Changes In Collision Frequency Research Articles

Neutral Ingestion Effects on Plume Properties of a Radio-Frequency Plasma Discharge

To enable comparison of thruster performance across different vacuum facilities, an understanding of the effect of operating pressure on plasma plume properties generated in a helicon ion thruster architecture is required. Plasma property measurements acquired at two different vacuum facility operating pressures are used to determine the effect of neutral ingestion on the operation of a replica of the Madison Helicon Experiment at low power. The ion velocity distribution function, electron temperature, ion number density, and plasma potential are measured along the thruster axis. Plasma plume property measurements made at the “high-pressure condition” ( corrected for argon) are compared to values recorded at the “low-pressure condition” ( corrected for argon) for thruster operation at radio-frequency forward power, 340 G source region magnetic field strength, and argon volumetric flow rate. Operation at the high-pressure condition has 31–79 V lower ion energy, 0.8–6.1 eV lower electron temperature, and higher ion number densities than operation at the low-pressure condition. Changes in collision frequency and ionization balance result from increased neutral ingestion at the higher operating pressure.

Investigating the accuracy of Bayesian techniques for before–after safety studies: The case of a “no treatment” evaluation

The main challenge in conducting observational before–after (BA) studies of road safety measures is to use a methodology that accounts for many potential confounding factors. However, it is usually difficult to evaluate and decide on the accuracy of the different safety evaluation techniques available in literature. This is mainly because the outcome of the comparison has no specific target (i.e., the effect of a specific treatment on safety is not precisely known).The objective of this paper is to compare the accuracy of some of the commonly used Bayesian methodologies for BA safety studies by applying them to locations where no safety treatment has been implemented (making the target result to be no effect). This goal was pursued within the setting of a specific case study where a recent set of collision data was available for urban signalized intersections in British Colombia (Canada) with no safety treatments implemented during the time frame considered. An assessment of the temporal stability of the data set was undertaken to exclude the presence of significant BA changes as explanation of the results reported in this paper.Both the well-known empirical Bayes and the full Bayes method with non-linear intervention models were explored for comparison. Two types of selection of the hypothetical treatment sites were used in the analysis: random, to minimize the selection bias effect, and non-random, by selecting sites with abnormal collision frequency (hotspots). Several criteria were used for comparisons including variability among the index of effectiveness for individual treatment locations, the stability of the outcome in terms of the consistency of the results of several experiments and the overall treatment effectiveness.The results showed that when sites are selected randomly for treatment, all methodologies including the simple (naïve) BA study provide reasonable results (small statistically non-significant change in collision frequency). However, when sites are selected for treatment because of high collision occurrence, the estimated index of treatment effectiveness can potentially be biased by values up to 10%. This finding can have significant impact on estimating safety benefits of treatments, especially on those that have low collision reductions. As well, the FB method seems to perform better than other evaluation techniques including the most commonly used EB method. In particular, the FB method provides higher consistency in the estimated collision reduction among treatment sites.

Effect of self‐absorption on attenuation of lightning and transmitter signals in the lower ionosphere

AbstractThe attenuation of VLF signals from lightning and ground‐based VLF transmitters during transionospheric propagation has been the subject of recent interest, as discrepancies have been found between satellite data and model calculations. Previous modeling efforts, however, have not considered the self‐absorption effect due to nonlinear heating and ionization in the lower ionosphere. A self‐consistent model of ionospheric heating is presented here using a time‐domain model of VLF wave propagation through the ionosphere. The model is able to estimate the attenuation of signals due to heating below ∼100 km altitude. In this model, the ionospheric state is updated as the fields propagate, leading to changes in collision frequency and electron density, which in turn affect the wave propagation. We use this model for ground‐based VLF transmitters at different frequencies, amplitudes, and latitudes (i.e., magnetic dip angle), and for lightning‐generated sferics with different amplitudes, at different latitudes, and using a variety of ionospheric density profiles. We find that the inclusion of self‐consistent heating causes a change in the transionospherically propagating wave amplitude that varies considerably with the source amplitude and other parameters. Typical values for the heating contribution to wave attenuation are 1−2 dB for VLF transmitters, but greater than 10 dB for large amplitude lightning discharges. An interesting effect is observed for VLF transmitters and low‐amplitude lightning, where the signal is actually enhanced due to heating, rather than attenuated, in the direction propagating across the Earth's magnetic field.

