Number Density Ratio Research Articles

Resonant Vibrational Excitation and De-Excitation of N2(v) by Low-Energy Electrons

We have calculated cross sections and rate coefficients for low-energy electron impact excitation of the nitrogen molecule from vibrationally excited levels N2(v) 1-8. Calculations are performed in the 2Pig shape resonance energy region, from 0 to 5 eV. The cross sections are determined by using our recent integral cross section measurements of the ground level vibrational excitation and the most recent cross sections for elastic electron scattering, applying the principle of detailed balance. The rate coefficient calculations are performed for the Maxwellian electron energy distribution. By using extended Monte Carlo simulations, the electron energy distribution functions (EEDF) and the rate coefficients are also determined for the nonequilibrium conditions, in the presence of the homogeneous external electric field for the typical, moderate values of the electric field over gas number density ratios, E/N.

Mass and Auger electron spectroscopy studies of the interactions of atomic and molecular chlorine on a plasma reactor wall

We have investigated the interactions of Cl and Cl2 with an anodized Al surface in an inductively coupled chlorine plasma. The cylindrical substrate is rapidly rotated within a differentially pumped wall and is exposed to the plasma 35% of the time through a conical skimmer. On the opposite side of the substrate, a second skimmer and differential pumping allows the surface and desorbing products to be analyzed by Auger electron spectroscopy (AES), line-of-sight mass spectrometry (MS), and through pressure rise measurements. In a 600W Cl2 plasma at 5mTorr, the surface becomes covered with a layer with the overall stoichiometry of about Al2Si2O10Cl3, with Si being the result of the slow erosion of the quartz discharge tube. The surface layer composition (specifically Cl coverage) does not change as a function of the delay time (1ms–10min) between plasma exposure and AES characterization. In contrast to AES measurements, the MS signals from Cl2 desorption, resulting from recombination of Cl atoms, decrease by about a factor of 10 over the 1–38ms probed by varying the substrate rotation frequency. Substantial adsorption and desorption of Cl2 are also observed with the plasma off. Cl recombination coefficients (γCl) derived from an analysis of the time-dependent MS signals range from 0.01 to 0.1 and increase with increasing Cl-to-Cl2 number density ratio, suggesting a competition for adsorption sites between Cl2 and Cl.

Measuring the Ratio of 242Cm to 244Cm in Operating BWR Cores Using Start-Up Range Neutron Monitors

The purpose of this study is to compare the measured and calculated ratios of neutrons originating from 242Cm and 244Cm in commercial BWRs in order to evaluate the prediction error for a weakening start-up range neutron monitor (SRNM) signal due to the decay of in-core neutron sources over a ten year period. Measurements were carried out over a period of several months, using the least-squares method to fit the time series of SRNM signals with the theoretical model. Neutron sources of each fuel assembly were calculated by the macroscopic depletion model of a plant process computer. The ratios of neutrons originating from 242Cm and 244Cm detected by each SRNM were calculated by whole core fixed source calculation using the nodal expansion method. One BWR core measurement was taken at the beginning of the cycle and three BWR core measurements were taken in the middle of the cycle. A comparison of the measured and calculated ratios of 242Cm and 244Cm number densities showed reasonable agreement. The slight difference observed between the measured and calculated ratios will be discussed.

The full impulse response of two-dimensional jet/wake flows and implications for confinement

In this theoretical study, a linear spatio-temporal analysis is performed on unconfined and confined inviscid jet/wake flows in order to determine whether they are absolutely or convectively unstable. The impulse response is evaluated in the entire outer fluid, rather than just at the point of impulse, over a wide range of density ratios. This confirms that the dominant saddle point can validly migrate into the plane of diverging eigenfunctions. This reveals that, at certain density ratios and shear numbers, the response can grow upstream in some directions with a cross-stream component, even though it decays directly upstream of, and at, the point of impulse. This type of flow is convectively unstable when unconfined, but becomes absolutely unstable when confined. Other effects of confinement are described in a previous paper. Together, these articles have important implications for the design of fuel injectors, which often employ confined shear flows at high Reynolds number and large density ratios to generate strong mixing in combustion chambers.

