Transverse Diffusion Coefficient Research Articles

Remote sensing observations and analyses of cooling water discharges from a coastal power station

Abstract Results are reported from a series of remote sensing investigations into the marine discharge of cooling water from a coastal power station complex. Qualitative observations and analyses of the spreading behaviour of the thermal discharge are presented for a range of tidal states, using ATM data from a series of aircraft overflights of the site. It is demonstrated that, under ebb flow conditions, the discharges from two adjacent outfalls are merged into an integrated surface plume in the far-field, and that the plume behaves subsequently as if generated by a single source. It is shown that the lateral spreading of the plume is inhibited significantly by coastal attachment and local thermal forcing and cross-flow effects. Calibrated overflight data from the thermal infrared bands have been used to quantify the unimpeded offshore spreading, and estimates are presented of the transverse turbulent diffusion coefficient ϵt, for the site in question. Values of ϵt, thus derived are shown to be in satisf...

Reexamination of the Li+-He interaction potential.

Using various recently proposed interaction potentials for the Li + -He system, elaborate calculations of the mobility, longitudinal, and transverse diffusion coefficients of Li + swarms in helium have been made by high-speed Monte Carlo simulations (MCS's). In addition, the transverse diffusion coefficients for this ion-neutral-atom pair have been experimentally measured with total errors of ±3%

Free electron charging of ultrafine aerosol particles

Experiments of free electron charging of aerosol particles in the free molecule regime are reported. Monodisperse ultrafine silver particles of 5–30 nm were exposed to known concentrations of low energy electrons produced by ionization by α-particles in a unipolar aerosol charger. The kinetic energy of the electrons was varied by changing the electric field intensity in the charger (i.e. between 93 and 279 V cm −1). The range of Knudsen number for aerosol charging (i.e. the ratio of the electron mean free path to the particle radius) was from 30 to 261. The charged fraction was measured as a function of particle size in high-purity helium and nitrogen under different charging conditions. The experimental results for the combination coefficient between neutral particles and electrons suggested a free-molecule diffusion charging mechanism which was dependent on the electron mobility, transverse diffusion coefficient, mean free path, and mean kinetic energy (i.e. electron temperature). The functional dependence was similar to that given by the ionic charging theories of Natanson (1960, Sov. Phys. 5, 538–551), and Fuchs (1963, Geofis. Pura Appl. 56, 185–193) when the appropriate electron properties were used. The electron charging models of O'Hara et al. (1989, J. Aerosol Sci. 20, 313–330), and Zagnit'ko et al. (1989, Russ. J. Phys. Chem. 63, 883–888) did not fit the experimental results because they were not derived for the free-molecule regime. A modified Fuchs charging theory that uses an empirical accommodation coefficient for the electrons at the surface of the particle was used to fit the experimental results. Good agreement was found by using an accommodation coefficient of 0.4 for both helium and nitrogen.

Spin diffusion in dilute, polarized 3He-4He solutions

Spin dynamics for arbitrarily polarized and very dilute solutions of 3He in liquid 4He are described. We began at a very fundamental level by deriving a kinetic equation for arbitrarily polarized dilute quantum systems based on a method due to Boercker and Dufty. This approach allows more controlled approximations than our previous derivation based on the Kadanoff-Baym technique. Our previous work is here generalized to include T-matrix interactions rather than the Born approximation. Spin hydrodynamic equations are derived. The general equations are valid for both Fermi and Bose systems. By use of a well-known phenomenological potential to describe the 3He-3He T-matrix we calculate longitudinal and transverse spin diffusion coefficients D ⊥ and D ¦ and the identical-particle spin-rotation parameter Μ. We confirm that these two diffusion constants differ at low T with D ⊥ approaching a constant as T → 0, and D¦~1/T2. Estimates of errors made by our approximations are considered in detail. Good agreement is found in comparison with data from both Cornell University and the University of Massachusetts. We find that the s-wave approximation is inadequate and that mean-field corrections are important. Comparison is also made between theory and the recent UMass viscosity measurements.

