Table of contents

Describes the use of two frequencies for measurements of the electron density N and the collision frequency nu in overdense plasma cylinders produced inside capillary tubes. The determination of N and nu is based on the measurement of the reflection coefficient in a rectangular waveguide crossed by the plasma.

The effects of target surface impurities on the laser-produced plasma were studied by means of Thomson ion spectrograms and time-of-flight measurements of the plasma ions. Hydrocarbons were believed to be the most likely surface impurity. Consequently, polyethylene was used to characterise an impure target. For comparison, pure targets (5 to 20 ppm impurities) of C, Ti, and Ta and a standard quality Ti target were used. The fastest C^z+ ions from the plasma expansion using a polyethylene target had the same peak velocity. When the hydrogen impurity was removed from a carbon target, the fastest C^Z+ ions had an energy proportional to their charge in agreement with the isothermal expansion model. Also, the presence of hydrogen affected the lateral transport of the plasma.

The emission of repetitive streamer corona was observed by a photon counting method with a high time resolution. The half-width of the main pulse of the primary wave corresponding to the emission from the 0.014 mm length part in the direction of the electrode axis is found to be only 0.8 ns. This primary wave emission has the nature of transient developing glow discharge which has the cathode glow, cathode dark space and negative glow. The faint but steady build-up of emission which continues from the cathode toward the cathode glow, suggests a considerable secondary electron emission from the cathode during the period of the primary wave propagation.

Photoconductivity in solution grown thin films of pure and iodine-doped polyvinylchloride (PVC) films has been studied as a function of the applied voltage. The photoconductivity of PVC films is increased on doping with iodine. This is interpreted on the basis of the formation of photo-induced charge transfer complexes in the film.

Faraday rotation of FeBr₂ crystals was measured at room temperature from 500 to 850 nm in pulsed magnetic fields up to 20 T generated by a 2 mm internal diameter copper solenoid. Specific magnetic rotation was 65 degrees cm^-1 T^-1 at 500 nm, decreasing smoothly to 15 degrees cm^-1 T^-1 at 850 nm. The linear absorption constant was at a minimum between 600 and 800 nm, reaching 2 cm^-1. For fields larger than 2 T the rotation/attenuation of FeBr₂ at room temperature is greater than that of FeBO₃. The use of FeBr₂ as a rotator or isolator for pulsed lasers is discussed.

The total energy spread of thermal field-emitted electron flows from an electrochemically etched LaB₆ single crystal is measured and compared with theoretically expected curves under constant emitted current densities and constant applied electric field strength. It is confirmed that the energy spread of thermal field-emitted electron flow is much wider than both cold field and thermal-emitted electron flows, as expected from the theoretical analysis. The work-function is also determined from the Richardson-Dushman equation and the tip radius is determined from the Fowler-Nordheim equation by using the determined value of the work-function.

The results and interpretations of Gupta et al. (see J. Phys. D: Appl. Phys., vol.12, p.L131 (1979)) are discussed. The expansion of crystals of NaCl by irradiation is not simply due to the formation of anion and cation vacancies.

The authors investigated the stimulated Brillouin scattering of right-handed circularly polarised (RHCP) laser radiation in a piezoelectric semiconductor when laser radiation propagates (i) in the direction of the applied magnetic field and (ii) perpendicular to the magnetic field. The nonlinearity in the low-frequency ion-acoustic wave arises due to the equation of motion of electrons and in the high-frequency scattered sideband through the equation of continuity. Typically for T₀ approximately=77K, B_s approximately=1 kG, n₀⁰ approximately=10¹⁴ cm^-3, scattering anglue theta approximately=30 degrees , laser power density corresponding to a CO₂ laser approximately=10 kW cm^-2, the growth rate for longitudinal propagation in n-InSb turns out to be approximately=10⁷ s^-1. For transverse propagation the growth rate is one order of magnitude smaller than for longitudinal propagation.

A microwave power of 0.2*10^-6 W is obtained with an ammonia maser. It is shown experimentally that the variation of the oscillation amplitude as a function of the oscillation frequency is quite different at a low level of oscillation and at a high level. A simple interpretation of the experimental results is presented.

Design and construction of a fast discharge-excited laser for operation at temperatures up to 600K is described. Output power from KrF was found to fall with elevated temperature due to build-up of absorbing species. In XeF output power was found to increase linearly with temperature. New laser transitions in the red wings of the previously observed spectra were found in KrF and XeF.

The parabolic approximation has been used most successfully for acoustic wave propagation to describe almost plane waves which are diffracted through small angles as they propagate forwards. An alternative to the plane wave description, in terms of normal modes, exists for the waves if the structure is that of a waveguide. If the waves are confined in the guide by a strong gradient in the wavespeed, the usual parabolic approximation becomes invalid and the normal mode description is a better foundation for a useful approximation. The author describes the development of a parabolic approximation for guided modes in a structure which varies in both lateral directions.