Reionization scenarios and the temperature of the intergalactic medium

We examine the temperature structure of the IGM due to the passage of individual ionisation fronts using a radiative transfer (RT) code coupled to a particle-mesh (PM) N-body code. Multiple simulations were performed with different spectra of ionising radiation: a power law (/ o i0.5 ), miniquasar, starburst, and a time-varying spectrum that evolves from a starburst spectrum to a power law. The RT is sufficiently resolved in time and space to correctly model both the ionisation state and the temperature across the ionisation front. We find the post-ionisation temperature of the reionised intergalactic medium (IGM) is sensitive to the spectrum of the source of ionising radiation, which may be used to place strong constraints on the nature of the sources of reionisation. Radiative transfer effects also produce large fluctuations in the HeII to HI number density ratio ´. The spread in values is smaller than measured, except for the time-varying spectrum. For this case, the spread evolves as the spectral nature “

Rate coefficients for resonant vibrational excitation of CO

Rate coefficients for vibrational excitation of the carbon-monoxide molecule, via the 2Π shape resonance in the energy region from 0 eV to 5 eV have been calculated. Calculations are performed for a Maxwellian electron energy distribution by using our recent experimentally measured differential cross sections for excitation of the first 10 vibrational levels. By using extended Monte Carlo simulations the electron energy distribution functions (EEDFs) and rate coefficients are determined in non-equilibrium conditions, in the presence of a homogeneous electric field. Calculations are performed for typical, moderate values of the electric field over gas number density ratios, E/ N. A difference between Maxwellian and non-equilibrium rate coefficients was found due to a specific shape of the electron energy distribution function under the considered conditions.

Growth mechanism of lanthanum phosphate particles by continuous precipitation

Lanthanum phosphate, a matrix material of green phosphor, was synthesized by using a T-shaped mixer-type tubular reactor which enabled continuous precipitation of the particles having a uniform diameter. Effects of precursor concentrations and residence time on the yield, number density, diameter, and aspect ratio of particles were investigated for discussing the particle growth mechanism. Particularly, in order to observe the initial stage of particle growth, the residence time was varied over six orders ranging from 0.4 ms to 120 s by employing tubes of 250 μ m in inner diameter and a newly developed fast particle sampling device. The lanthanum phosphate yield was proportional to the concentration product of the precursors, lanthanum ion and dihydrogen phosphate ion, at the inlet of the reactor, while the number density of the particles was proportional to the 2.3th power of the precursor concentration product. The particles formation was practically completed within 0.01 s. Then, the particles grew slowly up to a few micrometers in diameter by growth of radiating acicular crystallites.

Mixed Traffic Flow in Anisotropic Continuum Model

An extended speed gradient model is developed to study the mixed traffic flow of two types of vehicles with different velocities and different lengths. Two new terms, representing the reaction of one type of vehicle to the other, are proposed and built into the dynamic speed evolution equations for fast cars and slow vehicles. With the developed model, the relation between the steady-state behavior of the mixed traffic flow system and two system parameters is analyzed, including the number density ratio R of the fast cars to the total vehicles and the vehicle length ratio α of the slow vehicle to the fast car. The formation and propagation of traffic jams, the dissolution process of the jams, and the local cluster effect of the mixed traffic system are also studied. The simulation results are reasonable and consistent with daily experience.

Nighttime ozone profiles in the stratosphere and mesosphere by the Global Ozone Monitoring by Occultation of Stars on Envisat

The Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument on board the European Space Agency's Envisat satellite measures ozone and a few other trace gases using the stellar occultation method. Global coverage, good vertical resolution and the self‐calibrating measurement method make GOMOS observations a promising data set for building various climatologies. In this paper we present the nighttime stratospheric ozone distribution measured by GOMOS in 2003. We show monthly latitudinal distributions of the ozone number density and mixing ratio profiles, as well as the seasonal variations of profiles at several latitudes. The stratospheric profiles are compared with the Fortuin‐Kelder daytime ozone climatology. Large differences are found in polar areas and they can be shown to be correlated with large increases of NO2. In the upper stratosphere, ozone values from GOMOS are systematically larger than in the Fortuin‐Kelder climatology, which can be explained by the diurnal variation. In the middle and lower stratosphere, GOMOS finds a few percent less ozone than Fortuin‐Kelder. In the equatorial area, at heights of around 15–22 km, GOMOS finds much less ozone than Fortuin‐Kelder. For the mesosphere and lower thermosphere, there has previously been no comprehensive nighttime ozone climatology. GOMOS is one of the first new instruments able to contribute to such a climatology. We concentrate on the characterization of the ozone distribution in this region. The monthly latitudinal and seasonal distributions of ozone profiles in this altitude region are shown. The altitude of the mesospheric ozone peak and the semiannual oscillation of the number density are determined. GOMOS is also able to determine the magnitude of the ozone minimum around 80 km. The lowest seasonal mean mixing ratio values are around 0.13 ppm. The faint tertiary ozone peak at 72 km in polar regions during wintertime is observed.