A Community Initiative to Increase Use of Seat Belts in Northern British Columbia: Impacts on Casualty Crashes

Objective: The study aimed to establish an association between seat belt ticketing by police, seat belt wearing rates, and decreases in casualty rates following the implementation of a community seat belt initiative in a northern region of British Columbia. Methods: Annual and monthly violation ticket rates and the percentage of casualties unbelted in collisions were computed for the North Central region and a comparison region, the Southern Interior. The trends in annual seat belt ticket rates, seat belt use among injured victims, and injury data from 2001 through 2007 were examined by descriptive/univariate methods and with intervention time series analysis. Use of a casualty rate measure controlled for changes in collision frequency over time. The primary outcome measure was injury claim incidents involving injuries other than to soft tissue. Injury claims involving only soft tissue were examined as a control series, because it was reasoned this subset of casualties would be less impacted by seat belt use. Results: Seat belt tickets per capita increased in the North Central (NC) region over the study period, exceeding levels in all other regions. The percentage of unbelted occupants in casualty crashes fell by about 3 percent per year in the NC region after the initiative was introduced compared to about 1 percent per year in the SI region. The time series models revealed a significant reduction in non soft tissue casualties per 100 collisions per month. No significant reduction in the soft tissue only injury criterion was detected. Conclusions: A strong community initiative backed by support at the provincial level can be successful in a largely rural and sparsely populated northern region despite the challenges faced in such regions.

Changes in behavioural response of Mediterranean seabass (Dicentrarchus labrax L.) under different feeding distributions

Captive-induced behavioural deviations may involve many aspects of fish behaviour such as swimming activity and enhancement of individual aggressiveness. We studied seabass (Dicentrarchus labrax) behaviour as a function of manual and automatic feeding distribution modes. Under manual mode, the food is distributed over an extended area for a longer period, and its precise location is not always predictable, while with pneumatic automatic feeders, fish receive the same amount of resource, which is concentrated in the same surface area over a shorter period. We compared seabass behaviour under automatic and manual conditions collecting video image recordings before, during, and after feeding distribution, in the morning and in afternoon, on two different days, and analysing data within independent sessions of measurements. Feeding modes significantly affected swimming behaviour: automatically-fed fish were characterised by vertical movements through the water column (towards the surface and bottom) and by horizontal swimming. Manually-fed fish were instead characterised by sharp direction changes during their swimming, mostly towards the surface. Feeding distribution induced changes in collision frequency and elicited aggressive behaviour. In particular, agonistic behaviour (i.e. a fish attacks another fish) was almost exclusively recorded during the feeding under automatic distribution, whereas it was constantly expressed during all the distribution phases under manual mode.

Characterization of aggregation and protein expression of bovine corneal endothelial cells as microcarrier cultures in a rotating-wall vessel.

