Table of contents

A method is presented to solve mixed boundary value problems of Laplace's equation. Several important examples in electromagnetic flowmeters are given.

The possibility of forming solitary coherent electromagnetic wavetrains in a bulk medium has been investigated theoretically. The process of creating such wavetrains as a result of the interference between electromagnetic waves emitted by two moving charges and two dipole oscillators has been described by a pair of differential equations. A conclusion has been made about the possibility in principle of electromagnetic fields existing in a vacuum in the form of a simple Fermi-Pasta-Ulam recurrence which can be interpreted as an interference between rectangular time and space impulses of different electromagnetic fields. An experimental electronic model, constructed to generate and detect Fermi-Pasta-Ulam recurrence wavetrains, showed that the character of the interaction of such wavetrains with a metallic shield differs from the analogous process with a traditional electromagnetic wave.

A series of life test measurements has been performed in He-Ne gas laser discharges using several different types of laser discharge cells. These studies have explored the mechanisms that lead to the failure of the discharges in these cells. Specifically, the dependence of the life of the discharge on the operating parameters (discharge current, cell temperature, gas composition and total and partial gas pressures) and on the materials used to construct the cathodes and discharge cells have been studied. The gas removal rates for He and Ne have been found to be very different from each other, and to depend on the operating conditions and cell construction materials. Positive ion-molecule reaction studies have been performed in a separate drift-tube-mass-spectrometer system in which the mechanisms leading to molecular impurity gas clean-up in these He-Ne discharges have been explored.

Presents work which examines the effects of contaminants upon the performance of fibre-fibre connections. Comparative measurements were made of the transmission loss characteristics of fibre junctions constructed from clean cleaved fibres, clean polished fibres and cleaved fibres with quantifiable amounts of debris placed on the face. Cleaned cleaved fibre junctions exhibited a smaller transmission loss per micrometre when separated than did an equivalent cleaned polished junction. It was found that a cleaved fibre, though superior in transmission to a polished fibre whilst clean is prone to pick up dust and liquidous debris during the cleavage process. The principle losses caused by such debris are due to scattering, absorption, separational misalignments and Fresnel diffraction in addition to the Fresnel loss already present. Experimental values for the insertion loss due to debris are reported and the various contributions to this are assessed. Absorption losses are the most significant by an order of magnitude.

The authors present a simple demonstration of the so-called 'breath test' (i.e. the deliberate fogging of a surface with the intention of showing areas of non-uniformity) on laser-irradiated colloidal silica coatings. An explanation of the observed effects is made, based on modifications to the coating structure induced by the laser irradiation.

The authors present a mathematically exact formation of the initial field striking the aperture of an absorbing optical waveguide, via an integral equation description. An analysis of the residual scattering by the waveguide is presented. Because of the presence of the non-negligible absorption coefficient, the solution for the fraction of the confined power is obtained by performing non-trivial integrations of Bessel and Hankel functions with complex arguments. In order to carry out a numerical analysis a specific computational method is applied by using an approximated polynomial expansion whose validity is also discussed.

A comparative study is presented of the optical damage induced by green light in LiNbO₃ waveguides prepared by Ti in-diffusion, proton exchange and ion implantation. The methods used were monitoring the out-coupled light power as a function of time and as a function of in-coupled power, and writing and erasing a holographic grating inside the guide. With the first method, the optical damage stability of Ti-diffused guides is greatly inferior to the other two types, optical damage appearing at in-coupled intensities of 6 W cm^-2 in Ti-diffused guides but not until 500 W cm^-2 in exchanged and implanted guides. With the holographic method, however, a similar photorefractive behaviour is observed for diffused and implanted guides which contain similar levels of Fe²⁺ impurities, whereas much less damage is apparent in proton-exchanged guides. Main photorefractive parameters are given for the three types of guides and their physical implications briefly discussed.

The effects of attenuation and dispersion on the waveform and source characterisation of AE signals at the epicentre due to a vertical force have been estimated. These effects have distorted the AE waveform by increasing the rise time and decreasing the amplitude. The source waveform obtained by deconvolution of the AE signal in a highly dissipative and dispersive medium with the Green function calculated under the elastic assumption have led to erroneous results for source characteristics. The experimental observation of AE signals has been presented for a PMMA plate using an optical probe.

A collisional-radiative model with an extended region of applicability is developed for an argon atom plasma. Atom-atom inelastic collisions and diffusion losses of the metastable states along with the electron-atom inelastic collisions and radiative processes are considered in this model, taking into account 65 effective levels. Among the analytical expressions used for the corresponding cross sections, special attention is paid to those determining the set of cross sections for excitation by electrons from the ground state, owing to the possibility of utilising the formulae recommended in kinetic modelling studies of discharges in argon or in mixtures including argon atoms. The numerical method developed makes it possible to investigate the mechanisms by which the excited levels are populated in a non-equilibrium argon plasma characterised (even in the case of a non-Maxwellian electron distribution) by a set of parameters, such as the electron kinetic temperature T_e, the atom temperature T_a, the ion temperature T_i, the electron number density n_e, the ground state atom population n₁, the discharge tube (or the plasma column) radius R and the optical escape factors Lambda _mn and Lambda _m, which are dependent only on the quantities T_a, n₁ and R in many cases of practical interest.

