Kevin F. Ness

Development of swarm transport theory in radio-frequency electric and crossed electric and magnetic fields

The advancements associated with modern day technology demands incorporation of the best physical understanding and the most accurate modelling of charged particle motion in gases. In recent times there have been major advances in the fundamental swarm transport theory, and this is the subject of the present paper. We start from 1986, when “multi-term” solutions of Boltzmann’s equation for static electric fields had been developed to a sophisticated level, and proceed through to the present day, where the theory is motivated far more by application to industrial processes, which involve both electric and magnetic fields , either static or time varying. We present a unified time-dependent multi-term solution of Boltzmann’s equation, emphasising the common methods and techniques underlying the treatment of all these situations, whether they be for electron or ion swarms. New and significant numerical results are presented to highlight the rich and diverse range of phenomena which are observed.

Multi-term solution of the Boltzmann equation for electron swarms in crossed electric and magnetic fields

Using the moment method, a multi-term solution of the Boltzmann equation for electron swarms in neutral gases under the influence of crossed electric and magnetic fields is presented. The electric and magnetic fields are assumed to be uniform and static. Both model and real gases are considered.

Electron drift velocities in He and water mixtures: measurements and an assessment of the water vapour cross-section sets.

The drift velocity of electrons in mixtures of gaseous water and helium is measured over the range of reduced electric fields 0.1-300 Td using a pulsed-Townsend technique. Admixtures of 1% and 2% water to helium are found to produce negative differential conductivity (NDC), despite NDC being absent from the pure gases. The measured drift velocities are used as a further discriminative assessment on the accuracy and completeness of a recently proposed set of electron-water vapour cross-sections [K. F. Ness, R. E. Robson, M. J. Brunger, and R. D. White, J. Chem. Phys. 136, 024318 (2012)]. A refinement of the momentum transfer cross-section for electron-water vapour scattering is presented, which ensures self-consistency with the measured drift velocities in mixtures with helium to within approximately 5% over the range of reduced fields considered.

Transport coefficients and cross sections for electrons in water vapour: comparison of cross section sets using an improved Boltzmann equation solution.

This paper revisits the issues surrounding computation of electron transport properties in water vapour as a function of E/n(0) (the ratio of the applied electric field to the water vapour number density) up to 1200 Td. We solve the Boltzmann equation using an improved version of the code of Ness and Robson [Phys. Rev. A 38, 1446 (1988)], facilitating the calculation of transport coefficients to a considerably higher degree of accuracy. This allows a correspondingly more discriminating test of the various electron-water vapour cross section sets proposed by a number of authors, which has become an important issue as such sets are now being applied to study electron driven processes in atmospheric phenomena [P. Thorn, L. Campbell, and M. Brunger, PMC Physics B 2, 1 (2009)] and in modeling charged particle tracks in matter [A. Munoz, F. Blanco, G. Garcia, P. A. Thorn, M. J. Brunger, J. P. Sullivan, and S. J. Buckman, Int. J. Mass Spectrom. 277, 175 (2008)].

Long-term current prediction in the central Great Barrier Reef

Abstract Long-term current records in the Great Barrier Reef region are needed to address major ecological problems such as periodic outbreaks of the coral predator, Crown of Thorns Starfish (COTS, Acanthaster sp.). In situ current meter data are sparsely distributed and typically available for deployments shorter than 1 year. A suite of linear systems models has therefore been developed which allow low-frequency along shelf currents to be specified using readily available meteorological and oceanographic forcing data. The models which are essentially statistical, nevertheless reflect our understanding of regional hydrodynamics. Using optimally lagged multilinear regression they allow predictions to be made quickly and economically from input time series and a few specified parameters. Models were calibrated using current meter mooring data obtained from a transect across the central Great Barrier Reef in 1985 and validated using data from similar deployments in 1987 and 1990. The models which are simplest to implement, perform well and are statistically efficient are those based on the geostrophic across-shelf momentum balance. Using as inputs coastal sea levels or, when available, offshore sea level differences, they can precisely predict currents over time spans of up to 25 years. They accurately respond to fluctuations at weather time scales and, when offshore differences are used, at seasonal and inter-annual scales. The predicted currents are being used to drive advective models of COTS larvae dispersal and to set boundary conditions for more complex numerical hydrodynamic current simulation models.

