Table of contents

Using a Fourier-Bessel expansion of the magnetic field generated by currents flowing on a cylinder, the relationship between current and field may be inverted. Hence the current density required to generate a specified target field B_z on the surface of a cylinder of smaller radius may be evaluated. Gradient, solenoidal and shim coil designs based on this approach provide exceptionally large usable volumes while remaining compact and, if necessary, of low inductance.

A special wall-stabilised arc facilitates the appropriate input of atoms of a number of high-boiling metals with sufficient concentration and homogeneity into the arc plasma. The element of interest is applied as an oxide to the inner wall of the cascaded arc by electrodeposition. Using an MnO₂ coating the capability of this arc for hook measurements on excited states of neutral and singly ionised manganese becomes possible. The temperature for the axis of the arc is obtained by a Boltzmann plot of Mn I hook data. Relative oscillator strengths for five Mn II transitions are determined and compared with those found by other authors.

The short circuit duration in MIG short-circuit welding is evaluated using the Bless relationship which refers only to mass of drops and surface tension to molten metal. In a typical experiment, the experimental values of short circuit duration fit with the expected ones, considering acceptable value for the surface tension of molten iron. Therefore, the Bless relationship should be a good mean for the prediction of the short circuit duration in the MIG welding process.

The authors have investigated the combined effect of the addition of TCA (trichloroethane) and an intermediate annealing step to reduce the density of defects that develop in thin oxides during the oxidation process. Thin oxides (85-95 AA) were grown at 950 degrees C in dry O₂ with varying amounts of Ar dilution and with or without TCA. Oxide breakdown voltage, current against voltage (Fowler-Nordheim parameters) and both high-frequency and quasi-static capacitance against voltage measurements were used to characterise the electrical properties of thin oxides. They show that the addition of TCA is most effective in improving the electrical properties of thin oxides when combined with an intermediate annealing step.

A theoretical model is proposed for the frequency response of ultra-thin pyroelectric elements produced by the Langmuir-Blodgett technique. The pyroelectric film is assumed to have a negligible thermal mass compared with the substrate on which it is deposited and the radiation absorbing layer with which it is coated. The authors show that, for high resistance films, the pyroelectric voltage is expected to be proportional to ( omega ₀)^-12/, where omega ₀ is the modulation frequency of the incident radiation, and present experimental data which support their theory.

The interpretation of thermal neutron absorption cross section measurements with Czubek's two-medium method has so far only been carried out with diffusion theory. In order to investigate the validity of this approximation, a two-medium plane case has been studied in transport theory for neutrons of one speed. The resulting coupled integral equations were solved by an iterative method. The deviation between the transport theory and diffusion theory results is less than one per cent for the thicker reflectors, increasing to about 1.5% for the thinner ones. The results suggest that the size of the inner region in the measurements is adequate. However, for the thinnest reflectors used, a correction may be necessary if high accuracy is striven for.

The efficiency of energy transfer from the first excited electronic singlet state of donor toluene to fluorescent indoles as activators in different types of scintillator systems comprising neat toluene, mixture of toluene and cyclohexane and mixture of toluene and paraffin is determined as a function of temperature in the range 293-353K. The results clearly demonstrate that the dependence of the energy transfer efficiency on dilution, temperature and viscosity is essentially governed by the influence of these factors on the probabilities of decay of excited donor molecules through energy-transfer processes as well as processes involving no energy transfer which exhibit compensatory changes.

Analytic formulae are presented for the radius of the emitting area and the current in rotationally symmetric electron guns. They are based on an approximation of the field strength in front of the cathode by a series expansion up to the fourth order in the radial position. The coefficients of the series expansion are calculated with bicubic spline interpolation from the geometry. The initial velocities of the electrons may be taken into account (Langmuir's law) or ignored (Child's law). The calculated currents are compared with experimental ones for various drives, cathode loadings, and geometries. The accuracy, using Langmuir's law is better than 10% for currents between 0.05 and 5 mA. The calculated currents, using Child's law, are systematically too low for currents smaller than 0.5 mA. Also presented is a method to derive the distance between cathode and first grid from a measurement of the visual cut-off potential as a function of the potential of the second grid. It is based on the series expansion of the field strength and similar to a method presented earlier by Hasker (1966).

Emittance diagrams are measured in a triode electron gun for various geometries, drives, and cathode loadings. Important properties of the electron beam, like the aperture angle and the crossover sizes caused by aberrations and the initial velocities of the electrons, are derived from the emittance diagrams. The aperture angle depends linearly on the perveance to the power one-third. The crossover size caused by aberrations depends quadratically on the radius of the emitting area. The crossover size caused by the initial velocities is described well by the law of Hemholtz-Lagrange with a cathode temperature 15% higher than the operational temperature. With these relationships the aperture angle and the crossover size caused by aberrations and initial velocities can be estimated over a broad range of geometries, cathode loadings and drives. Their accuracy is about 10%.

