Table of contents

A review of the main approximations to the induced current on a two-dimensional perfectly conducting scatterer at high frequency is presented. Special attention is given to the on-surface radiation condition (OSRC) technique. The technique gives reasonably accurate currents with relative ease. An iterative refinement may be done using the bi-conjugate gradient method in connection with a proper integral equation of second order. The inclusion of the OSRC current overcomes the non-uniqueness associated with the integral equations at the internal resonances.

The electric and magnetic field components in a partially plasma filled coil as it is used for laser pumping lamps is calculated. The longitudinal field component E_z is greater than the azimuthal field component E_phi in the plasma below a certain value of the plasma conductivity, while E_phi exceeds E_z above this value.

The EIT image domain is expanded in a complete set of orthogonal basis images. The number of basis images is given by the number of independent measurements (104 for 16 electrodes). They are ordered according to their sensitivity to data noise, with centrally located features and higher spatial frequency components appearing towards the higher terms in the series. This indicates that the resolution can be improved at the expense of degrading signal to noise in the reconstructed image. The technique offers a flexible way of assembling a reconstruction protocol. A specific reconstruction algorithm similar to the algebraic reconstruction technique (ART) is analysed to show its regularization properties.

The electrical characteristics of the charge (or capacitor) transfer circuits, which are used for the excitation of pulsed gas lasers, are investigated. The formalism that is followed is based on the solution of the differential equations that are obeyed by the currents that flow in the two loops of the circuit. The theoretical predictions are confirmed by experimental data taken with a nitrogen laser. The method can be applied to any type of gas laser using a charge transfer circuit, so that the shape of the voltage and current waveforms can be predicted from the circuit parameters and vice versa. Two other important parameters, the resistance and inductance of the discharge plasma, can also be found.

This paper describes a successful combination of resonance ionization spectroscopy (RIS) with laser ablation of atoms from a solid sample and time-of-flight (TOF) mass spectrometry for isotope separation. For the first time, to the authors' knowledge, RIS signals have been detected from Cs atoms ablated from a solid target. Clear resonances were detected at both of the 6²S to 7²P transitions with signal-to-noise ratios that can be greater than 100:1. This technique differs from previous recorded techniques in its ability to resolve ion peaks from a single laser shot rather than requiring integration over many shots, and in the ease with which it can be used to analyse unprepared samples.

Si, Ge, ZrO₂ and Cu were irradiated by air by 1.06 mu m 10 ns laser pulses with a high intensity of 6.4*10⁹ W cm^-2. Detailed scanning electron microscopy (SEM) and ablation crater profile measurements were made. The results indicated that the ablation rates of these materials under intense pulsed laser irradiation are quite high. The refractory oxide ZrO₂ has the highest material removal rate. Evidence of liquid material flushed out of the ablation craters was observed for all the materials studied with the exception of Cu. Gas dynamic velocities of transient thermal vaporization were calculated using self-similar solutions of the gas dynamic equations. Using these parameters, material removal rates were calculated and the results compared with the experimentally measured values. Possible ablation mechanisms of these materials under the irradiation conditions used in the present work were discussed.

Contact mechanics theories for the normal and tangential interactions between two non-adhering elastic spheres are well established and the tangential sliding criterion is given by Amontons' law. For micron-sized particles, however, surface adhesion effects due to van der Waals forces become significant. Theories to describe the particle interactions in the presence of adhesion have been developed but a rigorous analytical solution is intractable since the stress distribution in the spheres depends on the surface interaction which depends on the exact shape of the deformed surfaces and that is only known if the stress distribution is known. Accepting the approximate nature of existing analytical solutions, the authors have re-examined the tangential behaviour and conclude that, in the presence of adhesion, the contact must 'peel' before sliding can occur. This leads to a new sliding criterion which shows encouraging agreement with experimental results, even for negative normal loads.