Ultrasonic absorption and velocity measurements are reported for a range of polymers filled with glass beads. The data were obtained over a frequency range 15-35 MHz and a temperature range 298-348K with a variety of bead sizes and concentrations. In all cases the ultrasonic absorption and tan delta were observed to be larger in the filled than in the corresponding unfilled polymer. Comparison of the observed excess attenuation with that predicted by simple and multiple scattering theories indicates that the major influence on the acoustic propagation is from scattering effects. An additional contribution to the attenuation observed in one of the systems is attributed to a modification of the polymer relaxation spectrum by the inclusion of the glass beads.

Caustics formed in the field diffracted by defects are explored theoretically as a possible approach to the inverse scattering problem for ultrasonic non-destructive evaluation. The case of crack-like defects is considered in detail using the geometrical theory of diffraction. The involute of the far-field caustic reproduces the projection of the crack edge in the incident beam direction, for a plane incident wavefront. This purely geometrical inversion is carried out uniquely for the asteroid and its involute, the elliptical edge. For a general edge shape, the complete inversion requires one further length measurement, which may be carried out in some cases by further experiments with caustics. Useful limitations on the possible shapes of caustics are explained on the basis of catastrophe theory.

Recent interest in the use of ultrasonics for monitoring flow rates in highly concentrated suspension of particles points to the need for a multiple scattering treatment of the propagation of ultrasound. Techniques developed for the electronic structure of liquids and disordered alloys are applied to this problem. The importance of a self-consistent solution of the equations is emphasised and the example of a medium with two components of the same density and shear modulus but different longitudinal wave velocities is discussed in detail. It appears that a measurement of the frequency dependence of the velocity offers a method of particle sizing.

The authors determine the influence on the measurement of an ultrasonic wave of the fluid layer which couples the transducer to the solid medium investigated. The response of a broadband receiving transducer is studied by an impulse method, from 1 to 20 MHz, as a function of the thickness of the coupling layer between the transducer and an aluminium plate where a step plane wave is propagating. The filtering effect of the layer is observed and perfectly interpreted by a classical theoretical model. Then it is shown that a small angle between the transducer and the surface of the aluminium plate considerably modifies the response of the receiving transducer. The experimental results are well understood by considering the wedge-shaped coupling layer as an association of elementary parallel-sided plates of different thicknesses. Finally, the influence of the static force exerted on the receiving transducer is determined for several coupling liquids.

A theoretical description is given of a noise cancelling microphone using a piezoelectric plastic film as a combined diaphragm and transducer element. The essential components of the microphone are two identical and symmetrically placed Helmholtz cavities and the plastic diaphragms. It is shown that a careful consideration of the coupling of the oscillations of each component is necessary to understand the frequency response of the microphone.

The author is concerned with the theory of a domed piezoelectric plastic film which can be used as a combined diaphragm and transducer element in an earphone. In his model the diaphragm is held in a dome shape by a pressure, P, applied to one side. It is shown that the centre of the dome is displaced from the perimeter plane by a distance which varies as P^1/3 at high pressures. At low pressures the displacement depends on the state of the diaphragm before pressure is applied. An electric field applied across the film thickness changes the dome displacement through the piezoelectric effect. For large P and small electric fields the change of displacement is linearly proportional to the field and varies as P^-1/3. As P to 0 the behaviour is more complicated.

Previous papers, in which the oxidation theory of mild wear has been applied to the wear of low-alloy, medium-carbon steels, have shown in outline how a knowledge of the heat flowing in a pin-on-disc configuration can be related to a surface model and to the parabolic oxidation of contacting asperities. A detailed heat flow analysis is developed and the derivation of the oxidational theory of mild wear is shown.

A force balance approach to particle entrainment is adopted and leads to a model which is in good agreement with experimental data and correlations. The model can be written in dimensionless form involving four groups. Because of its mechanistic basis it has several advantages over correlations of (non-dimensionalised) experimental data.

Electron swarm development in nitrogen is studied for E/N=85-1131 Td by a Boltzmann equation method using eighteen recently published electronic excitation cross-sections (those given by Cartwright et al. (1977) and Chutjian et al. (1977), summarised in Cartwright (1978)). The results show that the ionisation coefficient and the electron drift velocity are in good agreement with experiment if the electronic excitation cross-sections for the ten 'valence' states (Cartwright 1978), with threshold below 12.5 eV, are reduced by 15% and the same elastic momentum transfer, vibration, dissociation and ionisation cross-sections as in a previous work of Taniguchi et al. (1978) are used. The excitation frequencies for the respective electronic states are obtained and discussed.