The output wavelength of a small pulsed Nd:YAG laser can be converted with good efficiency to an eye-safe region at 1.54 mu m using stimulated Raman scattering in compressed methane. In the presence of limiting factors such as medium depletion, stimulated Brillouin scattering and gas breakdown, the conversion is strongly intensity dependent and the focusing geometry must be chosen with care. Experimental and theoretical results are presented to illustrate and quantify these effects.

Any depression of the refractive index along the axis of the core of a conventional optical fibre tends to confine waves transmitted along it to the concentric layer of higher-refractive-index material. Moreover, the inner radius, in the transverse plane of this layer, proves to be a particularly important factor in determining propagation behaviour and this paper shows how the HE₁₁, the EH₁₁, the TE₀₁ and the TM₀₁ modes are affected. The EH₁₁ mode is found to be extremely sensitive to a minute filament of depressed refractive index along the axis and this apparently can provide for limited propagation of that mode at the bottom end of the frequency scale. The implication is that single-mode operation in the HE₁₁ pattern is only surely established when the core is of uniform refractive index extending right to the axis of the fibre.

A new ultrasonic interferometer for velocity measurement using optical diffraction and an optical heterodyne system is described. The velocity of pure water at 20 degrees C has been measured in the frequency range 0.5-5 MHz, and the velocity obtained increases with decreasing ultrasonic frequency. In order to clarify the behaviour, a new theory for the ultrasonic interferometer is given and the usefulness of the instrument is shown by using it to measure the velocity dispersion in benzene.

The evolution of finite-amplitude Rossby waves in an average zonal flow with weak shear and topography has been developed. The evolution equation developed is a general one and can be treated as an extension of Benney's equation to the time-varying case with topography. As an application to geophysical flow, the evolution equation has been applied to the problem of the formation of a monsoon onset vortex over the Indian region.

The active fluid volume in an electromagnetic flowmeter is taken as a finite circular cylinder. The magnetic potential and virtual-current potential in this volume are expanded in Fourier series in the variables theta (polar angle) and z (axial coordinate). Any magnetic field and virtual current is thus represented by the matrices a_nm and b_pq respectively (being the coefficients in the double-Fourier series). Expressions for the matrix elements for a four-pole electromagnet and for one or two pairs of point electrodes are derived. A method is given for obtaining the matrix elements representing a magnetic field in terms of values of the normal component of the flux density over a cylinder coaxial with the tube. The axisymmetric weight function (and hence the fully developed flow sensitivity) can be derived by evaluating a double sum involving the elements a_nm and b_pq and the coefficients W_nm(r) which are independent of magnetic field and virtual current. Calculations of the weight function and sensitivity have been made and examples of results given.

The radial variation of plasma parameters in a pulsed low-pressure arc (100-150 Pa) in helium is studied by various methods. For an on-axis electron density of N_e=1.2 and 3.3*10²¹ m^-3 the density profiles are measured by interferometric and spectroscopic methods. The gas temperature is derived from the Doppler broadening of the helium line He I at 388.9 nm. Relative line intensities of He II at 468.6 nm and He I at 471.3 nm yield the electron temperature. Collisional-radiative (CR) models put forward by Mewe and Drawin and Emard take deviations from local thermodynamic equilibrium into account. The necessity of introducing a 'channel model' for this discharge is supported by measurements of the plasma conductivity.

A simple model of DC partial discharges running in cable and capacitor dielectrics is described in order to investigate the influence of the measuring circuit on the DC partial-discharge repetition rate. This model shows that the measuring circuit can only influence the discharge repetition rate temporarily. On reaching the stationary state the DC partial discharges are governed by the electric qualities of the dielectric used.

Statistical properties of Trichel pulses with AC voltages are studied, measuring simultaneously the frequency distributions of the apparent charge q and of the light intensity q_L. The occurrence of quasi-Trichel pulses and negative streamers, in addition to regular Trichel pulses, is observed. It is established that, along with the normal conditions necessary for the occurrence of regular Trichel pulses, a sufficiently intense Malter emission of electrons is essential. If this intensity is insufficient quasi-Trichel pulses appear and if there is no emission negative streamers appear. The above types of partial discharge differ in certain properties, namely the range of values of q; the values of the ratio q/q_L; the stability of the pulses with time and the spread of the times of occurrence on the negative half-cycle of the applied voltage.

Measurements have been made of the current pulses observed in a negative point-to-plane corona discharge in oxygen. The apparatus was constructed to ultra-high vacuum standards to ensure controllable conditions on the gas purity and the state of the electrode surfaces. The effects of cathode voltage, tip diameter and oxygen pressure on the current pulse shapes have been investigated. Results include the first report of a step on the trailing edge of corona pulses in pure oxygen. The dependence of the trailing edge plateau on several parameters has been measured. The results are compared with the theoretical predictions of Morrow (1985) where appropriate.

Poisson's equation is formulated for the flow of electrons and ions in a parallel plane device composed of an electron emitting cathode, an ion emitting anode and a series of intermediate ion emitting grids that are arbitrary in position. The equation is solved numerically for the diode, triode, tetrode and pentode configurations to give the positions of the grids that produce an overall maximum in the space-charge limited electron current. In the case of the pentode this current is a factor of 25.68 times the Child-Langmuir limit for the corresponding simple diode. These results are considered in relation to electron beam generators, modulator valves and thermionic energy converters.

