental anisotropy on the observed Lya emission by performing radiative transfer calculations for models of neutral hydrogen clouds with prescriptions of spatial and kinematic anisotropies. The environmental anisotropy leads to corresponding anisotropy in the Lya flux and spectral properties and induces correlations among them. The Lya flux (or observed luminosity) depends on the viewing angle and shows an approximate correlation with the initial Lya optical depth in the viewing direction relative to those in all other directions. The distribution of Lya flux from a set of randomly oriented clouds is skewed to high values, providing a natural contribution to the Lya equivalent width (EW) distribution seen in observation. A narrower EW distribution is found at a larger peak offset of the Lya line, similar to the trend suggested in observation. The peak offset appears to correlate with the line shape (full width at half maximum and asymmetry), pointing to a possibility of using Lya line features alone to determine the systemic redshifts of galaxies. The study suggests that anisotropies in the spatial and kinematic distributions of neutral hydrogen can be an important ingredient in shaping the observed properties of Lya emission from star-forming galaxies. We discuss the implications of using Lya emission to probe the circumgalactic and intergalactic environments of galaxies.

Multiple-composition hyperthermal atomic beams formed by a laser-induced plasma for planetary environmental studies

A multiple-composition beam with a velocity of 8 km s−1 was formed using a laser detonation source in order to simulate the physical and chemical effects of neutral gas collisions on a material in the sub-low earth orbit (LEO). A premixed target gas was applied in the laser detonation beam technique. It was found that atoms of different masses, O and Ar for Ar + O2 target gas, are accelerated to similar velocities, i.e. different translational energies. Promotion of O2 decomposition is confirmed by using a mixed gas target. It is concluded that the multiple-composition beam obtained using a mixed target gas is suitable for simulating a neutral gas environment in the sub-LEO region.

Monte Carlo modeling of neutral gas and dust in the coma of Comet 1P/Halley

The neutral gas environment of a comet is largely influenced by dissociation of parent molecules created at the surface of the comet and collisions of all the involved species. We compare the results from a kinetic model of the neutral cometary environment with measurements from the Neutral Mass Spectrometer and the Dust Impact Detection System onboard the Giotto spacecraft taken during the fly-by at Comet 1P/Halley in 1986. We also show that our model is in good agreement with contemporaneous measurements obtained by the International Ultraviolet Explorer, sounding rocket experiments, and various ground based observations. The model solves the Boltzmann equation with a Direct Simulation Monte Carlo technique (Tenishev, V., Combi, M., Davidsson, B. [2008]. Astrophys. J. 685, 659–677) by tracking trajectories of gas molecules and dust grains under the influence of the comet’s weak gravity field with momentum exchange among particles modeled in a probabilistic manner. The cometary nucleus is considered to be the source of dust and the parent species (in our model: H 2O, CO, H 2CO, CO 2, CH 3OH, C 2H 6, C 2H 4, C 2H 2, HCN, NH 3, and CH 4) in the coma. Subsequently our model also tracks the corresponding dissociation products (H, H 2, O, OH, C, CH, CH 2, CH 3, N, NH, NH 2, C 2, C 2H, C 2H 5, CN, and HCO) from the comet’s surface all the way out to 10 6 km. As a result we are able to further constrain cometary the gas production rates of CO (13%), CO 2 (2.5%), and H 2CO (1.5%) relative to water without invoking unknown extended sources.

Drag coefficient of the weakly ionized plasma in the high Knudsen number regime

The drag force acting on a micron-sized polystyrene particle in the high Knudsen number regime is investigated. Analysis of the particle trajectories in stationary neutral argon gas environment suggests the damping time constant τ∝p−1.20±0.04 and Epstein drag force coefficient δ=1.40. The neutral drag coefficient is compared with the drag coefficient measurement in dust-free plasma. The phenomena of the reduced drag in weakly viscous and weakly ionized rf plasma are also observed in this report. It is shown that the slight changes in rf power and pressure would enhance the reduced drag effect, which suggests that there is an additional electrostatic force acting along the particle motion in the plasma.

The path to stoichiometric composition of III–V binary quantum dots through plasma/ion-assisted self-assembly

Semiconductor III–V quantum dots (QDs) are particularly enticing components for the integration of optically promising III–V materials with the silicon technology prevalent in the microelectronics industry. However, defects due to deviations from a stoichiometric composition [group III: group V = 1] may lead to impaired device performance. This paper investigates the initial stages of formation of InSb and GaAs QDs on Si(1 0 0) through hybrid numerical simulations. Three situations are considered: a neutral gas environment (NG), and two ionized gas environments, namely a localized ion source (LIS) and a background plasma (BP) case. It is shown that when the growth is conducted in an ionized gas environment, a stoichiometric composition may be obtained earlier in the QD as compared to a NG. Moreover, the stoichiometrization time, t st , is shorter for the BP case compared to the LIS scenario. A discussion of the effect of ion/plasma-based tools as well as a range of process conditions on the final island size distribution is also included. Our results suggest a way to obtain a deterministic level of control over nanostructure properties (in particular, elemental composition and size) during the initial stages of growth which is a crucial step towards achieving highly tailored QDs suitable for implementation in advanced technological devices.