The observation of different propagating modes in the positive column of an argon discharge, the pressure being the varying parameter, shows the existence of a transition between two different types of ionisation waves. For a particular condition, the observed spectrum shows a coupling between these modes and it contains several harmonics of an apparently absent fundamental mode. This result is shown to agree with Pekarek's theory.

The crystallisation behaviour of several amorphous rare-earth-rich alloys of 3d transition metals, prepared by melt spinning, was studied by means of differential scanning calorimetry, X-ray diffraction and transmission electron microscopy (TEM). The thermal stability of these alloys is found to increase in the lanthanide series going in the direction from La to Lu. An increase is also observed in the sequence Fe, Mn, Co, Ni. The type of crystallisation taking place in these alloys is largely determined by the heat of alloying. This quantity is rather small in rare-earth iron alloys. The effect of this small heat of alloying is not so much as to lead to a comparatively low number of intermetallic phases into which the amorphous alloys can crystallise but rather to give rise to nucleation difficulties.

The dependence of the resistivity of various spinel ferrites upon firing temperature has been studied. It has been determined that the resistivity of spinel ferrites has a maximum, which occurs in samples fired at 1300 degrees C. From impurity doping, the conduction in spinel ferrites can be classified into two types, deficit and excess, by the valence-controlled action. Besides this action, there are two other mechanisms which can effect the resistivity of ferrite: grain-boundary and structure distortion. The effect of various mechanisms on the properties of the ferrite have also been studied.

This new Boltzmann equation approach to secondary electron emission does away with most of the simplifying assumptions generally encountered until now in similar studies. The main processes governing the phenomenon, such as elastic and inelastic scattering, the effect of the primary beam diffusion in the excitation function and the fundamental characteristics such as variation of electron density with depth inside the sample and energy dependence of the mean free paths, are simultaneously taken into account. The validity of the present method has been tested by comparing the theoretical results for polycrystalline aluminium with those predicted by previous models and with experimental data. The results obtained satisfactorily describe the energy as well as the angular distribution and the secondary electron yield of emitted electrons.

A variety of 'Universal Yield Curves' for the secondary emission process have been proposed. A series of precise measurements of the secondary emission properties of a range of related amorphous semiconducting materials, made under UHV on freshly vacuum-cleaved surfaces, and covering a wide range of primary energies, have recently made possible an accurate assessment of the validity of the various UYCs suggested. It is found that no truly universal curve exists; the atomic number of the target materials plays an important part in determining the secondary emission properties. Agarwal's semi-empirical expression, which takes account of the atomic number and weight, is found to give good agreement for all the materials studied. Further theoretical investigation is required.

The scattering of re-emitted electrons in various thin layers is directly simulated by a Monte Carlo method. The major processes taken into account in the present work are elastic scattering by atomic nuclei and inelastic scattering by atomic electrons. The effect of the cross-sections for the scatterings on the energy and angular distributions of both transmitted and back-scattered electrons is studied. The dependence of the distributions on the atomic number of the layers and their thicknesses is also investigated.

Charge storage on aluminium oxide films has been studied using the Kelvin probe technique. The experiments extend previous studies by considering oxide films ranging in thickness from 3 nm up to 240 nm. The band structure of the oxide film deduced from these measurements is discussed and related to the problem of arc root initiation in oxide covered cathodes.

The effusion method was used to determine the vapour pressure of tetrachloroisophthalonitrile at 13 temperatures in the range 90-145 degrees C, with a minimum of two results at each temperature. Measured vapour pressures were in the range 1-135 mTorr and are expressed by the following equation for the solid phase of this compound: lg p=12.65-5652.84/T), where p is the vapour pressure in Torr and T is the absolute temperature in K. The 95% confidence limits for the respective constants of the equation are +or-0.3 and +or-122. The average difference between the raw data and the fitted values was 3.4%. Experimental conditions studied included effusion hole diameter and effusion time. The compound undergoes no phase change or decomposition below approximately 167 degrees C, at which temperature there is a solid phase transformation. All the results are for the phase stable below 167 degrees C.

The one-dimensional conservation equations describing the gas transport, coupled with the multi-temperature model rate equations describing the molecular kinetics, have been solved numerically in the case of a transverse-flow, transverse-discharge carbon dioxide laser. The influence on the gain distribution and output power of the discharge power, flow velocity, cavity position, output coupling and carbon dioxide content has been investigated both numerically and experimentally. Typical results are presented. Using empirically derived current distributions, good agreement is obtained between theory and experiment if provision is made for the effects of changes in gas chemistry in the discharge. The model has proved a useful tool for optimising performance of the laser in terms of power output, efficiency and beam quality.