Table of contents

A new nonlinear-boundary-condition approach for deriving the dispersion relation on the surface of GaAs grating structures is presented. We have used the linear set of wave equations and introduced the grating nonlinearities through the boundary conditions only. We have observed that the presence of a grating splits the surface-plasmon modes into two parts and generates a band gap. This approach is simple and, in the limiting case, reduces to well known theoretical calculations. The dispersion relation is solved for thin-film and thick-film gratings, in the limit in which the dielectric constant is less than zero. The results thus obtained may be used for millimetre waves and are also applicable to the optical region.

We derive the equations of motion from which nonlinear field- and frequency-dependent magnetic susceptibilities defined by M = + + may be calculated for a Heisenberg ferromagnet. Explicit expressions are given, in the circular polarization frame, for the second- and third-order effects that arise for a single-frequency input wave, namely second-harmonic generation, optical rectification, third-harmonic generation and intensity-dependent susceptibility. Illustrative graphs are presented for a few of the coefficients and the paper concludes with a brief discussion of the implications for multistability of a magnetic Fabry-Pérot resonator.

According to Dicke's theory of superradiance, the relation between the delay time and the tipping angle for superradiant pulses can be expressed by , where is the radiation damping time constant. This relation is verified by nuclear magnetic resonance (NMR) experiments on a normal water sample. The radiation damping time and the delay time are independently measured, and the tipping angle is evaluated. The tipping angle can be as big as several degrees if the spin inversion is not perfect. It can also be as small as rad, when the residual coherence in the transverse plane is destroyed by a strong field gradient pulse.

Through concurrent employment of the first and second laws of thermodynamics, a set of optimum power expressions for the open irreversible Brayton and open Joule-Brayton heat-engine cycles has been obtained. These expressions are applicable to configurations with a finite thermal reservoir in which the values of the source outlet temperatures are forced to interact with the overall cycle during the power optimization of the cycle's working substance temperatures. Use of the concurrent law procedure simultaneously allows both the minimization of internal cycle entropy generation and the maximization of specific cycle work in order to provide the internal cycle operating temperatures necessary for power optimization. The study is carried out for open versions of these cycles in which the energy input is provided from an external source through a heat exchanger, the conductance of which is optimally allocated with respect to the cycle flow rate. The work includes a novel comparative study of the optimized power output of these cycles both with non-interactive and with interactive sources. The results of this study conclusively indicate the importance of considering variable (interactive) sink outlet temperatures in obtaining the power optimum for these cycles.

A plume consisting of vapour and ionized particles of the workpiece is usually formed during various types of laser materials processing. The characteristics of this plume depend on a large number of parameters such as the laser power, spot size, scanning speed, material properties and shielding gas. The height, radius, temperature and absorption coefficient of the plasma are calculated for various values of process parameters. The surface temperature of the melt pool and the vaporization rate are also calculated on the basis of the Stefan condition at the liquid-vapour interface. The absorption coefficient of the plasma given by the Kramers-Unsöld relation and the ionization fraction given by the Saha-Eggert equation are used to model the laser beam propagation through the plasma. A detailed analysis of the plasma stability indicates that absorption of the laser beam by the plasma affects the melt-pool surface temperature nonlinearly.

An optoelectronic multichannel detection system for the study of the visible light emission of a vacuum circuit breaker arc is described. The system consists of two multiple slit collimator assemblies coupled directly to the arc discharge chamber and an electronic detection part. The light emitted by the arc is collected by the two collimator assemblies and is transmitted through optical fibres to the electronic detection part. By using a new, simple computational method two-dimensional plots of the vacuum arc light emission at different times are obtained.

Self-closing corona-stabilized switches have been developed to operate at pulse repetition frequencies (PRFs) which extend well into the kilohertz regime. The performance of these devices illustrates that corona-stabilized plasma closing switches are an effective alternative to conventional uniform or quasi-uniform field switches which utilize gas-flow techniques, high gas pressures or both to achieve high-PRF operation.

Corona stabilization takes place in electronegative gases such as air and and requires the presence of a highly divergent electrical field. Under dc, or slowly rising charging voltage conditions, a space charge develops around the highly stressed electrode which prevents premature breakdown and allows full voltage recovery to take place. This means that PRFs extending into the kilohertz regime can readily be achieved. Self-firing corona-stabilized switches have been demonstrated to provide PRFs of up to 5 kHz and triggered corona switches have been shown to be capable of operation at 10 kHz. The lifetime of corona-stabilized switches has been demonstrated to approach about shots.

