Table of contents

A preliminary investigation has been carried out of the effect of London-van der Waals, viscous and electrostatic forces on the collision efficiency with Brownian coagulation of particles. The authors' conclusions suggest that for highly charged aerosols the electrostatic effect is the dominant one. On the other hand, for aerosol particles which have up to about ten unit charges, they note that van der Waals forces, electrostatic and viscous forces interact in a complex fashion which significantly alters the conventional rate of coagulation predicted by classical theory.

A method of measuring velocity gradients on the basis of a photon correlation technique has been developed. Depending on the scattering geometry, particular rows of the velocity gradient tensor Gamma can be determined for laminar flow without any knowledge of the flow symmetry. The method is also applicable to turbulent flow. With the assumption of a normal distribution of the tensor elements of one row of Gamma , the measured correlation functions for a tube flow can be fitted satisfactorily. Limits of the applicability of the method are discussed.

Laser-induced fluorescence spectroscopy is applied to beryllium evaporated from a Knudsen cell at T=1400 K. Beryllium atoms are excited at lambda -235 nm and lambda =166 nm. The pump radiation is generated by stimulated anti-Stokes Raman scattering of dye-laser radiation. The fluorescence signals are calibrated by micro-balancing with a quartz crystal. Concentrations as low as 10⁸ cm^-3 can be measured in this experiment.

The non-linear dynamics of the critical state in hard superconductors has been studied taking into account the effect of dispersion and dissipation. Stationary thermomagnetic shock waves have been found to exist in superconductors. Estimates have been obtained for the velocity and width of the shock-wave front. It has been shown that shock waves found are stable with respect to small disturbances of the wave profile. A possible version of thermomagnetic instability-i.e. magnetic flux jump-evolution is suggested.

The halogen donor depletion instability, which affects rare gas halide lasers, is discussed with reference to an LC discharge circuit. The derivation of a simple analytic relationship between discharge duration and discharge parameters is no longer possible for such a circuit. Indeed, disagreements between recent results and previously derived scaling laws have their origin in the reactive nature of the energy storage circuit.

Describes the computer simulation of acoustic wave scattering from rough surfaces. A moving average is performed on a set of uncorrelated random numbers giving a set of random correlated numbers representing the height of a rough surface at intervals along the surface. The distribution of the initial data, together with the correlation of the moving average weights, determines the statistics of the generated surfaces. Surfaces are rough in one direction only, i.e. 'furrowed', with random heights and spacings of the furrows. The scattering of acoustic waves from these surfaces is studied using Kirchhoff theory. This gives an expression for the total wave field at any surface point in terms of the local surface height and gradient. The Helmholtz scattering integral is then evaluated over the surface by summing the contributions from each surface point. This gives the amplitude or intensity of the far-field scattered wave.

A two-dimensional model for the numerical simulation of the inductively coupled radio-frequency (RF) ion source RIG-10 is presented. Due to the ambipolar characteristics of a discharge operating with hydrogen gas, the model consists of an equation for the space charge imbalance, Poisson's equation for the self-consistent presheath potential and the ion momentum transport equation. For a relatively broad range of operation and design parameters, the model allows the reproduction and prediction of the RF discharge behaviour in a systematic way and, hence, computes the 2D distribution of the ion current density within the source. By implementing relevant discharge physics, the model can provide an appropriate tool for ion source design with respect to an application in the field of neutral beam injection.

Formation of strong double layers has been observed experimentally in a magnetised plasma column maintained by a plasma source. The magnetic field is approximately axially homogeneous except in a region at the anode where the electric current flows into a converging magnetic field. For field strengths larger than about 200 G the double layer has a stationary position only in the region of non-uniform magnetic field or at the aperture separating the source and the plasma column. It is characterised by a negative differential resistance in the current-voltage characteristic of the device. The parameter space, where the double layer exists, has been studied as well as the corresponding potential profiles and fluctuation spectra. The electric current and the axial electric field are oppositely directed between the plasma source and a potential minimum which is formed in the region of inhomogeneous magnetic field. Electron reflection by the resulting potential barrier is found to explain the essential features of the current-voltage characteristics observed, but further investigations of waves and fluctuations are required to clarify the current transport.