The non-equilibrium vibrational kinetics of pure HCl is studied for typical conditions of a discharge sustained xenon-chloride laser, namely: HCl partial pressure range 1 to 15 Torr, electron density from 10¹⁴ to 10¹⁶ cm^-3 and reduced electric field from 2*10^-17 to 5*10^-17 V cm². This work emphasizes the role of processes which are not usually included in the kinetic models for this laser. Particular attention is devoted to V-V (vibration-vibration) energy exchanges, direct dissociation by electron impact and deactivation of vibrational quanta by V-T (vibration-vibration) collisions. The role of the various vibrational levels on the formation of Cl^- is also considered.

A model of non-local electron multiplication for numerical simulation of gas discharge in the cathode region is suggested. The model is based on a hydrodynamic description of electron 'beams', emitted from the cathode surface and from the plasma in the region of the cathode. Solution of the test problem, i.e. a normal glow discharge in helium under a pressure of 1 Torr, has shown good agreement with a well known Monte Carlo result of Tran et al. (1977). The model presented is 'fast' enough to be used in self-consistent simulations of non-stationary discharges of various types. A numerical simulation was performed with a sudden switch of the 'beam' ionization mechanism to a 'local' one in the steady-state helium glow discharge. It showed that a streamer-like ionization wave was generated. It is suggested that the stationary state of a low-pressure glow discharge is sustained by non-local ionization, whereas the local ionization 'destroys' the steady-state solution.

Based on the observation of time-resolved avalanche current waveforms in electronegative gases, three different types of avalanches can be distinguished. The first distinction is that between electron avalanches and ion-dominated avalanches. Electron avalanches are further subdivided into electron avalanches with and without delayed electrons. Delayed electrons are the result of consecutive attachment and detachment processes. Experimental identification of the different avalanche types requires an experimental set-up with a time-resolution in the order of 1 ns. The conventional avalanche model, which involves effective ionization and drift, gives an adequate description only for electron avalanches without delayed electrons. An extended model, which also incorporates electron detachment and ion conversion processes, in principle describes all three kinds of avalanches. When the extended model is applied to the evaluation of measured waveforms, the often-quoted abnormal pressure dependencies in the electron drift velocity and the effective ionization coefficient disappear for the gases studied in this paper.

A self-consistent set of electron impact cross sections is reported for 24 neon and 13 xenon levels. Semi-empirical expressions are used which allow cross section calculations for impact energies from excitation thresholds up to the relativistic energy range. Therefore, they are effective for modelling electric discharges as well as electron-beam produced plasmas in any mixtures containing neon and xenon atoms, such as excimer lasers. The calculated cross sections are in general agreement with the experimental data obtained in electron impact energy loss spectra experiments. In addition, the Boltzmann equation has been solved, in the hydrodynamic approximation, for reduced electric fields in the range 10^-3-10³ Td. The predicted transport parameters are in agreement with the recent experimental data except for the 1s₅ neon level. For this state, the measured excitation coefficient conflicts with that calculated.

Spectra of copper vapour arcs in a vacuum (20 A-1 kA) are compared with the previously reported spectrum of a copper vapour spark in vacuum. In contrast to the latter, the spectra of the arcs consist mainly of Cu I and Cu II lines but only of a few lines of Cu III. The energy balance of a vacuum arc is discussed in a simplified manner taking a 100 A copper arc as an example. The results are: (i) the input energy is mainly transferred into heating of the cathode, kinetic energy of ions, enthalpy of electrons, ionization energy of ions, and energy invested in electron emission (latent work function). All other contributions are negligibly small. (ii) The electron temperature relatively far away from the spot centre (>100 mu m) can hardly exceed 2 eV. (iii) Since the number of photons emitted by the arc spot plasma is relatively small, it is not possible to perform plasma diagnostics by emission spectroscopy with simultaneous spectral, spatial and temporal resolution.

The propagation of highly damped surface waves in plasmas, i.e. plasmas with large axial electron density gradients is dealt with. The WKB approximation adopted in previous models of surface wave discharges is not applicable in this situation. A key parameter of discharge behaviour is v/ omega , where v is the electron-atom collision frequency and omega is the excitation frequency of the wave. Discharges in which v/ omega <<1 are generally referred to as 'weakly collisional'. However, the range of validity of the WKB approximation depends also on the local value of the electron density. Even if v/ omega <<1, this approximation breaks down for sufficiently low electron densities (e.g. near the end of the discharge), while in some cases WKB may adequately model discharges in which v/ omega >>1. The limits of validity of the WKB approximation are examined, both for published experimental results and parameters relevant to the authors' experiments on fluorescent lamps. In a second paper they show solutions for cases where the axial density gradients are high, using formalism developed previously for electrostatic surface waves.

