Table of contents

The authors describe a primary electron avalanche which is accelerated by the induced electric field and develops in an electrodeless torus. Waveforms obtained by an optical method have two peaks, one corresponding to the first passage of the avalanche round the torus and the other to the stage in which the ionisation proceeds rapidly, resulting in the formation of a nearly uniform plasma.

The percentage ionisation in the arc of a duoplasmatron in helium is obtained from the intensity ratios of the ionic (4686 AA) and atomic (5047 AA) lines. When the gas pressure in the gap between the anode and the intermediate electrode is 0.3 to 0.2 Torr, the percentage ionisation is found to be 90-95% in the arc centre, but it is about 15% when the pressure is greater than 0.5 Torr. In the emerging plasma, the percentage ionisation is less than 10%.

A simple optical and X-ray diffraction study on a thallium bromoiodide mixed crystal, showing periodic optical anisotropy, is reported. From the observed laser diffraction pattern, the nature and orientation of the strain pattern resemble that of a coarse grating. The orientation of the anisotropic domains corresponds to the crystallographic axis (110) on the basis of X-ray diffraction measurements which also indicate a doubling of spacing. Although the exact origin of this periodic anisotropy is unknown, a possible cause is conjectured.

For pt.I see ibid., vol.12, p.1633 (1979). The iteration technique is used to find the relation between the linear functional and Pade approximants. Three problems in radiative transfer and reactor physics involving spherical geometry are solved as examples of application: luminosity or the total energy emitted by a sphere; neutron capture probability; the albedo problem. Numerical calculations are done and compared with the exact values found in the literature, for luminosity and the albedo. For the capture probability no exact values are available for c not=0. Therefore the author calculates the exact values by the Sobolev method (1972). The Pade approximants give very good results in comparison with the exact values.

For pt.I see Proc. R. Soc., A358, p.105 (1977). The authors are concerned with the development of radiation damage cascades initiated by fast incident particles. They derive a general probability balance equation for the number of vacancies and interstitials in given regions of space, subject to an initial atom of specified velocity. The scattering model is quite general but they do employ the radiation damage model due to Kinchin and Pease (1955). Introduction of a generating function enables the various moments of the vacancy and interstitial distributions to be obtained. Explicit equations for the vacancy and interstitial densities and their two particle correlation functions are derived. They introduce the straight-ahead approximation and solve the resulting density equations by Laplace transform. Explicit expressions are obtained for the two densities over the complete space and energy range. Calculations show the existence of a vacancy rich region near the point of origin of the cascade. The proportionality of the size of the vacancy rich zone to the logarithm of the incident energy is noted and a useful semi-empirical formula is obtained.

The authors have studied the self-trapping and self-focusing-defocusing of an elliptically shaped laser beam in a magnetoplasma. The critical self-trapping power of the beam for the ordinary mode is twice the critical power for the extraordinary mode. On both sides of the critical power required for self-trapping, there are separate values of the critical power for the x-dimension as well as for the y-dimension of the beam. At and above the critical value for the x-dimension, the beam defocuses in both directions while at and below the critical value for the y-dimension, it self-focuses in both directions. Self-trapping is also observed in the case of the ordinary mode at a critical value of the external magnetic field for any power value.

Optical fibre cables for telecommunications now rely upon HE₁₁ transmission as the basic mode. This is a 'hybrid' wave and as such has certain disadvantages compared with a 'pure' mode. Some of the larger fibres employ multi-mode operation and this tends to accentuate problems with auxiliary equipment. A form of optical fibre is suggested for the support of a coaxial TE₀₁ mode and, as a 'pure' wave in a fibre of comparatively large diameter, it could offer some attractive features for future development. Requirements for launching and transmission of this TE₀₁ mode by itself, on a suitable optical fibre, are discussed.

Electron attachment coefficients, alpha _a have been measured in NF₃-N₂ and NF₃-rare gas mixtures over E/N range of 0.303<or=E/N<or=30.34 Td using a pulsed drift tube. As E/N increases for each of these mixtures, the value of alpha _a/(NF₃) initially increases, reaches a peak value and decreases thereafter. The two-body attachment rate coefficients have been estimated by assuming the drift velocity of electrons in the mixtures to be the same as that in the buffer gas.

