Table of contents

The operation of an ammonia beam maser with plane parallel and conical types of open resonator is described. Oscillation has been achieved with both types of resonator. The complex lineshapes of emission are correlated qualitatively with resonator microwave field distribution and molecular beam divergence.

High current arcs in air at atmospheric pressure, initiated between a rotating anode and a stationary cathode, have been observed to change to a stable stationary mode. The transition, which occurs over a narrow range of currents, is characterized by a drop in arc voltage and cessation of ablation of the anode surface.

In works on acoustics and two-phase flow the wave propagation velocity is regarded as a variable and not simply dependent upon temperature; whereas in gas dynamics it is usual to consider the adiabatic sonic velocity as the basis for defining a Mach number, resulting in the anomaly that for non-adiabatic flow processes choking occurs at numbers other than unity. However, by using a Mach number based on a defined wave propagation velocity appropriate to the expansion process, then choking of the flow always occurs at a Mach number of unity. However, such a concept is hypothetical for it is shown that real processes approach isentropic conditions on choking and here the appropriate Mach number is unity based on the conventional velocity of sound in a gas.

The equation developed for expansion through a tube takes the form (ρ/p)/(dp/dρ) = n where n is not normally constant. Thus the equation cannot be integrated to give the usual polytropic expansion law p/ρⁿ = constant. In two special cases, however, n becomes constant; these are for isothermal flow when n = 1 and adiabatic flow when n = γ. For such processes, therefore, the equation integrates to the familiar form.

A Debye theory is introduced and used in conjunction with a mathematical theory to predict a linear shock speed, U, particle speed, u, relationship. The basic result obtained is that the slope dU/du is directly proportional to the Grüneisen index. For a number of metals the theoretical numerical value for the slope agrees remarkably well with experimental results.

An analysis of the modulation of an electromagnetic wave propagating through a magnetoplasma disturbed by an acoustic wave has been given. The analysis of the modulation of the transmitted and reflected waves from the boundary between the magnetoplasma and free space has also been given. Numerical calculations have been made to study the variation of modulation with electron density, collision frequency, magnetic field, acoustic power and distance from the acoustic wave source.

The fm laser consists of a laser with a phase modulator in the cavity, driven at a frequency of approximately the axial mode spacing. A passive two-mirror cavity and a passive ring cavity containing such a modulator are analysed using travelling plane waves. The axial modes of the cavities are shown to be phase modulated and their centre frequencies and peak phase deviation are determined in each case.

For n and k in the ranges 1·0less-than-or-eq, slantnless-than-or-eq, slant4·0, 1·0less-than-or-eq, slantkless-than-or-eq, slant4·0, a computer study has been made of the way in which large errors in n and k may arise from errors in reflection coefficients, when the latter are measured at angles of incidence other than 74°. It is shown, as an example, that at near normal incidence errors in the product nk of ±76% are possible for n=2·00, k=3·00 when the measured reflection coefficient is in error by ±0·001.

It is shown that the coefficients K and S of the Kubelka-Munk reflectance theory are equivalent to 2Σ_a and ¾Σ_s, where Σ_a and Σ_s are, respectively, the coefficients of absorption and scattering for the medium concerned.

This paper presents an analysis of displacement damage produced in niobium, which has potential use as the first wall material of a fusion reactor.

Lindhard's formalism was used together with cross sections taken from the UK nuclear data file on niobium. Data sets describing damage energy created at various neutron incident energies were generated. It was calculated that:

1. The ratio of damage density (ie mean damage energy × cross section) at 14 MeV to that at 1 MeV is 3·5 to 1·00.

   The damage energy data sets were integrated over two different neutron spectra in order to compare the total effects. The spectra were taken from: (a) the first wall of a model fusion reactor, with a total flux of 2·8 × 10¹⁵ neutrons/cm² s; (b) the core centre of the Dounreay Fast Reactor (DFR), total flux 2·5 × 10¹⁵ neutrons/cm² s.

   Analysis showed that the number of atomic displacements per atom per year is

2. 165 displacements for spectrum (a);

3. 78 displacements for spectrum (b).

The energy balance of a steady-state, laminar flow electric arc in constant pressure axial flow has been solved by an approximate method. It is shown that such arcs can be represented by a single non-dimensional characteristic which takes account of the variation of voltage gradient with arc current, length, flow velocity and pressure. Arc voltages have been measured in constant pressure nitrogen flow up to 50 A current and velocities up to 300 m s⁻¹ at 1 and 2 atmospheres' pressure. These results show good agreement with the theoretical characteristic.

The Paschen curves of sparking potential as a function of the pressure × gap length product (pd) are given for normal pentane, hexane, heptane, octane and decane. The measurements of sparking potential have been made from pd about 30 Torr cm through the minimum sparking potential about 1 Torr cm to approximately 3 × 10⁻³ Torr cm. A previous empirical equation giving the sparking potential for values of pd greater than minimum sparking potential has been modified by the addition of two constants and an extra term, so that it now fits the Paschen curves for the whole of the extended range of the measurements while retaining the constants introduced by previous workers. Values of the constants are tabulated for the hydrocarbons listed above and a detailed comparison between the sparking potentials calculated from the equation and experimental results is given for n-octane.