A quantitative model is developed for determining the onset voltages of fore and back coronas in point-plane gaps. The back corona occurring in electrostatic precipitators is studied taking into account, instead of a dust layer, samples of bakelite plates with a pinhole. The onset voltage of the fore corona is the critical value of the applied voltage which yields a self-sustaining discharge from the stressed point. The model for determining the onset voltages of the back corona involves the solution of the governing set of equations; namely, Poisson's equation, the equation for current density, the current continuity equation and the breakdown condition of the dust layer. The calculated onset voltages are compared with those measured experimentally. Results of other investigators are discussed in the light of the present findings.

Results of measurements made on a vacuum arc centrifuge are reported. The rotational velocity of the plasma column has been deduced by cross correlating the floating potential detected by two Langmuir probes inserted into the plasma. The main result is that the rotational velocity is slightly lower than the Alfven critical velocity, for the four different elements used as cathode material to form the plasma (Mg, Zn, Cd, Pb). A tentative explanation for the results is proposed based on an extension of the global energy balance argument which describes a conventional plasma centrifuge.

In a tangential magnetic field the electrons flowing from the cathode spot towards the anode are deflected by the Lorentz force. The surplus electric charges accumulate at the plasma-electrode boundary (positive at the retrograde side of the spot and negative at the opposite side), thus forming a Hall electric field. This field increases the cathode voltage drop at the retrograde side and reduces it at the amperian side; the difference between the cathode voltage drops between these two sides being delta V_c. It is shown that since the Hall parameter omega _e tau _e increases with the distance from the spot, the main contribution to delta V_c is made by the rather distant (up to a distance of the order of the interelectrode gap) additional electric charges. The formula describing arc velocity is obtained on the basis of the hypothesis of the leading role of the cathode voltage drop asymmetry formed by the Hall field. This formula describes the experimental data on the velocity in the weak magnetic field as well as on the magnitude of magnetic field for which the velocity saturates. It also describes, in accordance with the experiment the velocity decrease with interelectrode gap decrease, in addition to the external gas and superimposition of magnetic field normal to the cathode surface.

The apparent emissivity of a parallelepipedic hollow radiation source is computed to all orders of multiple reflection. An iterative method is used to solve accurately the apparent emissivity integral equation, and a special discretization scheme is constructed to handle exactly all geometrical factors. Explicit expressions are given which allow the difficulties associated with the singularities appearing near the cavity edges to be avoided. The case of a flat five-plate cavity is considered in detail. It is shown that this particular shape is adequate to produce high and uniform apparent emissivity on the face opposite to the aperture.

Ternary CuInSe₂ thin films were prepared by electrodeposition from a high pH seleno-sulphate solution. The films were characterized by EDAX, X-ray diffraction and SEM. EDAX analysis shows that the stoichiometry of the films can be controlled by independently varying (i) the deposition current density, and (ii) the electrolytic bath composition. The films deposited at J=1.1 mA cm² with InCl₃ (15.12 mM) content in the bath mixture and at J=0.8 mA cm^-2 with InCl₃ (18.6 mM) content have a stoichiometric chalcopyrite structure with c/a approximately=1.97. Electrical measurements show that deposited films are p-type and have a resistivity of about 5*10³ Omega cm.

Field effect devices have been formed in which the active layer is a thin film of poly(3-methylthiophene) grown electrochemically onto preformed source and drain electrodes. Although a field effect is present after electrochemical undoping, stable device characteristics with a high modulation ratio are obtained only after vacuum annealing at an elevated temperature, and only then if the devices are held in vacuo. The polymer is shown to be p type and the devices operate in accumulation only. The hole mobility in devices thermally annealed under vacuum is around 10^-3 cm² V^-1 s^-1. On exposure to ambient laboratory air, the device conductance increases by several orders of magnitude. This increase may be reversed by subjecting the device to a further high-temperature anneal under vacuum. Subsidiary experiments show that these effects are caused by the reversible doping of the polymer by gaseous oxygen.

