Table of contents

The first application of tunable, coherent VUV radiation to the measurement of absorption lineshapes in a dense (n_e=10²⁴-10²⁵ m^-3) plasma is reported. The coherent VUV radiation was produced by frequency tripling the output of a powerful dye laser at the Rutherford Appleton Laboratory UV Radiation Facility. The VUV radiation (tunable between 1203 and 1236 AA) was used to obtain absorption profiles of hydrogen Lyman- alpha in the cool, (T_e=2-2.5 eV), near non-Debye Z-pinch plasma. The absorption profiles indicated a possible red wing asymmetry observed previously in emission on the same device.

The author presents a useful but simple graphical method that defines the conditions for single-mode operation in channel diffused waveguides at different operating optical wavelengths.

The phenomenon of cavitation occurs when a tension, applied to a liquid, exceeds a certain critical value known as the breaking tension or cavitation threshold. The present review article considers various ways in which liquids have been subjected to tension and how the accompanying cavitation has been studied. The importance of these matters in medicine and botany is also discussed.

A nonlinear formulation of the displacement cascade is described using the Boltzmann equation with a stochastic collision kernel. For the special case of the very hard sphere model the resulting exact solution is used to assess the limitations of the linear theory for calculating sputtered atom spectra and yields. Significant differences are found in the two cases, amounting to factors of 2 in some energy regions. The extension of the method to more realistic kernels is outlined.

Considers different solutions for the scalar potential, vector potential and components of the magnetic field due to a toroidal conductor of rectangular cross-section. The series expansions obtained independently by Garrett (1967) and Caldwell (1979) agree and further agreement is obtained by considering a new and more natural approach which involves the use of a Green function expansion in cylindrical polar coordinates. This Green function approach is extended to obtain computer results for a particular coil geometry with iron present-a problem which cannot be solved by Garrett's approach. The results are shown to agree with those of Garrett for the iron-free case.

The first-order corrected eikonal approximation (FCEA) has been used as an approximation to the Mie theory for particle sizing by the forward lobe scattering intensity-ratio technique. Particle diameters in the range 0.2-5.0 mu m are considered. Intensity ratio curves are presented as a function of particle size parameter at angle pairs 5 degrees /0 degrees and 10 degrees /5 degrees . A comparison with similar curves based on the exact scattering theory has been made and the percentage error in size parameters obtained using FCEA has been tabulated for relative refractive indices m=1.05 and 1.15.

Two numerical solutions of the Boltzmann equation are developed and applied to the calculations of swarm time-of-flight parameters in molecular model gases. It is shown that the two methods not only give near similar results but are in good agreement with calculations carried out by some other authors for the same situations. Errors already pointed out in some other works are confirmed and the need of using high-order methods of solution of the Boltzmann equation is stressed.

The importance of the attachment cross-section for F^- formation for the effective ionisation coefficients in SF₆ is discussed. The values of the dissociative attachment cross-section q_a for F^- formation for electron energies from 2 to 15 eV are very small compared with the values of q_a for SF₆^- formation at low electron energies. However, the effect of the values of q_a for F^- formation on the effective ionization coefficients is very great. Calculations were carried out by a two-term Boltzmann equation analysis. It is pointed out that the measured grand total electron-SF₆ cross-sections were inconsistent with the sum of elastic and inelastic cross-sections used by many workers.

The vacuum ultraviolet emission from a low pressure hydrogen plasma has been used to study the production mechanism for vibrationally excited ground state molecules thought to be important in negative hydrogen ion production. The present results are in agreement with theoretical calculations by Hiskes (1980). The rate of production of molecules with v>or=5 has been estimated and is sufficient to allow H^- production in such plasmas to be via these molecules.

The problem of the propagation of a slow high-frequency ionising electromagnetic wave along a thin plasma cylinder surrounded by a dielectric tube is considered, assuming that the characteristic diffusion and thermal conductivity lengths are small in comparison with the discharge length. The wave phase characteristics as well as the radial and axial profiles of the wave field and of the plasma electron temperature and density are determined. The obtained results suggest an approach for explanation of the experimental data.

A new probe has been designed to measure the electron and ion energy distributions in a tokamak edge plasma; it may be operated in both the retarding field analyser and E*B modes. In this type of probe aperture design is important and must include detailed consideration of Debye length, selective transmission, heat flux and space charge effects. Results obtained in the retarding analyser ion mode show that the ion parallel velocity distribution function as measured by the probe is a displaced Maxwellian in accordance with theoretical prediction. The electron parallel velocity distribution on the other hand is seen to be cut off by the wall sheath. Ion temperatures obtained under similar conditions in the E*B mode are in agreement with values obtained by fitting theoretical distributions to retarding field ion mode characteristics. The results which are presented were obtained in the edge region of discharges with line average densities of 10¹⁹ m^-3. In this particular case approximately equal ion and electron temperatures of order 15 eV were measured. There is experimental evidence that at the current levels used space charge effects are negligible.