The theoretical analysis of the energy distribution function of electrons in weakly ionised gases is developed in a relatively high E/N by using the three-term approximation of the distribution function in the Boltzmann equation. The continuity equation of the electron swarm with an anisotropic distribution function is also derived. The present analysis is applied to the steady-state electron swarm in neon: the energy distribution functions and the swarm parameters are computed numerically over a range of E/N from 141.3 to 1130 Td. Also the distribution functions are solved from the usual two-term Lorentz approximation. The range of the validity of the Lorentz approximation is investigated by the comparison between both results.

The motion of elastically colliding monoenergetic electrons in a field-free gas confined between two infinite plane parallel electrodes that emit and collect electrons respectively is analysed assuming first-order diffusion theory. The treatment aims to evaluate the dependence of the collected current on the initial angular distribution of the emitted electrons. This effect is shown to be quantitatively significant. The accuracy of this approach has been verified by comparing the results of diffusion theory with those derived from a Monte Carlo simulation of the electron motion.

A Michelson interferometer was used to study steady-state DC discharges in a point-to-plane gap of about 4 mm in ambient air. This interferometer made it possible to observe rectilinear fringes in the spark gap system. Isothermal curves for such discharges have been derived from the measurement of the displacement of the fringes due to heating of the system by the discharge. Various types of discharge were studied, mainly in the steady-state regimes with currents up to 1 mA, for both positive and negative points. Temperature rises exceeding 100 degrees C were found for the largest values of current and for currents exceeding 40 mu A the cathode was always hotter than the anode. The electric wind participates in the heat exchange process. These results are of interest because they supply information on energy balances and on glow to arc transitions for which a 'heating time' plays an important part.

The authors describe observations on the negative pulseless corona in air between plane and needle electrodes and reports the characteristics of the transition from the negative pulseless corona to spark breakdown by measuring the luminosity and the space potential of the discharge gap. For the measurement of the space potential, a triode tube electrometer was used. The luminosity near the plane electrode is strongest just before the spark breakdown, and the transition from negative pulseless corona to spark breakdown starts from the plane electrode. At the spark breakdown, the field intensity in this region is constant for gap length ranging from 8 to 16 mm. This field intensity is about 43 V cm^-1 Torr^-1. It is the condition of the transition from negative pulseless corona to spark breakdown that the relation alpha >or= eta is accomplished near the plane electrode.

The authors give an experimental account of how additions of calcium chloride to the etchant, consisting of cadmium chloride and methanol, modifies the etch pits formed on the freshly cleaved surfaces of potassium chloride crystals. As the concentration of calcium chloride is increased in the etchant, the etched samples show the following essential features. A progressive increase in the degree of surface roughness. A gradual increase in the shallowness of etch pits. A systematic variation in the morphology of the etch pits and finally the orientation changes from the (100) directions to the (110) directions. Also, the effect of decreasing the cadmium chloride concentration in the etchant is observed. These observations suggest that calcium chloride inhibits the action of the poison, i.e. cadmium chloride, during etching.

In order to give a theoretical determination of the diffusion coefficient of argon in 5A molecular sieves, the authors computed the potential interactions in the structure. The diffusion coefficient depends on the potential type chosen, but the most important parameter is the distribution of the cations in the different sites.

Radioactive silver was diffused into GaAs at 1000 degrees C. The diffusions were carried out over a range of diffusion times and with different amounts of extra arsenic in the ampoule. Profiles were plotted. It is proposed that the silver diffuses primarily by the movement of interstitial atoms but that most of the silver in the GaAs occupies a substitutional site. Electrical measurements were also carried out on diffused specimens, using capacitance-voltage and Hall techniques. All samples were p-type and a reasonably good correspondence was found between the silver and hole concentrations. It was established that the p-type conductivity was not due to thermal conversion and it was concluded that the silver acts as an acceptor in GaAs, occupying the gallium site.

