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Collision effects on line shapes using quantum mechanical description of atomic center of mass motion, considering pressure effects in gas lasers
Based on the idea that local conditions within a galactic disk largely determine the region's evolution time scale, researchers built a theoretical model to take into account molecular cloud and star formations in the disk evolution process. Despite some variations that may be caused by spiral arms and central bulge masses, they found that many late-type galaxies show consistency with the model in their radial atomic and molecular gas profiles. In particular, researchers propose that a scaling law be used to generalize the gas distribution characteristics. This scaling law may be useful in helping to understand the observed gas contents in many galaxies. Their model assumes an exponential mass distribution with disk radius. Most of the mass are in atomic gas state at the beginning of the evolution. Molecular clouds form through a modified Schmidt Law which takes into account gravitational instabilities in a possible three-phase structure of diffuse interstellar medium (McKee and Ostriker, 1977; Balbus and Cowie, 1985); whereas star formation proceeds presumably unaffected by the environmental conditions outside of molecular clouds (Young, 1987). In such a model both atomic and molecular gas profiles in a typical galactic disk (as a result of the evolution) can be fitted simultaneously by adjusting the efficiency constants. Galaxies of different sizes and masses, on the other hand, can be compared with the model by simply scaling their characteristic length scales and shifting their radial ranges to match the assumed disk total mass profile sigma tot(r).
Improved mathematical models based on fundamental principles of conservation of mass, energy, and momentum developed for use in computer simulation of atomization of jets of liquid fuel in rocket engines. Models also used to study atomization in terrestrial applications; prove especially useful in designing improved industrial sprays - humidifier water sprays, chemical process sprays, and sprays of molten metal. Because present improved mathematical models based on first principles, they are minimally dependent on empirical correlations and better able to represent hot-flow conditions that prevail in rocket engines and are too severe to be accessible for detailed experimentation.
The phenomenon of current sheet canting in pulsed electromagnetic accelerators is the departure of the plasma sheet (that carries the current) from a plane that is perpendicular to the electrodes to one that is skewed, or tipped. Review of pulsed electromagnetic accelerator literature reveals that current sheet canting is a ubiquitous phenomenon - occurring in all of the standard accelerator geometries. Developing an understanding of current sheet canting is important because it can detract from the propellant sweeping capabilities of current sheets and, hence, negatively impact the overall efficiency of pulsed electromagnetic accelerators. In the present study, it is postulated that depletion of plasma near the anode, which results from axial density gradient induced diamagnetic drift, occurs during the early stages of the discharge, creating a density gradient normal to the anode, with a characteristic length on the order of the ion skin depth. Rapid penetration of the magnetic field through this region ensues, due to the Hall effect, leading to a canted current front ahead of the initial current conduction channel. In this model, once the current sheet reaches appreciable speeds, entrainment of stationary propellant replenishes plasma in the anode region, inhibiting further Hall-convective transport of the magnetic field; however, the previously established tilted current sheet remains at a fairly constant canting angle for the remainder of the discharge cycle, exerting a transverse J x B force which drives plasma toward the cathode and accumulates it there. This proposed sequence of events has been incorporated into a phenomenological model. The model predicts that canting can be reduced by using low atomic mass propellants with high propellant loading number density; the model results are shown to give qualitative agreement with experimentally measured canting angle mass dependence trends.
The space radiation environment is a complex mixture of particle types and energies originating from sources inside and outside of the galaxy. These environments may be modified by the heliospheric and geomagnetic conditions as well as planetary bodies and vehicle or habitat mass shielding. In low Earth orbit (LEO), the geomagnetic field deflects a portion of the galactic cosmic rays (GCR) and all but the most intense solar particle events (SPE). There are also dynamic belts of trapped electrons and protons with low to medium energy and intense particle count rates. In deep space, the GCR exposure is more severe than in LEO and varies inversely with solar activity. Unpredictable solar storms also present an acute risk to astronauts if adequate shielding is not provided. Near planetary surfaces such as the Earth, moon or Mars, secondary particles are produced when the ambient deep space radiation environment interacts with these surfaces and/or atmospheres. These secondary particles further complicate the local radiation environment and modify the associated health risks. Characterizing the radiation fields in this vast array of scenarios and environments is a challenging task and is currently accomplished with a combination of computational models and dosimetry. The computational tools include models for the ambient space radiation environment, mass shielding geometry, and atomic and nuclear interaction parameters. These models are then coupled to a radiation transport code to describe the radiation field at the location of interest within a vehicle or habitat. Many new advances in these models have been made in the last decade, and the present review article focuses on the progress and contributions made by workers and collaborators at NASA in the same time frame. Although great progress has been made, and models continue to improve, significant gaps remain and are discussed in the context of planned future missions. Of particular interest is the juxtaposition of various review committee findings regarding the accuracy and gaps of combined space radiation environment, physics, and transport models with the progress achieved over the past decade. While current models are now fully capable of characterizing radiation environments in the broad range of forecasted mission scenarios, it should be remembered that uncertainties still remain and need to be addressed.