ENA detection in the dayside of Mars: ASPERA-3 NPD statistical study

The Analyzer of Space Plasma and EneRgetic Atoms (ASPERA-3) on board Mars Express is designed to study the interaction between the solar wind and the atmosphere of Mars and to characterize the plasma and neutral gas environment in near-Mars space. Neutral Particle Detectors (NPD-1 and 2), which form part of the ASPERA-3 instrument suite, are Energetic Neutral Atom (ENA) detectors which use the time-of-flight (ToF) technique to resolve the energy of detected particles. In the present study, we perform a statistical analysis of NPD ToF data collected between 14 March 2004 and 17 June 2004 when Mars Express was located at the dayside of Mars looking toward the planet. After pre-processing and removal of UV contamination, the ToF spectra were fitted with simple analytical functions so as to derive a set of parameters. The behavior of these parameters, as a function of spacecraft position and attitude, is compared with a model, which describes ENA generation by charge exchange between shocked solar wind protons and extended Martian exosphere. The observations and the model agree well, indicating that the recorded signals are charge-exchanged shocked solar wind.

The Analyser of Space Plasmas and Energetic Atoms (ASPERA-4) for the Venus Express mission

The general scientific objective of the ASPERA-4 (Analyser of Space Plasmas and Energetic Atoms) experiment is to study the solar wind–atmosphere interaction and characterise the plasma and neutral gas environment in the near-Venus space through energetic neutral atom (ENA) imaging and local charged particle measurements. The studies to be performed address the fundamental question: How strongly do the interplanetary plasma and electromagnetic fields affect the Venusian atmosphere? The ASPERA-4 instrument comprises four sensors; two ENA sensors, electron and ion spectrometers. The neutral particle imager (NPI) provides measurements of the integral ENA flux (0.1–60 keV) with no mass and energy resolution but relatively high angular resolution. The neutral particle detector (NPD) provides measurements of the ENA flux, resolving velocity (0.1–10 keV) and mass (H and O) with a coarse angular resolution. The electron spectrometer (ELS) is a standard top-hat electrostatic analyser in a very compact design. These three sensors are located on a scanning platform providing a 4 π coverage. The instrument also contains an ion mass composition sensor, IMA (ion mass analyser). Mechanically, IMA is a separate unit electrically connected with the ASPERA-4 main unit. IMA provides ion measurements in the energy range 0.01–36 keV/ q for the main ion components H + , He ++ , He + , and the ion group with M / q 20 – 80 amu / q .

The Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) for the Mars Express Mission

The general scientific objective of the ASPERA-3 experiment is to study the solar wind – atmosphere interaction and to characterize the plasma and neutral gas environment with within the space near Mars through the use of energetic neutral atom (ENA) imaging and measuring local ion and electron plasma. The ASPERA-3 instrument comprises four sensors: two ENA sensors, one electron spectrometer, and one ion spectrometer. The Neutral Particle Imager (NPI) provides measurements of the integral ENA flux (0.1–60 keV) with no mass and energy resolution, but high angular resolution. The measurement principle is based on registering products (secondary ions, sputtered neutrals, reflected neutrals) of the ENA interaction with a graphite-coated surface. The Neutral Particle Detector (NPD) provides measurements of the ENA flux, resolving velocity (the hydrogen energy range is 0.1–10 keV) and mass (H and O) with a coarse angular resolution. The measurement principle is based on the surface reflection technique. The Electron Spectrometer (ELS) is a standard top-hat electrostatic analyzer in a very compact design which covers the energy range 0.01–20 keV. These three sensors are located on a scanning platform which provides scanning through 180∘ of rotation. The instrument also contains an ion mass analyzer (IMA). Mechanically IMA is a separate unit connected by a cable to the ASPERA-3 main unit. IMA provides ion measurements in the energy range 0.01–36 keV/charge for the main ion components H+, He++, He+, O+, and the group of molecular ions 20–80 amu/q. ASPERA-3 also includes its own DC/DC converters and digital processing unit (DPU).

Charge-induced gelation of microparticles

A highly efficient coagulation process of microparticles was discovered during experiments that were carried out on the International Space Station. Charged microspheres injected into a low pressure neutral gas environment form mm-sized aggregates, provided the initial number density is high enough. The coagulation occurred at a rate ∼104–105 times faster than that usually observed with uncharged grains. Theoretical estimates show that this process is so fast that it cannot even be explained by a coagulation process with charge enhanced cross-section. The observations can, however, be described by a ‘gelation phase transition’. We summarize the salient measurements, outline the theoretical description and show their compatibility.