The author analysed the effects of inhomogeneity in the measurements of thermoelectric and photothermoelectric power. Physical lumped circuit models have been used to simulate longitudinal and layered inhomogeneities commonly found in insulating semiconductors. The results indicated a marked difference between the measured thermoelectric power and that calculated from conductivity data for inhomogeneous samples. The measured values were always too small. A quantitative comparison between theory and experiment gave a very good correlation for longitudinal inhomogeneity found in a CdS single crystal. The effects of layered inhomogeneity were demonstrated in the case of surface absorption in a CdS-CdSe sample. The results thus seem to explain some of the anomalous effects frequently observed in the measurements of thermoelectric power in light-sensitive single crystals.

The mechanism of electrical conduction in paper has been studied by means of a gas discharge. The current-voltage characteristics of the discharge were found to be modified by the paper medium. The present analysis indicates the presence of two types of conduction: one being ohmic, the other consisting of successive non-destructive discharges in the paper pores. From these results, a modification of the previously published equivalent circuit for paper is proposed.

The one-gap electronic dispersion model of Wemple and DiDomenico (1969) has been extended, using physically meaningful parameters, to take into account additional dispersion of refractive indices near the lowest energy gap, and applied to the case of chalcopyrite crystals. Dispersion parameters of some chalcopyrite crystals are presented. The location and polarisation of the average energy gap of these crystals is compared with other known energy gaps. Some salient features of the model are discussed with particular reference to the electronic contribution to the birefringence.

The model recently developed for aluminium is applied to secondary electron emission from polycrystalline copper and gold using appropriate noble metals data. The comparison between theoretical and experimental distribution energy curves, normalised for the maximum peak height, may lead to the conclusion that the present approach can be successfully used to describe secondary electron transport. However, the secondary electron yield is underestimated; a more realistic approach to the primary electron diffusion must be made in order to achieve better overall agreement.

The inelastic differential scattering cross-section of 80 keV electrons from amorphous carbon has been calculated for energy losses from 100 eV to the K-shell threshold using a model based upon single excitations from LCAO states to OPW states. Agreement with experiment is obtained at energy losses approaching the K-shell threshold. It is concluded that the model will allow accurate subtraction of the valence background and hence improved microanalysis.

Consideration has been given to the angular dependences of the coefficient of ion-electron emission from the (110) face of a Cu single crystal in a broad energy range. The energy of disappearance of the axial minimum has been found to decrease with increasing the distance between atomic rows.

Dendritic microneedles develop over a period of hours on the surface of a 10 mu m tungsten wire maintained at high potential (in the range 5-15 kV) in benzonitrile vapour. Auger spectra and X-ray diffraction patterns have been obtained from these microneedle-covered wires. The X-ray diffraction results for wires produced at high temperatures (>1000K) show that the wires themselves have been converted to tungsten monocarbide (WC). The Auger results indicate that the microneedles are composed of carbon; however, according to the X-ray diffraction, the structure is not that of graphite. The Auger spectra of microneedles grown with benzonitrile at room temperature also show only peaks due to carbon. A supply-type mechanism is put forward to account for the growth of these microneedles. It is proposed that the electric field focuses benzonitrile molecules on to the growing microneedle tips.

For pt.II see Phys. Status Solidi a, vol.57, p.30 (1980). An electrocodeposition method is described for obtaining films of a cationically substituted AgI-based superionic solid (NH₄Ag₄I₅) on a silver substrate. The electrical conductivity of various thickness films and films prepared under different conditions are reported at different temperatures. The best films are obtained when electrolysis is carried out at 40 degrees C. These results are compared with the results of best RbAg₄Ag₄I₅ and KAg₄I₅ superionic solid films obtained by the authors earlier using a similar method.

An optical study of long specimens of rock-salt that have been down-quenched shows the presence of slip lines that can only be produced by transverse stresses in the material. Some indication of the distribution of residual transverse stresses and the corresponding strains is obtained by examination using polarised light.