The paper describes the design of a corona-stabilized switch, including detailed electrostatic field analysis of the electrode geometry. This analysis was undertaken to establish the effect of the electrode geometry on the field distribution within the switch. The switch design was flexible and allowed a study of the effect of certain parameters on the switching performance. These parameters include the gap spacing, the gas pressure and the profile of the electrodes. The switch was operated under single-shot conditions to establish the breakdown voltage against pressure (V-p) characteristics and in a continuous repetitive mode to assess the PRF operation. The results show that the best repetitive performance is produced using electrodes which generate a high field over a region sufficient to generate a stabilizing corona.

The ion energy distribution in oxygen plasma in a planar inductive discharge was measured in the pressure range 3-20 mTorr using a quadrupole mass spectrometer. The ion energy distribution is broad and bimodal at low pressures, indicating that there is capacitive coupling between the rf coil and the discharge. At higher pressures multiple peaks generally related to charge-exchange processes in the sheath are apparent. The mean ion energy increases with decreasing gas pressure, from about 10 eV at 20 mTorr to roughly 25 eV at 3 mTorr. The mean ion energy follows closely the d.c. plasma potential measured by a Langmuir probe. The mean ion energy of ions is generally 10-40% higher than the mean ion energy of ions.

The spatial profiles of the plasma density and plasma potential are important for plasma applications like semiconductor etching because they determine the particle fluxes to the discharge boundaries and the energy distribution of the particles reaching the walls. Langmuir probe measurements have been performed to get information on the electron distribution function (EDF), the plasma density and potential at different positions in the discharge. At low-pressure operation a non-local behaviour of the electrons has been observed; nevertheless, significant differences between the measured spatial profiles and the predictions of non-local theory have been found. Surprisingly, at higher pressures, when strong deviations of the EDF from non-locality occur, the spatial profiles conform to a non-local scenario. These experimental observations could be reproduced by a self-consistent kinetic plasma model.

The principles of scanning tunnelling microscopy (STM) are extended to the study of field electron emission from metal, semiconducting and semi-insulating materials. A specially designed, high-vacuum STM device called a scanning tunnelling field emission microscope (STFEM) is constructed, and new measuring procedures are developed to examine complex physical properties of emission centres. Providing high bias voltages and fast mapping of large squares, the STFEM allows one to obtain reliable statistical data on surface properties, namely topography, emission intensity, surface potential distribution and local electroconductivity. Results from a study of low-field electron emission from CVD diamond films are described to illustrate the functional capabilities of the new STM device. It was found that the diamond films studied are composed of nanograined phases distinguished by their physical properties. It has also been noted that the low-field electron emission from the studied samples is associated with the interfaces of these phases.

Radio-frequency conductivity measurements on ethylene glycol-water mixtures at 298, 308 and 323 K under a 1 MHz electrical field have been used to evaluate the semiconduction activation energy . Two mathematical expressions have been formulated to represent the temperature and concentration variations of the rf conductivity and to calculate the rf conductivity for a dipolar mixture like ethylene glycol-water in terms of the conductivities of its components. These relations can satisfactorily explain the observed experimental results. The dielectric relaxation time and the corresponding energy parameters (, and ) for the process of dipolar orientation have also been computed for the mixtures using rf conductivity data. Analysis of the evaluated values shows that the semiconduction activation energy and dielectric relaxation time for such liquid mixtures obey a simple mixing rule.

This paper is devoted to the development of a theoretical base for the retrieval of optical constants of large strongly absorbing particles dispersed in a non-absorbing host medium. Both single and multiple scattering regimes are studied. Particles can have a spherical shape or be non-spherical, but convex and randomly oriented. Analytical formulae relating Fresnel reflectivities for surfaces of particles to the intensity of the diffused reflected light are proposed. They are based on the single and quasi-single approximations of the radiative transfer theory. The geometrical optics approximation is used to find local optical characteristics (for example the phase function and the extinction coefficient) of turbid media under consideration.

This work is dedicated to the calculation of the electromagnetic field created by an arbitrary current source in the proximity of a cylinder consisting of a certain number of concentric layers made of materials with various electrical conductivities and magnetic permeabilities, using the method of the transfer matrix. The general results obtained are then particularized for the case of an eddy current transducer with a rotating magnetic field, having its reception part of a special construction. The inversion method used to determine the thickness of the protective coatings on a conductive cylinder is presented.

The effect of Li preadsorption on the kinetics of CO and oxygen adsorption on a polycrystalline Pt surface has been investigated using the reverse flash method. Li preadsorption hardly affects the initial sticking coefficients for CO and oxygen, while their saturation coverages, attained at room temperature, are substantially decreased. The results are compared with data reported for other alkalis, and the physical factors that determine the promoting ability of electropositive adsorbates in gas adsorption kinetics and catalytic reactions are discussed.