Breakdown potentials of water vapour are measured at room and elevated temperatures between plane parallel electrodes made of stainless steel. The water vapour pressure is varied in the range 22·5-573 Torr and the temperature of both the vapour and the metal electrodes from 298-374 K. A minimum breakdown potential of 2000 V is observed in the p0d range (normalized vapour pressure times electrode gap length) 18-23 Torr cm.

The generalized secondary ionization coefficient γ in water vapour is calculated, as a function of the ratio of electric field and gas pressure, from the measured breakdown potentials and using the recently published values of primary ionization and attachment coefficients.

Measurements of pre-breakdown ionization currents in uniform-field gaps in filtered atmospheric air have been undertaken for values of the parameter pd (pressure × gap length) up to 10 500 Torr cm, corresponding to a maximum breakdown voltage of 350 kV. Over the whole range investigated, the spatial growth of ionization in the gaps was found to be in accordance with the generalized Townsend equation, the main source of secondary electrons being due to the photoelectric effect at the cathode. For given values of E_s/p, where E_s is the breakdown field stress, there is good agreement between the values of d_s (sparking distance) obtained by experiment and those predicted by the Townsend breakdown criterion.

The duration of the transient glow discharge preceding the arc stage of uniform-field gas breakdown has been measured for nitrogen and air. These measurements have been made for gaps up to 3 cm at gas pressures of 300 and 500 Torr, in the overvoltage range 4%less-than-or-eq, slantΔVless-than-or-eq, slant20%. It has been shown that the glow duration is strongly dependent upon the equivalent series circuit resistance, and this explains the poor agreement between the results of previous investigations.

During spark conditioning, a form of breakdown consisting of a very diffuse glow, with no subsequent arc discharge, has been observed. Some of the factors affecting the occurrence of this interesting phenomenon have been studied.

The hollow-cathode glow-modulator tube has a cylindrical molybdenum cathode cavity and ring anode. An abnormal glow discharge is formed between these electrodes in a low pressure neon-argon gas mixture to create a high intensity source of light which passes out through the anode. The tube is used mainly in scanning and facsimile equipment.

Impedance-frequency measurements have been made over the range 200 Hz to 6 MHz which at low frequencies have been related to the static voltage-current curves of the discharge. The effects of sputtering over periods up to 200 h have been observed and the cathode cavity has been sectioned and photographed. The results for various depth/diameter ratios are shown, each of which forms a spherical hollow after many hours of operation. The noise characteristics in a 6 kHz bandwidth have been measured over the range 5 kHz to 6 MHz and are explained with reference to the basic gas processes and the impedance characteristics. Identification has been made of the spectral lines from the discharge which are linearly proportional to current for the gas atoms and proportional to current cubed for cathodic atoms.

The electrical dispersion of a jet of kerosene into a spray of monodisperse droplets is discussed. It is found that monodisperse spraying can be achieved within a small finite range of applied voltages, within which droplet size is almost independent of voltage and is a function of the flow rate of kerosene. An approximate theory is developed which shows that the controlling parameter is the field strength at the surface of the jet, and correctly predicts the specific charge acquired by the droplets as a function of this field strength.

The epitaxial crystal growth and surface form of fcc nickel electrodeposits from the nickel sulphamate bath, on smooth copper {100} faces, are studied systematically by electron diffraction. Current densities up to 600 mA cm⁻² and thicknesses up to 60 000 Å are investigated. The results with the bath at 20 and 50°C, and unstirred, show quantitatively the effects of the bath temperature, and additional results show the effects of vigorous stirring at 50°C.

At 50°C two main isostructural regions of current density and thickness are distinguished, subdivided into five subregions where the surface or subsurface form is different. In the single-crystal region, negligible twinning due to lattice misfit occurs, in marked contrast to the strong twinning in Ni on Cu {110} and {111} using the same bath and conditions. The development of oblique faces is studied and a new region is found at high current density where their development is again very slow or absent. Above a certain current density, varying with deposit thickness, co-deposition of Ni(OH)₂ occurs, leading to twinning and random Ni crystals.

A new formula for the shear modulus of a macroscopically homogeneous and isotropic composite material is derived. Geometries for which this formula is applicable are suggested and correspond to those which may be obtained in two-component composites consisting of two continuous phases.

Single crystals of five members of the I-III-VI₂ ternary semiconductors have been grown by iodine vapour transport and variations of growth are reported. The measured energy gaps are 2·44 eV (CuGaS₂), 2·06 eV (CuAlTe₂), 2·51-2·61 eV (AgGaS₂), 3·13 eV (AgAlS₂) and 2·55 eV (AgAlSe₂). The refractive index for CuGaS₂ varies from 2·825 at 525 nm to 2·660 at 661 nm with refractive indices of 2·720 and 2·724 for two polarizations at 589·2 nm.

