Table of contents

The effect of resistive ballasting on the discharge stability of a UV-preionised, discharge-excited, XeCl laser is described. It is demonstrated that the resistive ballasting of one of the transverse electrodes has a significant effect on the duration and the shape of laser output pulses. Pulses of 80 ns FWHM (120 ns foot-to-foot), with energies up to 15 mJ, have been observed using this technique.

This letter comments on a conclusion by Phillips (see ibid., vol.13, p.221, 1980).

Aggregates, rather than single anisometric particles, are put forward as the source of the magnetic birefringence observed by H.W. Davies and J.P. Llewellyn (see ibid., vol.12, p.311, 1979).

J P Llewellyn replies to P C Scholten's suggestion.

The radial distribution of xenon in irradiated (U,Pu)N and (U,Pu)C nuclear fuels has been determined. When xenon is contained in bubbles larger than 20 AA in diameter the emitted X-ray intensity is deficient and the magnitude of the deficit increases with bubble size. The subsequent errors in the measured gas concentrations are corrected using a computer programme that calculates the integral X-ray intensity for any given configuration of the gas bubbles within the volume of X-ray excitation. The correction model, which takes into account the probability of electron-gas bubble interaction, the depth distribution of X-ray production, and X-ray absorption, is valid for any randomly dispersed second phase with dimensions smaller than the volume of X-ray excitation.

The authors are concerned with a thin wire antenna mounted on a perfectly conducting sphere. They calculate the current density J on the wire and the radiation pattern of the antenna. They use the Green's tensor G appropriate to the sphere to reduce the equation for J to a one-dimensional integral equation, which can be solved reliably once G has been computed. They find that the series development of G converges extremely slowly, and that it is essential to sum the tail of the series analytically. Finally they describe the computational algorithm in detail and present numerous plots which should be useful in testing other algorithms.

A continuously tunable high-pressure electron-beam-sustained CO₂ laser has been used to optically pump carbon tetrafluoride at low pressures and temperatures. Tuning between the 9 mu m R(10) and R(14) CO₂ lines laser action was observed in CF₄ at virtually all pump wavelengths to within the 0.1 cm^-1 bandwidth of the pump laser, and strong superfluorescence was observed at five of these wavelengths.

The use of the Fourier transform inversion technique for sizing fibres is examined when both random and systematic errors are present in the light scattering data. The reliability of any part of a reconstructed size distribution is shown to depend on the precision of the measured light intensity at all scattering angles used in the inversion. A consideration of the effect of the systematic errors and the need to resolve the distribution determines the ideal angular range over which measurements of the scattered light intensity should be made. This angular range is dependent on the size distribution under consideration and must therefore be determined for each fibre sample investigated. These observations also apply to particle sizing in general where inversion techniques are used to obtain the size distribution from optical scattering data.

A technique for the generation of ultrasonic waves from highly disorientated CdS layers results in the simultaneous excitation of both the longitudinal and transverse acoustic modes in the substrate, so providing a means for the measurement of the two associated elastic constants. This technique therefore overcomes the difficulties encountered with the usual method, where it is necessary to work with two separate transducers, and further extends the range of applications for CdS films. Experiments have so far been performed on two single-crystal samples, Al₂O₃ and V₃Si (an A15 compound).

The author has recently reported a theory of a domed piezoelectric plastic film which can be used as a combined diaphragm and transducer element in an earphone (see ibid., vol.13, p.201, 1980). This paper describes some modifications to the theory which may be necessary when the diaphragm is actually used in the device. In practical earphones the diaphragm displacement may be influenced by reaction effects of the sound pressure generated. At sufficiently low frequencies the reaction effect is easily incorporated into the earlier theory and criteria are derived for when it becomes important. The sound pressure is then calculated for the limiting cases where the reaction effect is either dominant or negligible.

The structured release waves in 2024 Al, shock-loaded to stresses in the range 3.0-9.5 GPa, were recorded using commercial manganin gauges. These waves consist of two elastic waves followed by the plastic release. The first elastic release is due to the elastic precursor's reflection from the free surface while the second one is the result of the elastic part of the main release wave. The ability to record all three waves is attributed to appropriate gauge placement, minimum gauge encapsulation thickness and intermediate stress range where the three waves can be well separated in time.

A theory of fracture transitions consistent with fracture mechanics is outlined. Transitions occur when a certain characteristic length attains a critical value: in linear elastic fracture mechanics this characteristic length is that of an inhomogeneous stress field, as for instance the radius of a notch. The critical length is proportional to a material factor E Gamma /Y² (or (K_c/Y)²) where E, Gamma , K_c and Y are Young's modulus, fracture surface energy, critical stress intensity factor and yield stress in a uniaxial test. To a first approximation the constant of proportionality is characteristic of the test only, and ranges from approximately 10² in Hertzian indentation to approximately 10^-1 in notched bar fracture. Two types of transition are considered: at the boundary between elastic and plastic-elastic conditions, when the yield criterion is just exceeded, and for fully plastic-elastic conditions when the plastic zone attains some limiting size imposed by the test. In the first case the characteristic length of the stress field is that of a specimen dimension, in the second the length of the plastic zone. The analysis therefore relates fracture transitions in different tests and materials to each other, and is illustrated by experimental data on indentation of polymethylmethacrylate and notched bar tests of silicon iron. A new fracture diagram is proposed.