A cometary neutral gas simulator for gas dynamic sensor and mass spectrometer calibration

The goal of the European Space Agency's Rosetta mission is a rendezvous with the Edgewood‐Kuiper Belt comet 67P/Churyumov‐Gerasimenko. After the initial encounter the spacecraft will accompany the comet for one to two years toward the Sun and throughout the perihelion passage. For the calibration of the onboard comet mass spectrometers a calibration system was built to simulate the neutral gas environment around a cometary nucleus. This facility consists of a fully automated three‐chamber ultra‐high vacuum system with a separate gas‐mixing unit and a docking section with a five‐axis table for instrument positioning and support. The gas‐mixing unit produces mixtures of gases and also gas water vapor mixtures. The gas mixtures are fed into the vacuum system by a leak valve (static mode) or through a nozzle to form a molecular beam (dynamic mode). All flight and flight spare sensors of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) have been calibrated in the static mode of the Calibration System for the Mass Spectrometer Instrument ROSINA (CASYMIR). Preliminary calibration measurements in a pressure range from 10−9 to 10−6 mbar for carbon dioxide and for noble gases from helium to xenon show very promising results.

Measurement of Zeeman pattern with VLBI

Magnetic field is believed to play an important role in the evolution of compact HII regions where massive stars are formed. In fact, it exists very few methods to measure it. One of the best method used, is based on the Zeeman effect: under the effect of the magnetic field, a line can be split into two separated lines of opposite circular polarization. The separation in frequency between the lines is directly proportional to the intensity of the magnetic field. One of the difficulty of the method, is to made a correct identification of the Zeeman pairs. The very high resolution achievable by VLBI allows us to demonstrate whether the pairs are spatially coincident (as expected in true Zeeman pairs) or not. The OH emission traces the densest pockets of the neutral gas environment. In addition, because of its non zero electronic angular momentum, the OH radical is a natural magnetometer allowing us to determine both the intensity and the direction of the magnetic field in the outer layers of the compact HII regions. In this poster I reported on the possibilities of VLBI observations of OH masers in the first excited state (J = 5/2) lying at 6 GHz.

The neutral gas environment of the young supernova remnant SN 1006 (G327.6+14.6)

Fil: Dubner, Gloria Mabel. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Instituto de Astronomia y Fisica del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomia y Fisica del Espacio; Argentina

On the Neutral Gas Content and Environment of NGC 3109 and the Antlia Dwarf Galaxy

As part of a continuing survey of nearby galaxies, we have mapped the neutral gas content of the low surface brightness Magellanic-type galaxy NGC 3109 and its environment, including the Antlia dwarf galaxy, at unprecedented velocity resolution and brightness sensitivity. The H I mass of NGC 3109 is measured to be 3.8 ± 0.5 × 108 M⊙. A substantial warp in the disk of NGC 3109 is detected in the H I emission image in the form of an extended low surface brightness feature. We report a positive detection in H I of the nearby Antlia dwarf galaxy and measure its total neutral gas mass to be 6.8 ± 1.4 × 105 M⊙. We show the warp in NGC 3109 to lie at exactly the same radial velocity as the gas in the Antlia dwarf galaxy and speculate that Antlia disturbed the disk of NGC 3109 during a mild encounter ~1 Gyr in the past. H I data for a further eight galaxies detected in the background are presented.

In vessel pressure measurement in nuclear fusion experiments with asdex gauges

Two crucial aspects of the operation of large existing and future fusion experiments are strongly influenced by the neutral gas environment of the plasma. High energy confinement and good performance of H-mode plasmas call for an as low as possible neutral gas density at the surface of the main plasma. The reduction of power load and erosion on the collector plates call for an as high as possible neutral gas density in the divertor. Both conditions can be fulfilled simultaneously only by appropriate mechanical structures and operational scenarios. The scenarios are close to operational limits. For safe operation, the neutral gas has to be controlled carefully and with high time resolution. At MPI für Plasmaphysik, Garching, a special type of hot cathode ionization gauge has been developed, the so-called ASDEX gauge, which can cope with the conditions inside the discharge vessels of fusion oriented plasma experiments : strong magnetic fields with direction changing by up to ±25°, and high levels of noise and background signals. The gauges are used successfully at several tokamaks and stellarators. The design of the gauges will be shown and the influence of the magnetic field on the operation of the gauge will be discussed.

Comparisons of experimental measurements and two-dimensional plasma-fluid numerical simulations of a magnetized plasma column

The density, the electron, ion, and neutral temperatures in dense (ne≳1013 cm−3) and cool (Te<8 eV) magnetized helium plasma columns submerged in a cool neutral gas (n0∼3×1015 cm−3) environment have been measured. These plasmas, of similar collisionality to those expected in the scrape-off layer of ITER (International Thermonuclear Experimental Reactor) [R. Parker, Contrib. Plasma Phys. 34, 422 (1994)], are found to obey the low-recycling behavior of approximate plasma pressure balance along the column axis. Density decreases by a factor of about 2 and Te remains isothermal. Computer simulations using the B2 [R. Schneider et al., J. Nucl. Mater. 196–198, 810 (1992)] plasma-fluid code have been performed and are in good agreement with experimental results. Measured ion temperatures are consistent with classical energy transfer from the electrons. Neutrals within the plasma are heated by elastic and charge-exchange collisions. This set of measurements represents the most detailed comparisons between experimental results and B2 predictions.