Rate coefficients for resonant vibrational excitation of N 2

Rate coefficients for vibrational excitation of the nitrogen molecule, via the 2Π g shape resonance in the energy region from 0 to 5 eV have been calculated. Calculations are performed for Maxwellian electron energy distribution by using recent experimentally measured differential cross sections for excitation of the first ten vibrational levels and for elastic scattering. By using extended Monte Carlo simulations the electron energy distribution functions (EEDF) and rate coefficients are determined in non-equilibrium conditions, in a presence of homogenous electric field for typical, moderate values of the electric field over gas number density ratios, E/N. A great difference between Maxwellian and non-equilibrium rate coefficients was found due to a very specific shape of the electron energy distribution function in nitrogen under the considered conditions.

Electron excitation coefficients of neutral and ionic levels of krypton in Townsend discharges

In this paper, we present experimental results for excitation coefficients of krypton atoms to several Kr and Kr + excited levels for E/ N (electric field to gas particle number density ratio usually in units of Townsend, 1 Td = 10 − 21 V m 2) values from 7 × 10 − 20 V m 2 to above 1 × 10 − 17 V m 2. The data have been obtained in two different parallel plate self-sustained Townsend discharge drift tubes. The spatial distribution of the emission intensities were recorded and then normalized to give excitation coefficients at the anode, by using the electron flux at this point. The values of these coefficients are placed on an absolute scale by using a standard tungsten ribbon lamp calibrated against a primary blackbody radiation standard. The ionization rates at different E/ N are obtained from the spatial emission profiles. The data for atomic krypton levels 2p 2, 2p 3, 2p 5, 2p 6, 2p 7, 2p 8, 3p 5 and 3p 6 (in Paschen notation) were converted to excitation coefficients by using quenching coefficients from the literature. The emission coefficients of eight 4s 24p 4 ( 3P)5p levels of Kr + have also been measured for E/ N values from about 1 × 10 − 18 V m 2 up to nearly 8 × 10 − 18 V m 2.

Satellite‐observed pollution from Southern Hemisphere biomass burning

Biomass burning is a major source of pollution in the tropical Southern Hemisphere, and fine mode carbonaceous particles are produced by the same combustion processes that emit carbon monoxide (CO). In this paper we examine these emissions with data from the Terra satellite, CO profiles from the Measurement of Pollution in the Troposphere (MOPITT) instrument, and fine‐mode aerosol optical depth (AOD) from the Moderate‐Resolution Imaging Spectroradiometer (MODIS). The satellite measurements are used in conjunction with calculations from the MOZART chemical transport model to examine the 2003 Southern Hemisphere burning season with particular emphasis on the months of peak fire activity in September and October. Pollutant emissions follow the occurrence of dry season fires, and the temporal variation and spatial distributions of MOPITT CO and MODIS AOD are similar. We examine the outflow from Africa and South America with emphasis on the impact of these emissions on clean remote regions. We present comparisons of MOPITT observations and ground‐based interferometer data from Lauder, New Zealand, which indicate that intercontinental transport of biomass burning pollution from Africa often determines the local air quality. The correlation between enhancements of AOD and CO column for distinct biomass burning plumes is very good with correlation coefficients greater than 0.8. We present a method using MOPITT and MODIS data for estimating the emission ratio of aerosol number density to CO concentration which could prove useful as input to modeling studies. We also investigate decay of plumes from African fires following export into the Indian Ocean and compare the MOPITT and MODIS measurements as a way of estimating the regional aerosol lifetime. Vertical transport of biomass burning emissions is also examined using CO profile information. Low‐altitude concentrations are very high close to source regions, but further downwind of the continents, vertical mixing takes place and results in more even CO vertical distributions. In regions of significant convection, particularly in the equatorial Indian Ocean, the CO mixing ratio is greater at higher altitudes, indicating vertical transport of biomass burning emissions to the upper troposphere.

