Table of contents

The modification of the nitrogen spectrum in a flowing dc discharge is studied in relation to the []/[] ratio. An interesting phenomenon of spectral `cleaning' is observed, which leads to a pure second positive spectrum.

X-ray photoelectron spectroscopy (XPS) has been used to characterize the composition of air-cleaved mica surfaces before and after treatment under a range of different conditions in the argon rf plasma discharge. The changes in the composition of the processed surfaces that have been identified are found to depend strongly on the plasma etching parameters used. When etching was conducted at relatively high power levels ( W), the XPS data demonstrate that the mica surfaces examined have a significant accumulation therein of the heavier elements present, namely aluminium and silicon, whereas the oxygen component on the surface is decreased dramatically. However, no obvious changes in the chemical states of the elements contained in the mica were found for the sample surfaces processed under these conditions. Similar phenomena were also observed when the plasma etching was conducted at higher gas pressures or over prolonged etching times. The various phenomena observed are attributed mainly to the disorganization of the mica surface during etching, namely sputtering followed by re-deposition of the component elements, with loss of oxygen from the surface becoming apparent when the etching parameters were changed as indicated. A relatively high carbon content measured by XPS is always observed on the plasma-etched mica surfaces for which a low silicon content is usually found. The reasons for this behaviour are discussed. The oxygen deficit observed on the argon plasma-etched mica surfaces may be even more significant if one makes allowance for the possible contribution that surface-adsorbed species containing oxygen can make to the oxygen signal.

The propagation of light in a symmetrical optical waveguide structure comprising two identical ion-exchanged or sol-gel waveguides with a thin water film between them has been analysed using numerical modelling. The results show that in the case of ion-exchanged waveguides there is an increase of times in the field intensity at the surface of the waveguides for liquid film thickness of the order of a few tens of nanometres and times in the case of sol-gel waveguides. In the case of sol-gel waveguides the fraction of light power propagating in the liquid film is of the order of 10% of the total power propagating in the waveguide structure even at small separations of the waveguides. Thus improved sensitivity of signal detection for Raman and fluorescence waveguide spectroscopy can be realized with the proposed waveguide structure with high spatial resolution.

The pole profiles of quadrupole magnets are often made circular, with a radius that can be so chosen as to reduce the error in the field due to the 12-pole contribution. In some magnets the pole face consists entirely of a few plane surfaces. We have obtained an analytical procedure which optimizes the lengths and the angles of the straight lines to eliminate the 12-pole, 20-pole and 28-pole contributions to the field error.

Results of the decay of magnetization or after-effect function, b(t) are presented, for three ferrofluids of saturation magnetizations 0.09, 0.06 and 0.04 T and corresponding approximate packing fractions of, 0.22, 0.15 and 0.1, respectively, which show b(t) to be little affected by the packing fraction. b(t) was similar for all three samples with an initial rapid decay up to an approximate time of s and a time constant of s. Data from this time region are realized by the transformation of the frequency-dependent complex magnetic susceptibility data of the samples, , measured in the gigahertz frequency range. Because the complex susceptibility data indicate the presence both of relaxational and of resonance mechanisms, is modelled in terms of its parallel, , and perpendicular, , susceptibility components with corresponding relaxation times, and , respectively. is suitably modified to include a distribution of particle sizes, r, and anisotropy constant, K. The decay of magnetization has essentially two components, that due to magnetic moments which relax by the Néel relaxation mechanism corresponding to and that due to the precession of magnetic moments which relax by means of the transverse time, . By application of a varying polarizing magnetic field to the samples, which effectively reduces the contribution of the component, the variation in is shown to be in accord with that predicted by the equations of Coffey et al.

The thermal neutron diffusion cooling coefficient is a macroscopic material parameter which is needed for a description of the decay of the thermal neutron pulse in a medium and gives information on the diffusion cooling of the thermal neutron spectrum in a bounded volume. Experimental results from various measurements for Plexiglas (a hydrogenous substance) are overviewed in the paper. A method for a theoretical calculation of the parameter is presented. The formula used utilizes other thermal neutron parameters and a cooling function which describes the deviation of the neutron spectrum in a bounded system from the distribution in an infinite one. The energy dependence of the function is obtained numerically from relations which result from the eigenvalue problem of the scattering operator when both the decay constant and the spectrum of the thermal neutron flux are developed in powers of the geometrical buckling. The calculation utilizes Granada's synthetic scattering function. The obtained value of the diffusion cooling coefficient for Plexiglas is at the temperature of C. The uncertainty is estimated to be within the physical model of the scattering kernel used. Results obtained using the same type of kernel in other theoretical methods are also included.