For pt.I see ibid., vol.22, p.623 (1989). The extensive collisional-radiative model for an argon atom plasma is applied to a low-pressure, hollow-cathode arc discharge and to the positive column of a low-pressure glow discharge in order to clarify the mechanisms by which the excited levels in these discharges are populated, the results being compared with experimental investigations in the literature. Computations are carried out for various sets of input parameters, such as the electron kinetic temperature T_e, the atom temperature T_a, the ion temperature T_i, the electron number density n_e, the ground state atom population n₁, the plasma column radius R and the escape factors lambda _mn and lambda _m, characterising the non-equilibrium plasmas under consideration. The predictions of the authors' model. i.e the populations in the excited levels as a function of the electron number density, the effective principal quantum number and the discharge current, are compared with the experimental results and in two cases also with the theoretical results of other authors. It is shown that all calculated dependences are fairly close to the corresponding experimental curves referring to both discharges. The results presented confirm the applicability of the so-called 'analytical top model' of van der Mullen et al. (1978, 1980) and Walsh's formula for Lambda _1n interpreted according to Mills and Hieftje (1984).

When a material object is inserted into the scrape-off layer of a tokamak, it creates a wake which extends parallel to the magnetic field. Experiments are described which are the first direct test of the concept of a collection length, i.e the parallel scale length for this disturbance. The disturbed zone is a consequence of the equilibrium between parallel and cross-field particle transport. Two adjacent probes on the DITE tokamak were used. The large 'disturber' probe is a rectangular graphite plate which creates the main particle flow pattern and it can be rotated to change its projected width normal to the magnetic field. The small 'search' probe is located 0.5 m away on the same magnetic field line. It consists of a back-to-back pair of single probes which are used to measure the poloidal variations of the ion saturation current and Mach number in the wake of the disturber probe. Results are presented from experiments in which the disturber probe is rotated to change its natural collection length. The disturber probe was also biased with respect to the torus. The data obtained demonstrate the importance of considering non-ambipolar transport processes which are seldom discussed in the literature.

A theoretical explanation is presented for the recent observation of a maximum in the production of NO₂ in onset streamers in (positive) point-to-plane gaps in air at atmospheric pressure. The observed NO₂ yield was an order of magnitude higher than in the steady corona obtained by increasing the gap voltage. In the exemplary case considered, the gap voltage was 7.0 kV, the mean current 1.43 mu A, the average time between consecutive streamers 0.64 ms, the height of the streamers 9.5 mm, the loss of energy about 390 eV/molecule and the yield of NO₂ was 1.0*10¹¹ molecules per streamer. According to the proposed model, the NO synthesis occurs mainly via the oxidation of vibrationally excited N₂. The small diameter of streamers and the diffusion are crucial in the suppression of reverse reactions. The theoretical results are sensitive to uncertainties in input data, for example, in the cross sections for vibrational excitation and for electron-ion recombination. The analytical method with closed-form expressions was adequate for the calculations. An auxiliary series of experiments aimed at maximising the ozone yield; the least expenditure of energy was 42 eV per ozone molecule.

Vacuum spark discharges produce small micropinches of electrode material at high electron densities and temperatures and are noted for the emission of spectra from high ionisation stages. For the evaluation of the pinch formation, a microchannel plate detector was combined with an optical multichannel analyser to record spectra from a single discharge. The observed anomaly of the intensity ratio of the fine structure components of the Lyman- alpha transition of Fe XXVI is discussed.

Measurements of TSD together with surface potential were used to study negative corona discharge effects on the charge stability of Teflon FEP electrets. During charging, liquid formation over the sample surface (clearly visible for long-time corona irradiation) was concluded to be responsible for a decreasing in the thermal charge stability. The authors also discuss the charge stability at room temperature, while the sample is exposed to the neutral activated species from a corona discharge source. A decay in the potential was observed and this was attributed to a surface charge compensation by positive ions, and not to neutral activated molecules coming from the corona tip as suggested previously.

The hydrostatic pressure and temperature dependence of velocities of ultrasonic modes propagated in natural crystals of iron pyrites FeS₂ have been measured. It is confirmed that C₁₂ is positive; the Poisson ratio shows the usual positive sign; it becomes more positive under pressure and remains positive down to low temperatures. Vibrational anharmonicity of iron pyrites is discussed in the Gruneisen parameter approach.