Rotating-wall vessels are beneficial to tissue engineering in that the reconstituted tissue formed in these low-shear bioreactors undergoes extensive three-dimensional growth and differentiation. In the present study, bovine corneal endothelial (BCE) cells were grown in a high-aspect rotating-wall vessel (HARV) attached to collagen-coated Cytodex-3 beads as a representative monolayer culture to investigate factors during HARV cultivation which affect three-dimensional growth and protein expression. A collagen type I substratum in T-flask control cultures increased cell density of BCE cells at confluence by 40% and altered the expression of select proteins (43, 50 and 210 kDa). The low-shear environment in the HARV facilitated cell bridging between microcarrier beads to form aggregates containing upwards of 23 beads each, but it did not promote multilayer growth. A kinetic model of microcarrier aggregation was developed which indicates that the rate of aggregation between a single bead and an aggregate was nearly 10 times faster than between two aggregate and 60 times faster than between two single beads. These differences reflect changes in collision frequency and cell bridge formation. HARV cultivation altered the expression of cellular proteins (43 and 70 kDa) and matrix proteins (50, 73, 89 and 210 kDa) relative to controls perhaps due to hypoxia, fluid flow or distortion of cell shape. In addition to the insight that this work has provided into rotating-wall vessels, it could be useful in modeling aggregation in other cell systems, propagating human corneal endothelial cells for eye surgery and examining the response of endothelial cells to reduced shear.

Variations, with peak emission altitude, in auroral O2 atmospheric (1, 1)/(0, 1) ratio and its relation to other auroral emissions

Spectral distributions of auroral optical emissions, peaked at distinctly different heights in the thermosphere, show significant variations, with altitude, in the O2 atmospheric (1, 1)/(0, 1) band ratio. The latter increases with height in auroras peaked between 110 and 150 km and then gradually decreases at higher altitudes. To minimize ambiguities associated with auroral height determination needed for investigating this effect, four independent height‐assessment methods are employed. The first one is based on the incoherent scatter radar (ISR) soundings of the auroral ionization profile from which the height, where precipitating particles dissipate most of their energy, can be determined. Concurrent spectroscopic observations of the thermalized rotational distributions of auroral band emissions yield the ambient air temperature, and hence an independent assessment of the height, of the thermospheric region where these emissions peak. Changes in O/O2 and O/N2 ratios with height lead to changes in the ratios of auroral emissions, from these species, peaked at different heights. Finally, changes in collision frequency with height lead to changes in the brightness of the auroral emissions, resulting from radiatively allowed transitions relative to those produced from radiatively forbidden transitions. The four methods yield comparable values for the height of the thermospheric region where emissions, from each auroral event, peak. The observed variations in O2 atmospheric (1, 1)/(0, 1) with auroral height is compared with that expected from O (1D) + O2 excitation source and quenching by O2 and O. The effects of electron impact excitation of O2(b1∑g+, v′) and high rotational levels of the P branch of O2 atmospheric (0, 0) band on O2 atmospheric (1, 1)/(0, 1) ratio are discussed. Quantitative ratios of various auroral emissions, from O, N2, and N+2, peaked at different heights, that can provide an assessment of auroral heights where these emissions peak, are listed.

Abrasion of suspended biofilm pellets in airlift reactors: Effect of particle size

The detachment of biomass from suspended biofilm pellets in three-phase internal loop airlift reactors was investigated under non-growth conditions, and in the presence of bare carrier particles. In the experiments the size of biofilm pellets and bare carrier particles was varied. Results show that an increase in particle size drastically increases the abrasion rate caused by particle collisions. This increase is larger than predicted by conventional collision theory, which accounts for changes in collision frequency and collision impact. However, collision theory was formulated for neutrally buoyant particles which follow the liquid flow. This condition does not hold for biofilm pellets and carrier particles. The difference might therefore be caused by differences in particle responses to flow fluctuations. An empirical relationship, including this flow response, was formulated. The collision impact is also strongly affected by the roughness of a bare carrier particle: sharp and edgy particles cause much more damage than smoother ones.

Mitigation of bird collisions with transmission lines through groundwire marking

A study was conducted to evaluate the effectiveness of groundwire marking in reducing bird mortality through collision at a power transmission line in southwestern Spain. Monthly flight intensity observations and weekly searches for dead birds were carried out at four sectors of the line comprising 28·2 km, during two consecutive winters, 1989–1990 and 1990–1991, respectively before and after groundwire marking with coloured PVC spirals. Flight intensity and collision frequency decreased respectively by 61% and 60% at marked spans compared to the same spans prior to marking, while there was no significant change in collision frequency at spans left unmarked. After marking, the percentage of birds flying between the cables decreased and that flying above them increased. Our results suggest that many birds avoided flying across the marked spans of the line or climbed while approaching them, and therefore collided less frequently. The percentage decrease in mortality observed in our study falls within the range of results of other groundwire marking or removal studies.