The paper presents a simplified model of the ion source chemistry, explains several details of the data reduction method used in obtaining atomic-nitrogen (N) densities from OSS data, and discusses implications of gas-surface reactions for the design of future satellite-borne mass spectrometers. Because of various surface reactions, N appears in three different forms in the ion source, as N, NO, and NO2. Considering the rather small spin modulation of NO and NO2 in the semi-open ionization chamber used in the OSS instrument, it is not surprising that these reaction products have not been previously identified in closed source instruments as a measure of the presence of atomic nitrogen. Warmup and/or outgassing of the ion source are shown to drastically reduce the NO2 concentration, thereby making possible reliable measurement of ambient N densities.
A combination of numerical modeling and analysis of the Pioneer Venus UADS data base is studied, specifically data from the orbiter ion mass spectrometer (OIMS), orbiter neutral mass spectrometer (ONMS), and orbiter electron temperature probe (OETP). A one dimensional model of the Venus nightside ionosphere was set up in which downward fluxes of atomic ions are introduced at the upper boundary to simulate transport of ions from the dayside. The model shows that the densities of mass-28 ions, CO(+) + N(2+), resulting from an influx of atomic ions from the dayside are quite small, due to the high ionization potentials of CO and N2 that make chemical production difficult. A look at the data reveals that the actual densities of mass-28 ions are quite variable, from values near 10 to more than 10(exp 4) cm(exp -3). The excess mass-28 ions are assumed to be produced by electron precipitation and that the presence of high densities of mass-28 ions is a signature of auroral precipitation. A discussion of the atomic oxygen green line in the nightglow of Venus, which is produced mainly by dissociative recombination of O(2+), is presented. Original calculations of production rates of excited states for models based on Pioneer Venus data are also presented.
We have used a three-dimensional model of recovery phase storm hot plasmas to explore the signatures of pitch angle distributions (PADS) in global fast atom imagery of the magnetosphere. The model computes mass, energy, and position-dependent PADs based on drift effects, charge exchange losses, and Coulomb drag. The hot plasma PAD strongly influences both the storm current system carried by the hot plasma and its time evolution. In turn, the PAD is strongly influenced by plasma waves through pitch angle diffusion, a microscale effect. We report the first simulated neutral atom images that account for anisotropic PADs within the hot plasma. They exhibit spatial distribution features that correspond directly to the PADs along the lines of sight. We investigate the use of image brightness distributions along tangent-shell field lines to infer equatorial PADS. In tangent-shell regions with minimal spatial gradients, reasonably accurate PADs are inferred from simulated images. They demonstrate the importance of modeling PADs for image inversion and show that comparisons of models with real storm plasma images will reveal the global effects of these microscale processes.
Context. NGC288 is a globular cluster with a well developed blue horizontal branch covering the so-called u-jump which indicates the onset of diffusion. It is therefore well suited to study the effects of diffusion in blue horizontal branch (HB) stars. Aims. We compare observed abundances to predictions from stellar evolution models calculated with diffusion and from stratified atmospheric models. We verify the effect of using stratified model spectra to derive atmospheric parameters. In addition we investigate the nature of the overluminous blue HB stars around the u-jump. Methods. We define a new photometric index sz from uvby measurements that is gravity sensitive between 8 000K and 12 000 K. Using medium-resolution spectra and Stroemgren photometry we determine atmospheric parameters (Teff, logg) and abundances for the blue HB stars. We use both homogeneous and stratified model spectra for our spectroscopic analyses. Results. The atmospheric parameters and masses of the hot HB stars in NGC288 show a behaviour seen also in other clusters for temperatures between 9 000K and 14 000 K. Outside this temperature range, however, they follow rather the results found for such stars in (omega)Cen. The abundances derived from our observations are for most elements (except He and P) within the abundance range expected from evolutionary models that include the effects of atomic diffusion and assume a surface mixed mass of 10(exp −7) M. The abundances predicted by stratified model atmospheres are generally significantly more extreme than observed, except for Mg. The use of stratified model spectra to determine effective temperatures, surface gravities and masses moves the hotter stars to a closer agreement with canonical evolutionary predictions. Conclusions. Our results show definite promise towards solving the long-standing issue of surface gravity and mass discrepancies for hot HB stars, but there is still much work needed to arrive at a self-consistent solution.