Conduction electron spin resonance (CESR) spectra have been measured for pristine and boron-doped vapour-grown carbon fibres. Pristine fibres annealed at temperatures between 2300 and 3025 K exhibit broad 'two-dimensional' powder patterns when the fibres axis is perpendicular to the magnetic field. The full g-anisotropy of single crystal graphite is not established until the fibres are annealed above 2800 K. The CESR spectra of fibres heat treated above 2750 K and having diameters of less than 20 mu m show a collapse of the two-dimensional powder pattern into a relatively sharp ESR line at the isotropic value of the pattern, (g_c+g_a)/2. This narrowing results from the large-scale motion of the electrons in the basal planes during the experiment. Due to the annular morphology of the vapour-grown fibres, the RMS migration of the electrons during the ESR experiment must be of the order of the fibre diameter for motional narrowing to occur. Electron migration is limited by electron-phonon scattering at room temperature. Products of the electron mean free paths and Fermi velocities determined from the narrowing of the ESR signals increase with increasing annealing temperatures. For fibres annealed at all temperatures this product is still slightly less than that estimated for pure graphite. Fibres doped with boron at 2975 K display a reduced anisotropy due to a shift in g_c. Boron-doped fibres exhibit motional narrowing behaviour similar to pristine fibres.

The linear electro-optic effect in GaP has been investigated in the wavelength range 0.552-1.2 mu m at applied electric field frequencies of 200 Hz and 9.45 GHz. The dispersion of the clamped and unclamped electro-optic coefficients is measured and the fitted constants in the formula describing the clamped electro-optic coefficient dispersion are determined. The values of the clamped electro-optic coefficient dispersion are estimated. Based on the results of the measurements and reported data obtained from experiments on Raman scattering as well as on the measured second harmonic generation coefficient, an attempt has been made to estimate the value and dispersion of the electronic and ionic contributions to the nonlinear susceptibility.

The stress-optical constants of cracked PMMA plates in liquid environments (c_tliq) under uniaxial tension were determined by a shadow-optical method, which made it possible to evaluate the contribution of Poisson's contraction to c_tliq from the reduction of caustic diameters. In the course of the experiments, the shadow image planes were located either on the liquid tank wall (1) or in air far from the tank (2). For case (1) the contribution of Poisson's contraction to c_tliq was found to be typically 8-34% depending on the type of liquid. For case (2) the contribution could be over 40% under the assumption that the optical effect of the liquid was negligible in the shadow-optical measurements. The dependence of the liquid refractive index on c_tliq is also discussed.

Pulsed laser melting of amorphous silicon in the nanosecond time regime exhibits various complex melting and solidification mechanisms depending on both material and laser parameters. Numerical modelling of explosive crystallisation mechanisms, where a self-propagating buried molten layer is observed, is attempted. It is shown that simple heat-flow calculations, including undercooling and overheating, can produce a self-sustaining melt front in agreement with experimental data.

The light emitted from the region close to the tip of an emitting liquid metal ion source has been observed. It is shown that at high currents the light intensity rises almost as the cube of the current, but that the optical emission also depends on the source characteristics. A model based on the break-up of charged droplets is proposed to account for these results, thermal evaporation being found to be insignificant. Measurements of the effects of source temperature on light emission are also presented. From these measurements it is demonstrated that field ionisation cannot be responsible for the distortion of the total ion energy distribution seen at elevated temperatures.

It is shown that a basic physical model of a plasma column sustained by a weakly damped surface wave (SW) can be formulated by combining Maxwell's equations for a SW propagating along a radially homogeneous plasma with a continuity equation for the charged particles, assuming that the electrons are locally in equilibrium with the SW electric field. Such a model is shown to be self-contained as it determines the SW dispersion characteristics, the spatial distribution of the SW field and the axial distribution of the electron density and wave power for given operating conditions. The present model can be extended to take into account in a self-consistent way the influence of the radial distributions of the field intensity and the plasma density on each other. Such an extension has already been worked out but it involves heavy numerical work making it difficult to obtain a clear physical picture.

The DC conductivity, sigma , of 10 Bi₂O₃-(90-x)V₂O_5-xFe₂O₃ (mol.%) glasses, where x=0, 1, 3, 5, 10, 15, 20, 30 and 40, has been measured in the temperature range RT-200 degrees C. It has been found that for the ion-containing glasses log sigma versus 1/T plots obtained show a change in the slope near 110 degrees C. The activation energy, and the pre-exponential factor in the temperature range T>110 degrees C increases significantly with increasing Fe₂O₃ content. On the other hand, in the temperature range T<110 degrees C, these parameters increase weakly with increasing Fe₂O₃ content. The results are discussed in view of Mott's theory of small polaron hopping in the oxide glasses.

The drying of latex paint films in a natural environment has been followed by measurements of the AC conductance of the films. Three stages in the drying have been observed with the first two being related to evaporation of water from the bulk of the film, in accordance with mechanical measurements. The third and final stage has been characterised by a power-law decay in the conductance and has been interpreted as the elimination of a percolation network as particles coalesce, thereby cutting the conduction paths.

Deals with quantitative Auger electron spectroscopy by considering the elastic phenomena of electrons in solids: the authors introduce the current of elastically reflected Auger electrons and give some details of the backscattering R factor.

The author shows that the temperature variation of an activation energy is consistent with the demonstration of the thermodynamic compensation law.