The technique of electron diffraction has been used to investigate the optimal growing conditions for copper sulphide thin films. The authors have analysed the parameters which take part in the sulphurisation process. This analysis has allowed them to determine suitable conditions of thiourea concentration, solution temperature, dipping time and film thickness to obtain the different phases of copper sulphide. In particular, they give the best values of these parameters to obtain the chalcocite phase.

The fractal or Hausdorff dimensionality of an ideal random structure exhibiting self-similarity implies that the intensity of radiation scattered at small scattering vectors should have a power-law dependence on the magnitude of the scattering vector. However, for any real system, the scattering law must be modified by the introduction of a correlation length which reflects the finite overall size of the system. This concept can also be incorporated into the outer (Porod) part of the small-angle scattering where a non-integral power-law dependence can arise from interfaces of fractal character. The scattering law for the inner region of the small-angle scattering (corresponding to larger distances) is proportional to Q^-D where D is the fractal dimensionality of the structure. The scattering law for the outer region (corresponding to smaller distances) is proportional to Q^D-6, where in this case D is the fractal dimensionality of the interface. This reflects the crossover from scaling based on mass per unit volume to one based on mass per unit area as the length of the examining probe (the inverse scattering vector) becomes shorter. An inverse correlation is predicted between the fractal dimensionality of the interface and the correlation length obtained using a nonfractal (Debye) model over a restricted range of scattering vectors. Measurements on shaly (argillaceous) rocks exhibit deviations from Porod scattering indicative of microscopically rough interfaces with D>2, and show the negative correlation between interfacial dimensionality and correlation length.

In an unstressed orthorhombic material the shear wave velocity is invariant under an interchange of the propagation and polarisation directions, if these directions lie along the principal texture axes of the material. In the presence of a stress with principal stress directions coincident with the principal texture axes, this symmetry is broken and the change in the velocity when the propagation and polarisation directions are interchanged is given by the difference in stresses along the propagation and polarisation directions. In this paper, the effect of a misalignment of the symmetry and sample planes on this relationship has been examined. For austenitic welds, an error in the determination of stress of the order of yield can occur for tilts of 5 degrees . However, for structural steels with textures very much less than the theoretical maximum, a determination of the stress present to an accuracy of 10% of yield remains possible using this technique.

Expressions are presented for the determination of the activation energy, the frequency factor and the order of kinetics for single thermoluminescence peaks recorded with hyperbolic heating. A 'general order of kinetics' equation of the form dn/dt=-const. n^gamma exp(-E/kT) is assumed to apply. The peak widths at signal levels equal to ¹/₄, ¹/₂ and ³/₄ of the peak height are used for the determination of the energy and frequency factor, while the asymmetry of the peak gives the order of kinetics parameter gamma .

Thin polycrystalline films of chalcocite ( approximately Cu₂S), djurleite ( approximately Cu_1.96S) and digenite ( approximately Cu_1.84S) have been obtained by sputtering in an Ar/H₂ atmosphere and their optical constants n( lambda ) and alpha ( lambda ) have been determined in the 400-1000 nm spectral range. Using the values of the optical constants found previously, the determination of diffusion lengths and lifetimes of minority carriers of both heterojunction components and the surface recombination velocity is achieved. The method used adjusts a theoretical expression of the spectral short-circuit current to the experimental values by a minimisation 'Simplex' method. The results are compared with those obtained by Partain and co-workers (1983) during degradation of Cu_xS/CdS solar cells.

MoS₂ films have been prepared by RF sputtering in a planar magnetron system. Their chemical composition was analysed by energy dispersive X-ray spectroscopy. The morphology and growth characteristics were examined by scanning and transmission electron microscopy and by electron diffraction. Correlation between process conditions, film stoichiometry and morphology has been pointed out by a systematic variation of the experimental sputter parameters with emphasis on a stable substrate temperature during deposition. It has been shown that by a precise control of the substrate temperature specific morphologies can be obtained which are independent of film thickness. In particular, at high deposition temperature the films exhibit a lamellar-type microstructure with the 002 planes of the MoS₂ crystallites perpendicular to the substrate. The crystallite size strongly decreases for deposition at lower temperature. The increase of power density was found to give higher sulphur concentration in the films whereas substrate biasing results in a pronounced excess of the metallic element.

The pair generation rate, ideal characteristics and conversion efficiency of a solar cell with a specific thin-film design discussed previously are calculated and compared with those of a conventional design chosen as standard. Various factors affecting the efficiency are examined in order to find parameters that will improve the efficiency relative to that of the standard cell. It is found that for an n- on p-substrate Si thin film of thickness 2.5 mu m based on the present design, the efficiency can be improved significantly relative to that of the standard cell by exploiting the back surface field effect and by raising the acceptor impurity concentration to a level comparable to the donor impurity concentration.