The Advanced Rocket Injector/Combustor Code (ARICC) that has been developed to model the complete chemical/fluid/thermal processes occurring inside rocket combustion chambers are highlighted. The code, derived from the CONCHAS-SPRAY code originally developed at Los Alamos National Laboratory incorporates powerful features such as the ability to model complex injector combustion chamber geometries, Lagrangian tracking of droplets, full chemical equilibrium and kinetic reactions for multiple species, a fractional volume of fluid (VOF) description of liquid jet injection in addition to the gaseous phase fluid dynamics, and turbulent mass, energy, and momentum transport. Atomization and droplet dynamic models from earlier generation codes are transplated into the present code. Currently, ARICC is specialized for liquid oxygen/hydrogen propellants, although other fuel/oxidizer pairs can be easily substituted.
During the tenure of this grant, we have been looking into the chemistry of the nightside ionosphere of Venus with a view toward elucidating the relative roles of electron precipitation and plasma transport as sources of the nightside ionosphere. Secondary goals have included determining the densities of minor species on the nightside, and verifying the relative normalization of the Pioneer Venus orbiter ion mass spectrometer (OIMS) and orbiter neutral mass spectrometer (ONMS) in the photochemical equilibrium region. Our studies have involved a combination of numerical modeling and analysis of the Pioneer Venus UADS data base, specifically data from the OIMS, ONMS and electron temperature probe (OETP). We have set up a one-dimensional model of the Venus nightside ionosphere, in which downward fluxes of atomic ions are introduced at the upper boundary to simulate transport of ions from the dayside. Our model shows that the densities of mass-28 ions (CO+ + N+) resulting from an influx of atomic ions from the dayside are quite small, due to the high ionization potentials of CO and N2 that make chemical production difficult.
The performance of a broad-beam, three-grid, ion extraction system incorporating radio frequency (RF) mass discrimination was investigated experimentally. This testing demonstrated that the system, based on a modified single-stage Bennett mass spectrometer, can discriminate between ionic species having about a 2-to-1 mass ratio while producing a broad-beam of ions with low kinetic energy (less than 15 eV). Testing was conducted using either argon and krypton ions or atomic and diatomic oxygen ions. A simple one-dimensional model, which ignores magnetic field and space-charge effects, was developed to predict the species separation capabilities as well as the kinetic energies of the extracted ions. The experimental results correlated well with the model predictions. This RF mass discrimination system can be used in applications where both atomic and diatomic ions are produced, but a beam of only one of the species is desired. An example of such an application is a 5 eV atomic oxygen source. This source would produce a beam of atomic oxygen with 5 eV kinetic energy, which would be directed onto a material specimen, to simulate the interaction between the surface of a satellite and the rarefied atmosphere encountered in low-Earth orbit.
Atomic nitrogen number densities in the upper thermosphere measured by the open source neutral mass spectrometer (OSS) on Atmosphere Explorer-C during 1974 and part of 1975 have been used to construct a global empirical model at an altitude of 375 km based on a spherical harmonic expansion. The most evident features of the model are large diurnal and seasonal variations of atomic nitrogen and only a moderate and latitude-dependent density increase during periods of geomagnetic activity. Maximum and minimum N number densities at 375 km for periods of low solar activity are 3.6 x 10 to the 6th/cu cm at 1500 LST (local solar time) and low latitude in the summer hemisphere and 1.5 x 10 to the 5th/cu cm at 0200 LST at mid-latitudes in the winter hemisphere.
To resolve uncertainties in estimated LEO atomic oxygen fluence and provide reaction product composition data for comparison to data obtained in ground-based simulation laboratories, a flight experiment has been proposed for the space shuttle which utilizes an ion-neutral mass spectrometer to obtain in-situ ambient density measurements and identify reaction products from modeled polymers exposed to the atomic oxygen environment. An overview of this experiment is presented and the methodology of calibrating the flight mass spectrometer in a neutral beam facility prior to its use on the space shuttle is established. The experiment, designated EOIM-3 (Evaluation of Oxygen Interactions with Materials, third series), will provide a reliable materials interaction data base for future spacecraft design and will furnish insight into the basic chemical mechanisms leading to atomic oxygen interactions with surfaces.