Influence of gas pressure and magnetic field upon dc magnetron discharge

We have proposed a hybrid model for dc magnetron discharge. Simulations of the discharges under different applied voltages are performed using the hybrid model. We confirm that this hybrid model is able to predict a magnetron discharge under conditions of less than 1 Pa by comparing the results from the simulation with that from the experiment. We also perform simulations of dc magnetron discharges under conditions of different gas pressures and different magnitudes of magnetic field. In so doing, we clarify the influence of gas pressure and magnetic field upon dc magnetron discharge.

Kinematics of the field hockey penalty corner push‐in

Abstract The aims of the study were to determine those variables that significantly affect push‐in execution and thereby formulate coaching recommendations specific to the push‐in. Two 50 Hz video cameras recorded transverse and longitudinal views of push‐in trials performed by eight experienced and nine inexperienced male push‐in performers. Video footage was digitized for data analysis of ball speed, stance width, drag distance, drag time, drag speed, centre of massy displacement and segment and stick displacements and velocities. Experienced push‐in performers demonstrated a significantly greater (p < 0.05) stance width, a significantly greater distance between the ball and the front foot at the start of the push‐in and a significantly faster ball speed than inexperienced performers. In addition, the experienced performers showed a significant positive correlation between ball speed and playing experience and tended to adopt a combination of simultaneous and sequential segment rotation to achieve accuracy and fast ball speed. The study yielded the following coaching recommendations for enhanced push‐in performance: maximize drag distance by maximizing front foot‐ball distance at the start of the push‐in; use a combination of simultaneous and sequential segment rotations to optimise both accuracy and ball speed and maximize drag speed.

Non-conservative electron transport in CF4 in electric and magnetic fields crossed at arbitrary angles

A Monte Carlo simulation technique is used to investigate electron transport in carbon tetrafluoride (CF4) for an arbitrary configuration of electric and magnetic fields. We investigate the way in which the transport coefficients and other swarm properties are influenced by the electric and magnetic field strengths and the angle between the fields. In addition, the sensitivity of transport data on the presence of non-conservative collisions (attachment/ionization) is analysed. It is found that the difference between the two sets of transport coefficients, bulk and flux, resulting from the explicit effects of non-conservative collisions, can be controlled either by the variation of the magnetic field strengths or by the angles between the fields. This study was initiated in order to obtain the transport data for input into the fluid models of magnetron and inductively coupled plasma discharges as well as several types of high energy particle detectors, and has resulted in a database of such transport data. Values and general trends in the profiles of mean energy, collision frequency, rate coefficients, drift velocity elements and diffusion tensor are reported here.

Stretch Shorten Cycle Performance: Detrimental Effects of Not Equating the Natural and Movement Frequencies

Abstract This study aimed to assess whether the benefits associated with stretch shorten cycle (SSC) movements required the movement frequency to be in resonance with the natural frequency of the elastic structures. Seventeen untrained participants performed SSC and concentric bench press throws. Further, quasi-static muscular actions were also performed in which a brief perturbation was applied to the bar with the resulting damped oscillations providing natural frequency data. It was observed that prior stretch did not facilitate concentric performance. Further, there were large significant differences between the natural frequency of the musculo-tendinous system and the frequency of the SSC movements. The authors hypothesize that the failure to achieve resonance contributed to the poor performance achieved in the SSC actions.

On the existence of transiently negative diffusion coefficients for electrons in gases in E ? B fields

A recent study (Raspopovic Z et al 2000 J. Phys. D: Appl. Phys. 33 1298) reported the existence of negative diagonal diffusion tensor elements for electrons in a neutral gas under the influence of crossed radiofrequency electric and magnetic fields. In this paper we demonstrate, using a time-dependent multi-term solution of the Boltzmann equation and time-resolved Monte Carlo simulation, that this phenomenon is a transient relaxation effect obtainable under dc crossed field conditions.