Table of contents

Temporal coherence difference has been shown to be very useful in electro-optical remote sensing when it is required to detect coherent emissions automatically in the presence of intense but largely incoherent light. Most coherence dependent sensors to date have relied on the sought-after signal emitting light which is significantly more temporally coherent than that from the interfering background. Such sensors in effect employ high-pass coherence processing. This paper describes a novel technique for rejecting unwanted signals which are either less or more temporally coherent than the required signal. The theory is given and an experiment described which shows that even in the presence of a turbulent transmission path, signals within the coherence length range 0.13-0.17 mm are readily detectable; emissions outside the range can easily be rejected. It is because the technique can be used to detect a range or band of coherence values that it is named bandpass coherence processing. Furthermore, the coherence band quoted does not represent a limit on the technique. Indeed the principle can be applied to almost any range and to any waveband where suitable components can be identified.

Following the author's modified model used in a previous paper for the calculation of the breakdown parameters of molecular gases, they examine the influence of stepwise ionisation processes, namely photoionisation and electron-impact ionisation, of electronic excited states on the electron energy distribution function and its parameters. Particular attention is devoted to molecular nitrogen at pressures of 7.6*10² and 3.286*10⁴ Torr irradiated with 10 ns pulse of Nd:YAG laser ( lambda =1.06 mu m) at peak intensities 3*10¹¹ W cm^-2 and 4*10¹⁰ W cm^-2. On the basis of his results he proves that the influence of stepwise collisional ionisation on the ionisation growth rate is weak at low pressure whereas there is considerable evidence that this process dominates at high pressure where vibrational losses are pronounced.

Possible rapid cooling of a higher temperature plasma is investigated in a gas-contact cooling method in which the effect of finite time for mixing the plasma and the contact gas is considered. The calculation has shown that cooling under these conditions is much more rapid than that achieved by radiative loss cooling and that the gain per unit length of the He II 164 nm line is approximately 2 cm^-1 for the laser oscillation under optimum conditions. The possibility of realising a laser at a shorter wavelength in a plasma with higher charge number Z is also discussed.

The response of CdS:Cu bistable optically controlled semiconductor switches to 10 ns laser pulses had been measured with green (532 nm), red (650 nm) and IR (1064 nm) excitation and pulse mixtures. In addition to light-stimulated conductivity the authors show a new mechanism: the stimulated switch-off with an infrared light pulse. With the present low-doped samples it was possible to realise turn-on times as fast as 15 ns (10-90%) and turn-off times shorter than 250 ns; the minimum switched-on time was 150 ns for 60 ns on-off pulse separation. Furthermore it was possible to demonstrate repetitive switching with a minimum switching period of 1 mu s.

An experimental study of the de-ionisation of a pulsed discharge has been performed in a noble gas plasma with low ionisation. The theoretical description includes a proper account of the ionic-molecular reactions within the framework of the collisional relative model. Calculations and experiment were compared and showed rather good agreement.

Shows the possibility of generating magneto-active plasmas in a waveguide by the use of electromagnetic waves launched transversely to the static magnetic field for ordinary and extraordinary polarisation. With extraordinary polarisation, plasmas of densities of about 10¹⁷ m^-3 can be obtained at low power levels. The working gas was argon and the static magnetic field about 0.09 T. Plasma density is limited by the frequency of the incident waves (2.45 GHz in the authors' case) and not by the mechanism of wave transformation.

The ionic and electronic reactions occurring in the cathode dark space (CDS) and negative glow (NG) of a hydrogen glow discharge with operating parameters typical of those used in surface treatments are modelled to calculate the angular and energy distributions of the ions incident on the cathode. The self-consistent model comprises three interactive sub-models, two of which are based on Monte Carlo simulations of electrons and positive ions, the third on conservation and mobility equations and is used for the determination of the ions' diffusion current from the NG to the CDS. Where available, reaction cross sections are taken from the literature with estimates being made for those such as H₃⁺ to H⁺ which are not documented. An experimental determination of the energy of H⁺, H₂⁺ and H₃⁺ ions arriving at the cathode of the discharge is described and it is shown that the hydrogen ion currents predicted by the model are in accordance with those measured experimentally.

Cobalt silicide formation by ion beam mixing has been studied, under bombardment with inert (neon) ions. Mixing is found to be impeded by the presence of an interfacial oxide layer. Stoichiometric silicide formation is dependent upon the use of optimum ion dose and energy. In contrast to argon and xenon bombardment high doses of neon lead to exfoliation of the surface due to migration and localisation of the gas in the sub-surface layer. The mixing behaviour is discussed in terms of both radiation enhanced diffusion and thermal spike models.