The glow phases of sparks in SF₆ have been studied at pressures around 1 Torr, and for discharge gaps of between a few cm and 1 m. Applied voltage pulses were maintained low enough so that the sparks did not proceed to their final arc phase. Measurements were made of discharge current, voltage and axial potential distribution as functions of time, and information on neutral particle density as a function of time was obtained using a polarization interferometer. The SF₆ spark was found to develop through two glow phases. The first is a low current (tens of mA) phase, in which little molecular dissociation occurs; it is dominated by electron attachment. Gas heating during this phase leads to dissociation in times of the order of 100 mu s, and a transition occurs to a higher current (around 1 A) glow phase. In this dissociated phase, electron attachment is not so important. The interferometer study showed the increase in discharge refractivity due to dissociation, which accompanies the glow-glow transition. Further evidence for the role of dissociation in the transition was obtained from comparison of the SF₆ spark with sparks in hydrogen, argon and SF₆/argon mixtures.

The authors have calculated numerically the distribution of polarisation energies for trapped charges in an amorphous matrix, either a polymer or an organic glass. The distribution is of order 1.5 eV wide and is skewed, having a significant tail at large energies. Despite the simplicity of the model, the authors argue that this is a reasonable estimate of the situation for the static fluctuations considered, and it may account for the wide distribution of trapping energies inferred from various experimental techniques.

Thermally stimulated current (TSC) studies on vapour-phase-grown (ZnSe)_x(CdTe)_1-x single crystals have been carried out for all the crystals with 0<or=x<or=1. The various traps have been identified and the trap parameters have been evaluated. The traps found individually in CdTe or ZnSe are not, in general, found in the mixed crystals. The activation energies of the traps are found to vary at random from 0.14 eV to 0.45 eV and no regular variation with x is observed. The TSC in these materials are independent of illumination conditions and hence they may be reliably used to achieve higher efficiencies in any photosensitive device.

Thomson scattering of laser light, which has been adopted for diagnosis of fusion plasmas, etc., is applied here to high pressure arc discharges as found in power circuit breakers, for measuring the electron density in the columns. The scattered light spectrum from the electrons in the high temperature air at 1 atm is theoretically calculated in the electron density range from 5.0*10¹⁹ to 1.8*10²³ m^-3. The notable aspect of light scattering under such high pressure conditions is that the scattered light spectrum has sharp satellite peaks on both sides of the incident laser wavelength, which are brought about by the plasma oscillation between electrons and ions. Actual measurements of the Thomson scattered light have been carried out on DC arcs of 10 to 40 A which are stabilised by axial air flow. The satellite peak is distinctly detected, as expected from calculations, at the wavelength 5.7 nm from the laser wavelength for 40 A arc current. The electron density around the arc axis is determined as 9.2*10²² m^-3 from its profile analysis. This value of electron density shows a good agreement with that estimated from the spectroscopic diagnosis.

A photoluminescence band with unusual spectral properties has been observed in MBE grown CdTe. It is particularly prominent in single crystal layers containing large concentrations of extended defects generated either by metallurgical imperfections or contamination at the substrate-layer interface. It is suggested that this luminescence results from electron-hole recombination at these extended defects. The weak LO phonon coupling may result from the delocalisation of the electronic binding potential between several nearly equivalent sites, while the spectral breadth of the no-phonon component may result from the influence of small environmental differences between these sites. Similar bands have been observed in ZnSe.

A method is suggested to obtain the order of kinetics from data on isothermal decay of thermoluminescence. The method starts from the May and Partridge equation (1964); and is easier to apply than the original method of May and Partridge. Application to peak XII in LiF-TLD 100 and a second-order peak in KCl:Ca yielded consistent results.

The emission of ions and other charged particles from sharpened solid needles wetted with a film of liquid metal is initiated by an electrically driven redistribution of the film at the needle's apex to form a Taylor Cone. The authors examine the effect of viscous fluid flow on the properties of the emitter, in particular the transient properties during redistribution and at onset. A purely inertial treatment is found to be inadequate and viscous effects are shown to dominate. Electric stress and surface tension pressure are not equal during emission and the liquid cone is in a state of tension with steady-state conditions maintained by the process of ion emission.

A theoretical model is developed for calculating the current-voltage curves of capillary-type liquid metal ion sources. The solution derived is analytic, given in terms of known parameters. The agreement with experiment is found to be encouraging.

Deals with densification of the thermally grown SiO₂ films resulting from intrinsic oxidation stressing. A quantitative model giving the stress levels within the oxide during and after growth evolves from a viscoelastic relaxation scheme. Amount of stress and related quantities such as wafer curvature, refractive index or density are calculated and compared with different kinds of experimental data. As an example, it is demonstrated that low temperature oxidation processes are basically responsible for high density oxides because of Si-SiO₂ intrinsic strain.