Velocity-dependent collision rates from light-induced drift experiments: C2H4-noble-gas mixtures.

Light-induced drift arises in a binary mixture when velocity-selective optical excitation is combined with a state-dependent kinetic collision frequency (or cross section). This phenomenon is studied experimentally for rovibrationally excited ${\mathrm{C}}_{2}$${\mathrm{H}}_{4}$ immersed in the atomic buffer gases Ne, Ar, Kr, or Xe at temperatures between 275 and 500 K. In several cases, the laser-frequency dependence of the drift is observed to be anomalous and to change dramatically with temperature. This means that the relative change in collision frequency upon excitation depends strongly on velocity. To evaluate the data, an analytic model description is presented and applied to the experimental data. It is found that for each particular buffer gas, all data are well described with a relative difference in collision rate depending on a single parameter only, i.e., the average relative velocity. This dependence is found to be different for the various noble gases. This suggests that the attractive part of the interaction potential is crucial for the occurrence of anomalous light-induced drift.

Change in Electron Mass and Collision Frequency Due to Acoustic Wave Propagation in Piezoelectric Semiconductors

AbstractA phenomenological theory is developed to study the relative change in electron mass and collision frequency when an acoustic wave propagates through piezoelectric semiconductors. The results show that the change in collision frequency is more than that in electron mass for all acoustic wave frequencies in n‐CdS, and is more appreciable at lower temperatures.

On Joule heating of the equatorial electrojet E-region

Simultaneous data on electron density, electron temperature and current density obtained from a rocket borne Langmuir probe, a glass-sealed Langmuir probe and a proton precession magnetometer flown from Thumba (geomag. lat. 0.99°S, geomag. long. 146.79°E, magnetic dip 0°47'S) have been used to calculate the Joule heating in order to assess whether it contributes significantly to the thermal imbalance in the E-region. It is envisaged that the changes in electron temperature are partially brought about by changes in collision frequency and the energy loss factor. It is found that the Joule heating alone is not sufficient to explain the observed differences in electron and neutral gas temperatures. The inclusion of photoelectron heating and adjustments of profiles of the collision frequency and the energy loss factor bring the computed temperature differences closer to the observed differences.

Mutual and self-diffusion in binary mixtures

The relationship between the self and mutual diffusion coefficients in binary mixtures of dilute and dense hard sphere fluids is discussed on the basis of Bearman's equations for diffusion, with the hard sphere friction constants derived by taking into account the change in collision frequency on passing from the dilute to the dense fluid. The connection with the equations of Chapman-Enskog, Thorne, Douglass and Frisch is also considered together with the relationship to the viscosity of systems of mixed hard spheres.

Nonlinear transmission characteristics of the plasma sheath

Abstract : A model is constructed of a nonlinear, one dimensional inhomogeneous multicomponent plasma slab which has the characteristics of the plasma sheath surrounding a typical hypersonic re-entry vehicle at 200,000 ft while traveling at about 18,000 ft/second. A plane wave incident upon the slab at arbitrary angle can induce changes in electron temperature without affecting the neutral gas temperature. The variations of electron temperature produce changes in collision frequency and the rate coefficients describing the various electron production and loss mechanisms. Such changes cause alterations in the effective dielectric constant of the medium. On the basis of a model of a plane-layered medium composed of a stack of linear homogeneous sheets, the reflection and transmission coefficients of the nonlinear slab may be computed by a step-by-step numerical integration of Maxwell's equations expressed in the form of difference equations.