A pulsed NMR study of 3He-4He solutions

We report a study of the Leggett-Rice effect in dilute 3He-4He solutions at a Larmor frequency of 71 MHz. We have measured the height and phase of the spin echoes following 2-pulse NMR sequences of three solutions with 3He concentrations of 0.56%, 1.4%, and 4.0%. The theory of Leggett fits the data extremely well and yields values of the parameter ΜM0 and the transverse diffusion coefficient D. We are also able to evaluate ΜM0 and D by analyzing multiple spin echoes and find good consistency between the two methods. From values of λ/(1+F 0 a ) at various concentrations, we are able to determine the scattering length for 3He quasiparticles, a, quite accurately. We find a = −0.97±0.03 A.

Longitudinal Dispersion Coefficients in Estuary

Field measurements of velocity and salinity on a transverse cross section of a straight reach in the upper Conwy estuary have been used to evaluate the temporal and intratidal variations of the transverse and vertical shear induced components of the longitudinal dispersion coefficient. The results yield clear temporal trends and an element of reproducability between tides. A comparison with earlier published results for a cross section further downstream in the same estuary suggests that changes in the channel width, cross-sectional shape, and bends probably contribute significantly to a longitudinal variation of dispersion coefficient values. A comparison of the values of the longitudinal dispersion coefficients with those obtained from a solution of the governing transport equations emphasizes the need for a better understanding of the spatial and temporal variation of the values of the vertical and transverse eddy diffusion coefficients.

Mobility and diffusion of lithium ions in neon

The mobility and the transverse diffusion coefficient have been measured for 7Li+ ions in neon at 295 K and field-to-density values E/n0 up to 200 Td. The experimental transport coefficients were compared with values calculated from a recent ab initio potential (Roeggen et al. 1991). The calculated mobilities were 0.5% higher than the experimental mobilities at low fields, and 1.8% lower at high fields. Slight adjustments to the ab initio potential (essentially a shift of the repulsive branch by 0.02 a0) were sufficient to reduce this difference to +or-0.3% or less at all values of E/n0. The long-range part of the 'mobility-modified potential' corresponds to a polarizability for the neon atom alpha =2.682 au. This is about 0.5% above other 'experimental values', and about 0.5% below recent theoretical values.

Boltzmann Equation Analysis of Electron Swarm Development in Model Gases with Power-Law Elastic Scattering

The electron swarm behaviors in model gases with elastic momentum transfer cross-section of the form q ( e )∝ e r ( r =-1/2, 0, 1/2 and 1) are analyzed by the time-of-flight method of a Boltzmann equation. The velocity distribution functions are apprroximated by three term expansion. Here e is the electron energy. The ratios D L / D T are calculated to study an anisotropy in real space. Here D L and D T are the longitudinal and the transverse diffusion coefficients to an electric field E . The values of electron-to-atom mass ratios m / M are changed from 10 -4 to 5×10 -2 and the values of E / N are taken as 2.8, 14 and 28 Td respectively. Here N is the gas number density. The present result shows that the anisotropy in real space depends upon m / M regardless of E / N . The discrepancy between the electron diffusion coefficient D V as defined in velocity space and D T , and the validity of the usual two-term method to calculate the time-of-flight parameters are discussed in detail.

Drift Velocity, Longitudinal and Transverse Diffusion in Hydrocarbons derived from Distributions of Single Electrons

A time of flight method is described which allows the simultaneous measurement of drift velocity w and the ratios of the longitudinal and transverse diffusion coefficients to mobility (DL/JL, DT/JL) of electrons in gases. The accuracy achieved in this omnipurpose experiment is comparable with that of specialised techniques and is estimated to be �1 % for w and �5% for the D / JL measurements .. Results for methane, ethane, ethene, propane, propene and cyclopropane for values of E/N (the electric field strength divided by the number density) ranging from 0�02 to 15 Td are presented and discussed (1 Td = 10-21 Vm2 ).