Experimental studies of a simple unipolar electrophoretic flow have revealed that, typically, between 10% and 25% of the electrical energy supplied was usefully converted into flow work. Certain experimental uncertainties prevent firm conclusions, but two mechanisms have been proposed to explain low mass transfer efficiency; (i) space charge effects at the collector which reduces the intensity of the 'useful' electric field in the region of the bulk flow and (ii) microscopic shear dissipation arising from the disperse nature of the electrophoretic point forces. The single streamer flows were generated in n-hexane by the localised injection of charge from discs of ion-exchange resin in a parallel plate electrode system. The mass transfer data were collected using a frequency shifted, purpose built laser Doppler anemometer employing specially developed polymethylmethacrylate light-scattering seed particles. Measured velocity profiles within the flow structure provided estimates of mass transfer efficiency at applied field strengths up to 3.75*10⁴ V m^-1.

Values of the electron drift velocity, the longitudinal diffusion coefficient and the transverse diffusion coefficient, for electron swarms in nitrogen over the range 50 Td<or=E/N<or=500 Td have been determined by a method of counting and analysing the photons produced by such an electron swarm in a drift tube. A measure of the nitrogen E state deactivation rate is presented.

Wave phenomena at a strong electric double layer are investigated experimentally in a magnetised plasma column where the electron gyro frequency is smaller than the plasma frequency. Fluctuations with frequencies below the ion plasma frequency and high frequency fluctuations, peaked close to the electron plasma frequency, are found to be dominating. The low and high frequency fields are shown to have characteristic spatial distributions with the maximum fields occurring in localised regions at the double layer. Spatial correlation measurements show that the high frequency field is associated with axially propagating waves. The phase velocity is nearly constant over a wide range of frequencies and 10 to 20% smaller than the velocity of the electron beam that is formed in the layer. The double layer is characterised by a steep potential gradient in a central region with a thickness of about 20 Debye lengths. This region is surrounded by 'presheaths' where free electrons and ions are accelerated towards the layer. The role of the ionisation in the anode plasma is also discussed.

Ions produced by the interaction of a neodymium-Q-switch laser on solid tantalum have been investigated according to their energy, charge state and angular distribution. The laser intensity was of the order of 10¹⁰ W cm^-2. It was found that the ion energy spectra have smooth shapes and that the total number of ions can be well fitted to a Boltzmann distribution. The angular distribution of the emitted current was essentially cos² and only slight differences were observed for different charge states of the ions. The energy dependence of the different charged ion species exhibits a selective acceleration mechanism.

A small, low-power, lightweight arc plasma generator is described which is highly efficient electrically. The design, which utilises a small permanent magnet, has enabled electrode erosion problems to be solved, and the generator can be tuned to operate continuously on a variety of gaseous feedstocks (for example N₂, CH₄, H₂) with high stability and without the necessity for any noble monatomic gas additives. The fine tuning also enables the generator setting to be optimised selectively for particular atomic states, while the interchangeable anode construction has allowed species generated within the arc to be expanded into a variety of pressures with low recombination losses. These facilities are of potential use both in academic studies (lasers, molecular beams etc.) and in practical combustion situations.

Peltier heating effects have been observed to cause significant errors in measurements of electrical resistivity in chromium. These effects occur in many experimental arrangements, and for both AC and DC techniques. The errors became particularly acute in the vicinity of the Neel transition.

Measurements of the magnetically induced birefringence, linear dichroism, Faraday rotation, Faraday ellipticity and circular dichroism in a cobalt and a magnetite ferrofluid are reported. These results are interpreted in terms of the permittivity tensors of the materials concerned and the elements of these tensors are evaluated. In the case of the cobalt fluid the derived values of the elements of the permittivity tensor are compared with the values that have been derived from measurements on thin films of cobalt by other workers and the agreement is considered reasonable.

Thermoluminescence of CaSO₄:Dy has been investigated for radiations with widely different stopping powers. The lower stopping power radiations produce wide variations in the glow curve and a substantial increase in TL response. With increase in stopping power the extent of such variations reduces and the TL response shows a decrease in supralinearity. A physical model based on 'quasi-continuous' distribution of trap depths is proposed which qualitatively explains the stopping-power dependent TL behaviour of CaSO₄:Dy.

The secondary electron yield delta ₀ due to primary electrons, the yield c eta due to back-scattered electrons, and the efficiency c of these last electrons, are estimated using the recently developed models for secondary electron emission and electron back-scattering, with appropriate assumptions and using the following relation delta = delta ₀(1+(c/ delta ₀) eta ). The values of c/ delta ₀ obtained for aluminium with primary energy of the order of 1 keV seem to agree in the low primary energy range with the more empirical calculations worked out so far. An analysis of the discrepancy which still remains between theoretical and experimental values suggests further modifications in order to achieve better agreement and to extend the models in the region of several keV where scanning electron microscopes are usually operated.

A projectile which is scattered through an angle theta by an atom with smaller mass can have two different energies after scattering. This phenomenon is treated and demonstrated by the scattering of 9 keV ⁸⁴Kr⁺ ions from a stepped (410) copper surface. The measured energy and intensity distributions agree very well with theoretical expectations.