BURNING RATES AND FLAME OSCILLATIONS IN GLOBALLY HOMOGENEOUS TWO-PHASE MIXTURES (FLAME SPEED OSCILLATIONS IN DROPLET CLOUD FLAMES)

The study of spray combustion in industrial equipment is difficult due to the multiplicity of interdependent variables. Therefore, a combustion vessel was used for fundamental studies of aerosol combustion, under strictly controlled conditions of pressure, temperature and equivalence ratio. The droplet clouds were characterized using laser diagnostics, variables such as Sauter mean diameter, number density, gas and liquid equivalence ratios were determined. Nearly mono-sized droplet clouds were obtained in this vessel being the droplet diameter a function of time. The phenomenon of flame oscillation, the cyclic variation of flame speed during flame development, has been addressed in the past using several approaches but there is no yet a unique accepted explanation. The combustion studies in the present work revealed the differences between the speed of droplets and the gas velocity near the flame front. These resulted in variations in local equivalence ratio, which in turn manifested in flame speed oscillations.

Spatial distributions of the number densities of neutral atoms and ions for the different elements in a laser induced plasma generated with a Ni-Fe-Al alloy

Spatially-resolved emission spectroscopy, including spatial devonvolution of the spectra, has been used to determine the three-dimensional distributions of the relative number densities of neutral atoms and ions of the elements present in a laser-induced plasma generated with a Ni-Fe-Al alloy. The method is based on the precise measurement of the local electronic temperature from Saha-Boltzmann plots constructed with Fe I and Fe II lines. The plasma was generated in air at atmospheric pressure using a 1064-nm Nd:YAG laser, and the emission was detected in the time window 3.0-3.5 micros. The ionization fraction was very high (above 0.9) for the three elements in the sample, only decreasing behind the expanding plasma front. The relative number densities were obtained from the emissivities of selected elemental lines as well as the temperature. The error in this procedure was estimated, and it was found that it is largely due to the uncertainties in the transition probability values used. The spatial distributions of the total relative number densities of the three elements were shown to coincide within the error, a result which is relevant to the development of models of plasma emission used in analytical applications. The ratios of the total number densities of the elements in the plasma were compared to their concentration ratios in the sample; however, the relatively high errors in the relative number densities did not permit any definitive conclusions to be drawn about the stoichiometry of the laser ablation process.

Escape of atmospheric gases from the Moon

The escape rate of atmospheric molecules on the Moon is calculated. Based on the assumption that the rates of emission and escape of gases attain equilibrium, the ratio of molecular number densities during day and night, n0d/n0n, can be explained. The plausible emission rate of helium and radioactive elements present in the Moon has also been calculated.

Detection of Point Sources on Two-Dimensional Images Based on Peaks

This paper considers the detection of point sources in two-dimensional astronomical images. The detection scheme we propose is based on peak statistics. We discuss the example of the detection of far galaxies in cosmic microwave background experiments throughout the paper, although the method we present is totally general and can be used in many other fields of data analysis. We consider sources with a Gaussian profile--that is, a fair approximation of the profile of a point source convolved with the detector beam in microwave experiments--on a background modeled by a homogeneous and isotropic Gaussian random field characterized by a scale-free power spectrum. Point sources are enhanced with respect to the background by means of linear filters. After filtering, we identify local maxima and apply our detection scheme, a Neyman-Pearson detector that defines our region of acceptance based on the a priori pdf of the sources and the ratio of number densities. We study the different performances of some linear filters that have been used in this context in the literature: the Mexican hat wavelet, the matched filter, and the scale-adaptive filter. We consider as well an extension to two dimensions of the biparametric scale-adaptive filter (BSAF). The BSAF depends on two parameters which are determined by maximizing the number density of real detections while fixing the number density of spurious detections. For our detection criterion the BSAF outperforms the other filters in the interesting case of white noise.

Revised equipartition and minimum energy formula for magnetic field strength estimates from radio synchrotron observations

AbstractThe commonly used classical equipartition or minimum‐energy estimate of total magnetic fields strengths from radio synchrotron intensities is of limited practical use because it is based on the hardly known ratio K of the total energies of cosmic ray protons and electrons and also has inherent problems. We present a revised formula, using the number density ratio K for which we give estimates. For particle acceleration in strong shocks K is about 40 and increases with decreasing shock strength. Our revised estimate for the field strength gives larger values than the classical estimate for flat radio spectra with spectral indices of about 0.5–0.6, but smaller values for steep spectra and total fields stronger than about 10 µG. In very young supernova remnants, for example, the classical estimate may be too large by up to 10×. On the other hand, if energy losses of cosmic ray electrons are important, K increases with particle energy and the equipartition field may be underestimated significantly. Our revised larger equipartition estimates in galaxy clusters and radio lobes are consistent with independent estimates from Faraday rotation measures, while estimates from the ratio between radio synchrotron and X‐ray inverse Compton intensities generally give much weaker fields. This may be explained e.g. by a concentration of the field in filaments. Our revised field strengths may also lead to major revisions of electron lifetimes in jets and radio lobes estimated from the synchrotron break frequency in the radio spectrum. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The precipitation reaction in a Ag-modified TiAl based intermetallic, as studied by TEM