Rare earth orthoaluminates (REOA) REAlO₃, the compounds with perovskite structure, are compared with garnets with a view to finding an effective working material for 4.2-20 K magnetic refrigerators. It appears from the meanfield calculations that orthoaluminates are more advantageous than Gd₃Ga₅O₁₂, Dy₃Ga₅O₁₂ and Dy₃Al₅O₁₂, the garnets which so far have been considered the most promising refrigerants.

High density recording measurements have been carried out on CoP thin film tapes produced by electroless deposition. Media have been produced with coercivities in the range 1260-1860 Oe. In general D₅₀ is found to be proportional to H_c. However, it is also found that the initial slope of the demagnetization remanence curve is a further factor in determining D₅₀, presumably because this characterizes the resistance of the tape to demagnetization, which is an important factor at high recording densities. Reasons for the high remanence squareness of the tapes are discussed.

The distributions of the parameters of thermally stimulated depolarization current peaks are determined for two systems: water-in-oil emulsion and hydrated methylcellulose. The technique of thermal sampling was used to establish the fact that the peaks studied are due to a distribution of peak parameters and to show that a linear relationship exists between the peak temperature of the peaks thus isolated, and their energy. The density functions of the distributions of the energy and of the logarithm of the pre-exponential time constant are determined, for the dipoles polarized and for those initially present in the material. This is done both for the whole curve and for the thermal sampling curves; the results are compared. Broad distributions are found for the peak parameters for both materials examined.

Expressions for the complex amplitude reflection and transmission coefficients together with those for the reflectivity and transmissivity of a single basal plane and a plane parallel crystal plate, are derived for absorbing crystals belonging to symmetry classes as high or higher than orthorhombic symmetry. These expressions are presented in a form that could be used readily for the experimental determination of the optical constants of absorbing crystals. Polarization of the waves parallel and perpendicular to the plane of incidence, which coincide with a crystallographic principal plane are considered.

The authors report the results of a preliminary study of opto-thermal impulse response of float-glass surfaces. Remarkable differences of response were observed from the float, nonfloat and fracture surfaces, which are ascribed to near-surface impurities. Of particular interest was the discovery that nonfloat surfaces emit characteristic thermal waves, propagating from the interior to the surface. No theoretical models are currently available for describing such optothermal signals and two new data analysis programs, based on empirical expressions, were developed for this study. These reduce the decay curves to two main fitted parameters, to aid characterization and identification of unknown samples. The results illustrate the promise of opto-thermal techniques for the rapid and nondestructive identification of glass fragments in forensic investigations.

When used for tissue-equivalent monitoring, in accordance with ICRU Reports 39 (1985) and 43 (1988), the thermoluminescence material LiF:Mg, Cu, P shows an under-response to low-energy photons. This paper explores a new way to increase its low-energy response effectively, and to make it adjustable to some extent, by adding elements with high Z_eff. At the same time its excellent TL performance is maintained. Different concentrations of heavy elements are needed to satisfy the requirements for the ICRU quantities H_p(10) and H*(10).

The electrochemical and optical properties in poly(trans-1,2-di(2-thienyl)ethylene), (trans-PTE), and poly(1,4-di(2-thienyl)-1,3-butadiene) (PTB), in comparison with those of polythiophene (PT), during electrochemical p-type doping have been investigated by cyclic voltammetry and in situ optical-absorption-spectrum measurements. The evolution of localized states is evidenced by the spectral change associated with doping. These results are discussed in terms of polaron and/or bipolaron models. The band gap of trans-PTE and PTB was evaluated to be both about 1.9 eV and smaller than that of PT by about 0.2 eV. This result can be interpreted in terms of a conformation change in the molecular structure, such as a change in the torsion angle between neighbouring thiophene rings. The polaron states in trans-PTE appear in the band gap, about 0.65 eV above the valence band but about 0.6 eV below the conduction band. The polaron states in PTB appear at about 0.7 and 0.6 eV below and above the conduction and valence bands respectively. The top of the valence band of both trans-PTE and PTB is located at a higher-energy state than that of PT by about 0.2 eV. This result satisfactorily explains the stability of p-type dopants in trans-PTE and PTB compared with PT. The bottom of the conduction band of both trans-PTE and PTB is in almost the same position as that of PT and successful n-type doping in trans-PTE and PTB is expected.