Molecular nitrogen, atomic oxygen, and helium densities obtained from the AE-C satellite are analyzed in magnetic latitude and magnetic local time coordinates and compared to OGO 6 data for various seasons and magnetic activity levels. A depletion region for atomic oxygen and helium with respect to molecular nitrogen at high magnetic latitudes in the postmidnight magnetic local time sector persists under both high and low solar activity conditions. A global model in magnetic coordinates, patterned after the mass spectrometer and incoherent scatter (MSIS) model, is developed for molecular nitrogen, atomic oxygen, and helium. It is shown to represent the data well, without any residual UT dependence, and with an accuracy comparable to that provided by the more complex MSIS model with longitude terms The advantage of using magnetic coordinates is that they are more directly related to the major energy inputs and momentum sources in the polar regions than are geographic coordinates and are more convenient to use in studies of high latitude energy deposition processes. This is important for comparison with theoretical models where the number of coordinates is limited.
The Evaluation of Oxygen Interactions with Materials 3 (EOIM-3) flight experiment was developed to obtain benchmark atomic oxygen reactivity data and was conducted during Space Transportation System Mission 46 (STS-46), July 31 to August 7, 1992. In this paper, we present an overview of EOIM-3 and the results of the Lyndon B. Johnson Space Center (JSC) materials reactivity and mass spectrometer/carousel experiments. Mass spectrometer calibration methods are discussed briefly, as a prelude to a detailed discussion of the mass spectrometric results produced during STS-46. Mass spectrometric measurements of ambient O-atom flux and fluence are in good agreement with the values calculated using the MSIS-86 model of the thermosphere as well as estimates based on the extent of O-atom reaction with Kapton polyimide. Mass spectrometric measurements of gaseous products formed by O-atom reaction with C(13) labeled Kapton revealed CO, CO2, H2O, NO, and NO2. Finally, by operating the mass spectrometer so as to detect naturally occurring ionospheric species, we characterized the ambient ionosphere at various times during EOIM-3 and detected the gaseous reaction products formed when ambient ions interacted with the C(13) Kapton carousel sector. By direct comparison of the results of on-orbit O-atom exposures with those conducted in ground-based laboratory systems, which provide known O-atom fluences and translational energies, we have demonstrated the strong translational energy dependence of O-atom reactions with a variety of polymers. A 'line-of-centers' reactive scattering model was shown to provide a reasonably accurate description of the translational energy dependence of polymer reactions with O atoms at high atom kinetic energies while a Beckerle-Ceyer model provided an accurate description of O-atom reactivity over a three order-of-magnitude range in translational energy and a four order-of-magnitude range in reaction efficiency. Postflight studies of the polymer samples by x-ray photoelectron spectroscopy and infrared spectroscopy demonstrate that O-atom attack is confined to the near-surface region of the sample, i.e. within 50 to 100 A of the surface.
The modified gasdynamic convected-magnetic-field MHD model developed by Spreiter and Stahara (1980) to simulate the supersonic flow of the solar wind past planetary magnetoionospheres is extended to account for cases (such as Venus and comets) in which significant numbers of neutral atmospheric atoms become ionized in the surrounding flow and add to its momentum, energy, and mass. The mathematical model and the solution procedures for the nose and tail regions are explained; typical computational grids are shown; and numerical results for a comet and for the Venus ionosheath and bow shock are presented graphically. It is found that the bow shock weakens and moves further upstream of the obstacle as mass loading is increased and the flow upstream of the bow shock becomes more compressed.
The shape of the velocity distribution of water group ions observed by the Giotto ion mass spectrometer on its approach to comet Halley is modeled to derive empirical values for the rates of ionization, energy diffusion, and loss in the midcometosheath. The model includes the effect of rapid pitch angle scattering into a bispherical shell distribution as well as the effect of the magnetization of the plasma on the charge exchange loss rate. It is found that the average rate of ionization of cometary neutrals in this region of the cometosheath appears to be of the order of a factor 3 faster than the `standard' rates approx. 1 x 10(exp -6)/s that are generally assumed to model the observations in most regions of the comet environment. For the region of the coma studied in the present work (approx. 1 - 2 x 10(exp 5) km from the nucleus), the inferred energy diffusion coefficient is D(sub 0) approx. equals 0.0002 to 0.0005 sq km/cu s, which is generally lower than values used in other models. The empirically obtained loss rate appears to be about an order of magnitude greater than can be explained by charge exchange with the `standard' cross section of approx. 2 x 10(exp -15)sq cm. However such cross sections are not well known and for water group ion/water group neutral interactions, rates as high as 8 x 10(exp -15) sq cm have previously been suggested in the literature. Assuming the entire loss rate is due to charge exchange yields a rate of creation of fast neutral atoms of the order of approx. 10(exp -4)/s or higher, depending on the level of velocity diffusion. The fast neutrals may, in turn, be partly responsible for the higher-than-expected ionization rate.