Until recently, surface wave discharges have been studied, both experimentally and theoretically, using the wave propagation constant calculated in the assumption of a flat electron density profile. This paper deals with determination of wave characteristics (wavenumber, attenuation and fields) taking into account a radial profile of the electron density. The consequences for the experimental diagnostics are presented. In the range of low electron densities, the authors show that some results deduced from experimental measurements are spoilt if the radial electron density profile is not taken into account. They present an argon surface wave discharge (390 MHz) at low pressure (about 1 Torr) in a 76 mm diameter tube. The influence of the radial electron density profile is studied on the following experimental results: electron density, effective electron-neutral collision frequency and the mean power needed to maintain an electron in the discharge.

The high-frequency fluctuations, superimposed on arc current, ion current, light intensity and arc voltage of a single cathode spot Cu-vapour vacuum discharge are investigated. For a current above approximately 40 A, fluctuations in arc current are smaller than 1%. In arc voltage, variations on a MHz frequency scale are observed in the order of 10% of the time averaged voltage. A temporary increase of arc voltage is correlated to an increase of up to 100% in the ion current, drawn by a grounded probe 3.6 cm away from the cathode spot. A delay between the two signals is interpreted as the time of flight of a quasi-neutral high-density front (at a velocity of 1.1*10⁴ m s^-1), resulting from a pulsed production of ionised mass in the cathode spot region of the discharge. A synchronous correlation is established between the HF component of arc voltage and light intensity, originating from the cathode spot. The time-averaged light intensity, however, is proportional to arc current. Analysis shows that the fluctuations in light intensity-reaching amplitudes up to 100% of the average level-are probably due to a varying cathode spot density, rather than to a varying electron temperature. In the frequency spectra of all the signals involved, no peaks are found.

The hydrodynamic or gas-dynamic theory of plasmas including space-charge effects and several species of ions involves singular points within the plasma if the pressure and inertia of the charged particles are taken into account simultaneously. These singularities, which occur if the mean velocity of one of the species of particles reaches the sound velocity, can be removed. The required smoothing conditions lead to a well determined, non-linear eigenvalue problem for the concentrations of the various species of ions. Three methods are elaborated to calculate the eigenvalues and to obtain smooth solutions. It is shown that even under quasi-neutral conditions similar mathematical problems occur if the plasma contains several species of positive ions. The methods described can be applied to plasmas in low-pressure discharges, in hollow cathodes and near electrodes and to fusion plasmas.

The work shows the crucial need for a careful choice of boundary conditions, particularly near the corona electrode, and revises the boundary conditions usually employed in classical methods. Commonly adopted assumptions which result in the corona radius and corona voltage drop being taken as constant have been shown to be inappropriate, since they lead to ambiguity in the mobility values. Values of positive-ion mobility based upon all three measured parameters have been calculated, using various models for the corona region, and these show a decrease in mobility with increasing humidity. This mobility is consistent with the field equations, and no dependence was found upon field strength or ion aging. The models yield an estimate of the dimensions and voltage drop for the corona ionisation region and the value of the field at the corona electrode.

Single-crystal silicon has been amorphised by bombardment with Si⁺, Ge⁺ or Sn⁺ ions. After conventional annealing (900 degrees C, 30 min) some disorder remained. The amount was measured by ion channelling and transmission electron microscopy and compared with the quantity of excess interstitials predicted by the Monte Carlo computer program TRIM. Both the predicted and measured integral disorder fell as the atomic weight of the projectile increased. Because lattice strain also leads to a reduction in ion channelling this had to be taken into account when estimating residual damage from back-scattering analysis. The results indicate that there are six scattering centres per interstitial.

The generation of electrons and holes in the polycrystalline super-ionic conductor Ag₂HgI₄ has been demonstrated through the measurement of electrical conductivity with an ion-blocking electrode, magnetic susceptibility and thermoelectric power. The contribution of the electrons and holes to the electrical conductivity is very small compared to the ionic contribution. While the electrons take part in the transport phenomena at all temperatures, the holes do so only at elevated temperatures. Although the compound changes phase at 50 degrees C, the electrical conductivity does not show a sharp change at this temperature. The similar mechanism of conduction in both the phases is due to the same number of tetrahedral sites available to the cations in the two phases.