Thin films of CuInSe₂ with varying copper content have been prepared using a solution growth technique. Optimum deposition parameters, such as pH of solution, composition of elements, deposition rate and temperature, for obtaining films suitable for semiconductor devices have been obtained. Optical properties of the films were studied extensively in the wavelength range 600-1400 nm. The variations of optical band gap, absorption coefficient, Sellmeier's parameters, refractive index and dielectric constant with copper composition x were studied. The present results agree well with the results of other studies.

A procedure is described for the determination of optical thin film parameters from oblique incidence photometric measurements with respect to substrate dispersion and absorption. The procedure has been used successfully for the determination of optical parameters n and alpha of amorphous carbon layers.

The synchronous rolling and asynchronous rolling of boron steel B₃F by positron annihilation have been studied. The experimental results show that asynchronous rolling can not only reduce the rolling pressure, but can also improve the mechanical properties of the materials. A significant relationship between the resist strain intensity and the average lifetime of positron annihilation has been found.

A model for predicting the friction of contacting rough surfaces is presented. Numerical simulation techniques are used to generate randomly rough surfaces with Gaussian statistics, An elastic Hertzian analysis is then used to calculate the contact of each generated surface with a rigid, perfectly smooth, half plane of specified applied load. The number, location and size of the contact points are determined. Contact is also calculated according to simple plasticity theory, and the authors show how the results from the elastic analysis tend to the plasticity results, as surface roughness increases. For each calculated contact profile, the friction coefficient for that contact is calculated, using two method, one 'microscopic' and one 'macroscopic'. For the microscopic method he calculates the adhesive force required to break each junction, and sum these to produce the total frictional force. This assumes that each contact point must be broken simultaneously, and is likely to overestimate the frictional force for a large number of contact points. In the macroscopic approach the total frictional force is related to the total true contact area, but does not take account of the details of each individual contacting junction. He presents results from the models, showing the effects of applied load, and of surface and material properties, on quantities such as the total contact area, mean contact pressure and mean surface separation, and hence on frictional force. The numerical approach allows one to study the typical variations in friction that might reasonably be expected to arise from the statistical nature of the surface contact. He finds that these variations are generally smaller than those measured in a typical friction experiment. Model predictions are compared with experimental measurements of the friction of MoS₂ coated substrates. The macroscopic model is shown to predict the variation of friction with roughness, whereas the microscopic model performs better when studying the effect of applied load on friction. In both cases, agreement is better for the metallic substrates than for the ceramic substrate. Reasons for these results, and possible extensions of the model, are discussed.

Small particle penetration depth in a random fixed bed of larger glass spheres has been studied experimentally under laminar flow conditions and in the absence of Brownian motion. The small particles, marked with a thin coating of gold, were tracked in the fixed bed made optically transparent by matching the index of refraction of the suspending fluid to that of the glass spheres. Small particle penetration depths were examined across different ratios of the small particle diameter to the large sphere diameter.

A new experimental technique is proposed for investigations into charge carrier movement in dielectric systems. VPITDD is characteristic of measurement of the depolarization current induced by voltage-pulse polarization with pulse duration much shorter than the depolarization time. This method is used in the investigation of thin films of organosilicon plasma polymers. A simple model is proposed for describing the observed results.

The variation of magnetization with concentration has been examined for powdered single crystals of Cr/MgO containing Cr concentration in the range 2500-5000 p.p.m. The magnetization measurements were made at room temperature (23 degrees C) and at high temperature (800 degrees C). Long heat-treatment has the effect of decreasing the magnetization for all the specimens examined.

Fluid microdrop collisions are important in both chemical and meteorological studies. A three-dimensional molecular model of colliding microdrops of water is developed and explored. Fundamental modes for coherent collisions are described and exhibited, as are results for noncoherent collisions.