Electron kinetics in simple liquids at high electric fields.

A model for the motion of an electron in a simple liquid is presented. It is used in the Monte Carlo simulation of the behavior of an assembly of electrons in liquid argon subjected to high electric fields. To implement this simulation, a set of scattering rates for electrons have been obtained. Electron-energy distribution, transport parameters (drift velocity, longitudinal, and transverse diffusion coefficients), and ionization rate have been calculated. The distribution function has been found to be far from the usually assumed Maxwellian form. For fields above 10 kV/cm, the drift velocity decreases with increasing field, changing to a positive slope for fields above approximately 500 kV/cm. At higher fields, the ratio of the longitudinal to transverse diffusion coefficient becomes greater than 1. These observations are a consequence of having, with increasing field, a maximum in the backscattering rate, and an increasing number of electrons with energies above that corresponding to the maximum in the total scattering rate. The threshold field for ionization has been calculated to be approximately 2 MV/cm.

A study of K+–He and K+–Kr atomic interaction potentials based on transverse diffusion measurements

The ratio of the transverse diffusion coefficient to the mobility, DT/K, has been measured for K+ ions drifting in helium and krypton in the presence of a uniform electric field. Results at 298 K are reported at E/N values ranging from 10 to 220 Td for K+–He and 5 to 420 Td for K+–Kr. Values of DT/K for these two systems were also calculated with a Monte Carlo simulation method which used a number of proposed potentials as input. The accuracy of these potentials has been evaluated by comparing the results to the experimentally determined values. For K+–He none of the available potentials seems to be completely accurate, but for K+ –krypton the potential of Koutselos, Mason, and Viehland appears to be a very good representation of the actual potential. Within the ranges studied, the estimated accuracy of the experimental measurements is ±2.5% and the accuracies for the Monte Carlo calculations are ±1% for K+–He and ±0.5% for K+–Kr.

Temporal evolution of ischemic damage in rat brain measured by proton nuclear magnetic resonance imaging.

We studied the effect of focal cerebral ischemia on the "state" of brain water using proton nuclear magnetic resonance imaging. Focal cerebral ischemia was induced in five halothane-anesthetized rats via tandem occlusion of the left common carotid artery and the left middle cerebral artery. The proton transverse relaxation time, the proton density, and the water diffusion coefficient were measured at various times from the same region of brain tissue from 1.5 to 168 hours after occlusion. Early measurements indicated significant changes in the transverse relaxation time (p = 0.004) and water diffusion coefficient (p = 0.002) of ischemic brain tissue compared with a homologous region from the contralateral hemisphere. However, the transverse relaxation time, proton density, and water diffusion coefficient in ischemic brain tissue showed different temporal evolutions over the study period. Diffusion coefficient weighting was superior to relaxation time and proton density weighting for the visualization of early cerebral ischemia. Our data suggest that nuclear magnetic resonance imaging is sensitive in detecting changes in proton-associated parameters during early cerebral ischemia and confirm significant changes (p less than or equal to 0.01) in the temporal evolution of transverse relaxation times, proton densities, and diffusion coefficients following middle cerebral artery occlusion.

A multi-term Boltzmann equation analysis of electron swarms in gases-the time-of-flight parameters

An accurate and efficient method for solving the Boltzmann equation is presented. Using a multi-term expansion technique, the time-of-flight (TOF) electron swarm parameters (such as the centre-of-mass drift velocity Wr, the longitudinal DL and transverse DT diffusion coefficients) can be evaluated. The method utilizes an amalgamation of the conventional two-term expansion and the Galerkin method. The first two terms of the Legendre polynomial expansion of the electron energy distribution are obtained by the two-term method, while the third-order and higher-order terms are deduced by the Galerkin method. The present technique is applied to determine the electron energy distribution using Fourier components to solve the Boltzmann equation and TOF swarm parameters in methane gas. Good agreement with Monte Carlo simulation is obtained and the method is also applied to a previously described model gas.