The figure of merit for energy conversion by a thermocouple involves a combination of the properties of the two branches. Although it often has a value that is close to the average of the branch figures of merit, this is not always the case. In particular, there is no positive thermo-electric material that matches the negative thermo-elements, made from bismuth-antimony, that are useful at low temperatures. It is shown that there is some merit in using a material with a very low Seebeck coefficient, but with a very high ratio of electrical conductivity to thermal conductivity as the positive branch. Consideration is given to the use of the new high-temperature superconductors for this purpose. Experiments on the latter in a magnetic field at liquid-nitrogen temperature are reported.

Thermally stimulated luminescence (TSL) and conductivity (TSC) in low-density polyethylene samples were measured simultaneously, while the sample was being heated to room temperature after X-ray irradiation at -190 degrees C. The TSC curves of samples from which absorbed air had been removed showed five peaks, labelled C₁-C₅ in order of increasing temperature, while the TSL glow curves showed only three peaks (L₁-L₃). The effects on these peaks of (i) immersing the samples in fuming nitric acid, (ii) annealing them in vacuum, and (iii) exposing them to an oxygen/ozone mixture, were investigated. It is concluded that the C₁ and L₁ peaks are associated with the same traps, these traps being formed by the polymer chain themselves in the chain-fold regions of the sample, and being broken up by the onset of side-chain motion. The same is true for the C₂ and L₂ peaks, except that a different set of traps and 'crankshaft' motion are involved. There is no obvious correlation between the L₃ peak and any of the TSC peaks; the L₃ traps are also formed in the chain-fold regions, and are broken up by the glass transition. The C₃, C₄ and C₅ traps are probably structural defects of various kinds in the crystalline regions of the samples, from which electrons escape by thermal excitation. When the samples contain absorbed air, a hitherto unreported peak is observed in the TSC curve, corresponding to the TSL 'gas' peak.

Transparent conducting films of both undoped and antimony-doped tin dioxide have been prepared by a novel technique. In this method, the substrate with a band of SnCl₂ paste painted near its lower edge is heated in a vertical configuration. The SnCl₂ undergoes oxidation to yield SnCl₄ (vapour) as one of the reaction products, which rises upward and is hydrolysed on the heated substrate surface to produce the oxide. The films are smooth and strongly adherent over a region approximately=5 cm in length. The best films obtained have a sheet resistance in the region of 100 Omega Square Operator ^-1 and 70% average optical transmission.

Hydrogenated amorphous silicon (a-Si:H) offers some technical advantages over crystalline silicon for optical position sensing devices since it can be grown on a large variety of substrates at approximately 250 degrees C, over large areas and at lower cost than crystalline Si. Its transparency to red light makes it suitable for applications which require superimposed detectors. The authors have examined a variety of layered a-Si:H structures using gold or Nichrome resistive contact layers and found that a multiple sequence of n-type/undoped/n-type thin films offers the best positional linearity and the minimum change by the Staebler-Wronski effect.

An explanation of the charge-density distribution on conductor surfaces and a comment on the work of Luo Enze (1986) is presented. Emphasis is placed on the fact that charge-density distribution functions, which only depend on the local geometry, may not exist.

The motion through air of a disc acted upon by an external constant force and by the air drag force is investigated. Experimental data are well fitted by assuming that, at high Reynolds numbers (Re> approximately 10³), the disc encounters, besides the usual added mass term and the quadratic damping term -Bv², an additional drag proportional to the product of acceleration and square velocity.

An experimental investigation is described in which it is shown that the piled-up region of a hardness indentation in metals also supports the indenter load and that the normal pressure is distributed uniformly over the projected area of the indentation. It is cautioned that if the piled-up region is not taken into account, as has been done by some investigators working with ultra-low hardness testing machines, then the estimated hardness values may have significantly high errors.

A new type of waveguide composed of a cylindrical quantum well is suggested. The effects of the response of the electron gas to a longitudinal electric field have been taken into consideration. Calculations of the transverse magnetic modes TM₀ show that the propagation constants are different from those of dielectric waveguides, and the phase and group velocities are higher than those of the dielectric waveguide.

Detailed AC and DC electrical resistivity measurements in Ba₂HoCu₃O_{7- delta} samples are reported. Although all samples have a very similar transition temperature to the superconductor state (about 92 K), their normal resistivity shows in some cases important differences. In the whole frequency range examined (up to 2 kHz), their AC resistivity is frequency independent and qualitatively similar to the DC resistivity. The behaviour of resistivity around T_c seems not be affected by the presence of the magnetic holmium ions. The excess conductivity above T_c strongly depends on the properties of the samples in the normal state.