Thermal expansion of ZnCr₂Se₄ was investigated from room temperature to 301 degrees C. Zinc chromium selenide, an antiferromagnetic spinel with a helical spin structure and of space group Fd 3m (O_h⁷), showed unusual behaviour above 340 degrees C on DTA. Thermal expansion of this compound in the temperature range 26-301 degrees C is reported in this paper. The average value of its thermal expansion coefficient in the specified temperature range is found to be 11.69*10^-6 degrees C^-1.

The linear superconducting OGMS system is described in terms of an analytical theory, computer simulation and particle stream analysis. The magnetic fields, force densities and equations of particle motion are established to yield the particle trajectories and deflections. These are obtained analytically and numerically, and their examination provides the optimum configuration of the system and the criterion for separation independent of particle size. The passage of the particle stream through the separator is also investigated. The effect of interparticle collisions is assessed and an estimate of the collisionless feed processing rate is obtained. Finally, air disturbances are considered and related to the minimum size of particles that the system can treat successfully.

The frequency dependence of the complex susceptibility of five colloidal suspensions of magnetite in Isopar M, with different particle concentrations, has been measured. From the results obtained it is deduced that particle to particle interaction has a negligible effect on the susceptibility over the frequency range 100 Hz to 10 MHz.

The effects of inhomogeneities of the drive field H_z and of the in-plane anisotropy on the generation of vertical Bloch line pairs in the stripe domain wall were investigated numerically. It was found that the inhomogeneity of H_z does not cause the generation of vertical. Bloch line pairs in the linear range of wall motion (i.e. H_z<H_zcrit). In the case of the inhomogeneity of the in-plane anisotropy, however, the generation of vertical Bloch lines appears also for H_z<H_zcrit, provided that such an inhomogeneity is sufficiently large.

Tin-doped indium oxide (ITO) films have been deposited by RF sputtering an oxide target onto substrates held at different distances from the target surface. The deposition rate is constant up to a target-to-substrate distance of 3.5 cm and decreases with further increase in substrate distance, this distance being associated with the thermalisation (virtual source) distance of the sputtered neutrals. The properties of the films are observed to be dependent on the target-to-substrate distance. Films deposited onto a substrate held 2 cm from the target have preferred orientation (440), lattice constant a=10.21+or-0.02 AA and band gap 3.5 eV; films deposited onto a substrate held 8 cm from the target have preferred orientation (222), lattice constant a=10.29+or-0.02 AA and band gap 3.7 eV. The observed differences in the properties are attributed to the different energies of the sputtered neutrals reaching the substrates.

The kinetic parameters of the first, third and fifth peaks of CaF₂:Tm (TLD-300) dosemeters were determined experimentally using methods based on initial rise, peak shape, different heating rates, isothermal decay and numerical fitting method and a comparison was made between the results obtained. The average activation energy values determined by the methods used here were 0.72, 1.16 and 1.77 eV, with mean values for the frequency factor of 2.58*10⁸, 2.89*10¹³ and 6.16*10¹⁶ s^-1 for the first, third and fifth peaks respectively. The values of activation energy obtained for each peak using the different methods covered a spread of about 4%. However, the spread for each of the frequency factors was very high.

Mossbauer absorber thickness has been optimised by considering Nowik's model of bounded diffusion. It is observed that optimum thickness T₀ is dependent on alpha , which is the ratio of harmonic force constant to damping force constant and the variation between T₀ and alpha has been obtained for various values of absorption cross section.

A new set of expressions has been derived for the determination of the activation energy E of a thermoluminescence (TL) peak. Unlike those previously existing in the literature, the present expressions take into account the variation of the symmetry factor mu _g(x) with U_m(=E/kT_m) where x is the fractional intensity, k the Boltzmann constant and T_m the peak temperature and prior knowledge of the order of kinetics b is not required. The applicability of the present expressions have been tested using both numerically generated and experimental thermoluminescence peaks.

An exact expression for solair temperature, a parameter widely used in solar-energy system modelling, has been presented which appropriately accounts for ground temperature variation (with depth). Numerical results illustrate the dependence of solair temperature on the ground temperature gradient. In addition it is erroneous to assume the thermal emittance of the ground to be unity, i.e. that of a black body, in the computation of solair temperature. The simplicity of the exact expression poses no problems for its practical acceptance.