An apparatus for measuring D T/K without in-vacuum mechanical motion

A new drift-tube apparatus has been developed for measuring DT/K, the ratio of the transverse diffusion coefficient to the mobility of ions in a neutral gas under the influence of an electric field. It harnesses existing technology in mechanical, electronic, materials, and computer engineering. The main features are the ion current detector, which comprises 65 parallel rods, and an external mechanical multiplexer, which was designed to have extremely low (fA order) leakage currents and noise levels. Unlike with other earlier systems, the need for precise mechanical motion or adjustments of experimental conditions inside the drift tube during data collection has been circumvented. Its design therefore simplifies construction and makes operation less error prone. Data collection is computer controlled. A value for DT/K can now typically be obtained in less than 20 min, which greatly increases the rate at which results can be obtained as well as reduces the chance of unwanted variations in experimental conditions occurring during data collection. Results are estimated to be accurate to within 3%.

Transverse diffusion and interaction potential for K+ drifting in Ar

Experimentally measured values of the ratio, DT/K, of the transverse diffusion coefficient to the mobility for K+ ions drifting in argon gas are reported. Results were obtained with estimated accuracies of +or-3% at E/N values ranging from 10-600 Td (1 Td=10-21 V m2) and have been adjusted to 298 K. These experimental values are compared with values obtained from Monte Carlo simulations using several proposed interaction potentials as input. The comparisons suggest that none of the potentials examined is a totally accurate representation of the K+-Ar interaction potential.

Longitudinal and transverse spin diffusion in polarized 3He - 4He solutions

Spin dynamics for arbitrarily polarized and very dilute 3He- 4He solutions are described. We generalize previous work to include 3He- 4He phenomenological interactions, and we calculate longitudinal and transverse spin diffusion coefficients and the identical-particle spin-rotation parameter. Good agreement is found in comparison with recent data. The s-wave approximation is found to be inadequate and mean-field corrections are important.

Boundary effects in transverse diffusion coefficient measurements

No abstract.

Measurements of DT/μ of Na+ ions in argon and neon

Room temperature measurements of the ratio DT/ mu between the transverse diffusion coefficient and the mobility of mass-identified ions in a neutral gas are reported for Na+ ions in argon and neon. A new apparatus was used which, in contrast to previous methods, does not involve mechanical movement which alters the geometry of the drift tube assembly during a measurement scan, or variation of the experimental conditions inside it. Results with an estimated accuracy of better than 3% were obtained at E/N values ranging from 10 to 260 Td with argon and 10 to 200 Td with neon.

Transport coefficients for lithium ions in nitrogen

The mobility mu and the longitudinal and transverse diffusion coefficients DL and DT have been measured for 7Li+ ions in N2 gas, at 295 K and field to density values E/n0 up to 600 Td. At low values of E/n0, clustering reactions distorted the measured current density distributions and arrival time spectra. The diffusion coefficients could therefore be determined accurately only for E/n040 Td, while the mobility was determined down to 12 Td. The experimental values of DL/ mu and DT/ mu were compared with values calculated from the experimental mobility values, using generalised Nernst-Townsend relations and neglecting inelastic collisions. Large discrepancies were found over the whole E/n0 range, showing the crucial importance of inelastic energy losses for the swarm behaviour.

Transverse diffusion coefficient and carrier density fluctuation derived from the fluctuation of the state occupancy function in semiconductors

We have very recently developed a theory of the fluctuations of the state occupancy function in semiconductors, including a new expression of the noise source term instead of the classical one which was shown to be erroneous. In the present paper, we show that this theory can apply to practical situations, since it allows us to actually compute the transverse diffusion coefficient versus the electric field intensity in the range 0–50 kV/cm, in p-type silicon at 300 K. Also the variance of the number of particles in a volume is computed.