The precipitation reaction of L1 2–(Al, Ag) 3Ti in a L1 0–TiAl(Ag) intermetallic compound with a nominal composition of Ti–54 at.% Al–2 at.% Ag has been studied by transmission electron microscopy (TEM), high resolution transmission electron micrograph (HRTEM), EDX and supporting microhardness test. TEM observations revealed that the alloy quenched from 1273 K is of L1 0 structure without any precipitates. After aging at 1073 K for 1 h, spherical L1 2–(Al, Ag) 3Ti particles were formed via homogenous nucleation. The compositions of the precipitates and the matrix have been determined. After further aging at 1073 K up to 65 h, growth and coarsening of L1 2–(Al, Ag) 3Ti precipitates and a change in the shape of the precipitates from spherical to elliptical were observed. The particle size distribution, number density and aspect ratio as functions of aging time have been determined. Microhardness tests showed that maximum hardness appeared in the sample aged at 1073 K for 17 h with an average precipitate diameter of about 40 nm. The results were discussed in terms of an anisotropy of the energy of interphase boundaries and the theories of coarsening and precipitation hardening.

Effects of Heavy Ions on Kinetic Alfvén Waves

Kinetic Alfvén Wave (KAW) is one of the low-frequency electromagneticfluctuations that are identified extensively in space plasmasby in situ observations of satellites and has been an interestingtopic for discussion widely in the fields of laboratory, space,and astrophysical plasmas because of its potential importancein plasma particle energization. Some satellite observationsshow that the number density ratio of the oxygen ionsto the ambient plasma is 30% ∼ 50%,sometimes, even as high as 80%. In this paper,effects of heavy ion species on KAWs are studied in a low-beta plasma.The results show that heavy ions not only considerablyreduce the propagation speed of KAWs, but also remarkablyinfluence the parallel component of perturbed electric field of KAWs(to the ambient magnetic field). The ratio of parallelto perpendicular components of perturbed field decreases(or increases) with the heavy ion abundance for KAWs dominated bythe electron inertial length (or by ion acoustic gyroradius).In particular, the resonant condition of KAWs with thermal electronsis modified by the heavy ion species.

Radial ICP characteristics for ICP-AES using direct injection or microconcentric nebulisation

Radial analyte signal intensities, fundamental plasma parameters, and non-spectral interference effects were characterized for inductively coupled plasma atomic emission spectrometry (ICP-AES) using a direct injection nebuliser (Vulkan DIN) or a microconcentric nebuliser (MCN) coupled to a cyclonic spray chamber for sample introduction. Radial analyte signal intensity profiles for atomic and ionic lines with energy sum (Esum) between 1.85 and 15.41 eV were used. When using the MCN system, for all lines the signal intensity profiles were parabolic with maxima at the axial centre of the plasma. For the Vulkan DIN, the shapes of the profiles were dependent on their Esum showing minimum intensities at the axial centre of the plasma for lines with high Esum values. The ionisation temperature, electron number density and magnesium ion-atom line intensity ratio determined indicated that ionisation and excitation capabilities were deteriorated at the centre of the plasma when using the Vulkan DIN compared with the MCN. This was found to be not simply a result of high aerosol load when using the Vulkan DIN, but because of poor plasma–aerosol interaction, possibly caused by the confined distribution of aerosol in the plasma and high nebuliser gas velocity. Indeed, for the Vulkan DIN, electron number density and ionisation temperature increased with liquid flow rate, which could be explained by plasma shrinkage, or the thermal pinch effect, which increased the aerosol–plasma interaction at increased liquid flow rates. As a consequence, when increasing the liquid flow rate in the range 20–90 μL min−1: (i) analyte sensitivity increased linearly; (ii) the plasma became unstable; (iii) the magnitude of matrix effects remained almost unaffected. These results are partly in contrast to observations made with other types of direct injection nebulisers and ICP instruments.