An equilibrium theory of adhesion between elastic solids is developed. Adhesion is shown to depend on the interfacial surface energy and on the geometry and elastic constants of the adherent bodies. The theory is used to interpret the well-known adhesion tests - the pull-off, peel and scratch tests - and the spontaneous peeling of thin films is discussed. Experiments on contacts between Perspex and gelatine surfaces support the analysis.

Copper-nickel alloy films of about 450 cm² geometric area for studies of gas/surface interactions have been prepared by simultaneous evaporation of the components from hairpin filaments, serving effectively as point sources, onto a heated Pyrex vessel wall at background pressures of about 8×10⁻⁹ Torr. Conditions for the preparation have been developed so as to minimize the subsequent annealing times needed for complete homogenization. Homogeneity was ascertained by x-ray and electron diffraction.

The theory developed by Englman for the propagation of temperature waves in insulators has been extended and some of his results obtained in simpler form. By analogy with problems already met in sound wave and neutron wave propagation, a method is described which enables the concept of a normal mode wave to be extended to a wider frequency range than was hitherto possible.

The total emissivity of four samples of silicon of different resistivities is measured in the temperature range 880-1550 K. The emissivity depends on carrier concentration in the silicon samples. As the temperature increases the emissivity increases, shows a maximum at about 950 K and then starts decreasing. After showing a broad minimum at about 1200 K, it starts increasing again. The actual values of the emissivity vary from about 0·42 to 0·78 depending upon the temperature and the resistivity of the sample.

A description is given of a Monte Carlo method for the computation of electron trajectories in solids. The calculations are based on the Bethe energy loss law and the Rutherford scattering equation. By assembling a large number of these trajectories, predictions are made on the transmission, backscattering, angular distribution and energy spectrum of electrons passing through thin films. The results are in good agreement with experimental data on the passage of electrons, initially of energy from 5 to 30 keV, through aluminium, aluminium oxide, copper and gold.

The spinel FeCoCrO₄, on which very little work seems to have been done so far, has been studied using the powder method and x-ray spectroscopy. It has been found that this compound is cubic with a=8·34 Å. The u parameter of this spinel is found to be 0·383. The oxidation states of the iron, cobalt and chromium ions in this compound appear to be three, two and three respectively from x-ray spectroscopic results. Consideration of the site preference energies show that the spinel has the ionic structure Fe³⁺[Co²⁺Cr³⁺] O₄.

Measurements of the complex initial permeability of several specimens of Mumetal (77% Ni, 14% Fe, 5% Cu, 4% Mo) have been made from 0-30 MHz. The samples were polycrystalline rings 50 μm in thickness; the mean grain size ranged from 9 μm to 140 μm. The results were analysed in terms of the domain model previously employed by the authors in 1969 and 1970, yielding the following main conclusions.

Despite the low anisotropy (of order 100 erg cm⁻³), there exists a well-defined domain structure. Except near the surface, where the domain size is independent of grain size, the scale of the domain structure is found to be nearly proportional to crystallite size to the power 0·4.

Some of this domain structure persists in polarizing fields as great as 20 times the coercive field. There is no evidence for an independent contribution to the permeability from rotational processes which, it is suggested, are instead strongly coupled to wall motion. There is evidence that the behaviour is beginning to be influenced by intrinsic spin phenomena at frequencies greater than about 30 MHz.

The real and imaginary parts of the eddy current limited permeability of a ferromagnetic sheet, calculated on the basis of the Polivanov domain wall displacement model of the magnetization process, are tabulated as functions of frequency and domain size.

The temperature spectrum of disaccommodation for manganese-zinc ferrous-ferrite toroidal specimens is studied using two methods of magnetic shock. One of these methods, namely, disturbance using a capacitor discharge (ac demagnetization), produces a broad maximum in the disaccommodation against temperature characteristic. The other method of disturbance, using the field provided by a permanent magnet, is shown to give a discrete disaccommodation spectrum, which is used to analyse three room-temperature diffusion effects in terms of the partial pressure of oxygen during firing. One of these effects, which depends only on the oxygen partial pressure (and hence cation vacancy concentration), has not previously been reported at room temperature. The permanent-magnet disturbance is also used to investigate the magnetization states in the toroidal specimens.

Single crystals have been grown based on optical quality lithium fluoride with the addition of magnesium and titanium. The content and distribution of titanium has been determined by x-ray fluorescence analysis and related to the intensity of an optical absorption band near 200 nm. The variation of thermoluminescence intensity within each crystal has been studied and related to titanium distribution.

Taking the current waveform of return strokes of a lightning discharge given by Bruce and Golde in 1941, the impulse magnetostatic flux density close to the discharge channel has been calculated. The magnetic flux density decreases with the inverse square of the distance of observation.