A stack of Makrofol-N (Bayer Co., Germany) plastic track detectors has been exposed to a well collimated beam of ions with initial energy of 16.34 MeV/n at UNILAC, Darmstadt (Germany). The effect of etching temperature on the etching characteristics of some solid state nuclear track detectors (SSNTDs) has been investigated for the registration of ion tracks. An empirical relation between the etching temperature and the various etch rates has been established from the experimental data. A correlation between track etch rate and energy loss rate has been established which provides us with an understanding of the mechanism of track formation. The maximum etched lengths of ions have been experimentally measured using Makrofol-N track detectors. The corrected track lengths have been compared with the theoretical energy range calculations and are found to conform to them reasonably well.

We report on the operation of a 6 cm bore copper HyBrID laser which incorporates a high-magnification (M = 280) negative-branch confocal optical resonator and utilizes a magnetic pulse compression excitation circuit. 125 W total output was obtained at a pulse repetition frequency of 12.5 kHz, of which 101 W (i.e. 80% of the total) was high beam quality output. `Power in the bucket' measurements revealed that of the high beam quality output was within the diffraction limit (62 rad), and within the diffraction limit.

The asymptotical limits for light fluxes scattered by large spherical particles with various refractive indices into a fixed range of scattering angles are obtained. Calculations are performed within the framework of the Mie theory and the geometrical optics approximation. The geometrical optics approach, combined with approximations of the radiative transfer theory, allows one to make quick estimates of reflection and transmission functions of disperse media with large particles. The results obtained can be used for studies of light fields in random media with large particles within the framework of the small-angle and quasi-single-scattering approximations of the radiative transfer theory.

Modern society demands a stable electricity supply, the reliability of which is partly determined by the dielectric strength of the outdoor high-voltage insulation systems. This paper concentrates upon the experimental study of one particular discharge phenomenon that occurs when such a system is subjected to rain, namely the inhibited discharge. This is a discharge whereby the energy input into the discharge channel is limited by the presence of a large series resistance in the discharge circuit; in practice water drops or runnels can act as this resistance. The experimental arrangement was a rod-to-plane electrode gap of up to 1 m subjected to a switching impulse. A water resistor (up to ) was placed in series with the rod electrode. The experimental results reveal the negative feedback characteristics of such an inhibited discharge; that is, as the discharge propagates, the discharge current charges the rod electrode and lowers the gap voltage. This negative feedback gives an increase in disruptive discharge voltage, a low discharge current and a prolongation of the final jump phase.

The inhibited discharge is a discharge in which the input of energy into the discharge channel is limited by the presence of a large series resistance in the external circuit supplying the discharge current. One situation in which such a discharge occurs is in outdoor high-voltage insulation systems when the system is subjected to rain. The inhibiting resistance is caused by layers or runnels of water on the insulator's surface. This paper concentrates upon the numerical modelling of the inhibited discharge. The core of the modelling is the interaction between the discharge itself and the external circuit which supplies the discharge current. Physical models of the corona streamer development, the propagation of the streamer-leader system and the final jump were used for the calculation of the discharge current. The calculations show that propagation of the streamer is not influenced by a large series resistance, whereas the propagation of the streamer-leader system and the final jump may be significantly inhibited.

On real cathode surfaces which are in contact with plasmas, micrometre-sized protrusions are abundant. A two-dimensional numerical model has been developed to investigate the interaction of such a cathodic micro-tip with a low-temperature plasma. The model allows integration and analysis of the various physical effects and an evaluation of their specific role in the heating, emission and erosion of cathode surfaces. In particular, the influence of heating due to the intense ion flux from the plasma sheath onto the cathode surface has been investigated in detail. Special attention was paid to the simultaneous action of strong electric fields in the cathode fall region. The plasma-cathode interaction is analysed for a range of parameters typical for high-current-density, low-pressure gas discharges and vacuum arcs. The temporal evolution of the thermophysical properties of the micro-tip, its interaction with the resulting metal vapour and its shape are modelled on a nanosecond time scale. Special cases of the evolution of microemitters have been identified, such as quasi-stationary emission on a microsecond time scale and extremely fast, explosive-like behaviour with emitter lifetimes on a nanosecond time scale. It is found that ion bombardment heating plays an essential part in the inception of emissive and erosive behaviour for a wide range of parameters.

Here we present an equivalent circuit model for the study of the surface discharge induced plasma chemical process (SPCP). The simplified model consists of a series of gaseous and ceramic capacitors. The discharge characteristics of the SPCP device are investigated based on this model. The discharge current, the average length of the streamer filaments, the inception breakdown voltage, the density of the surface charges, the dynamic capacity and the effective energy associated with the discharge process are simulated. The computed results are in good agreement with the experimental data.

The distribution profile of sputtered atoms in the region above a hollow-cathode bore is calculated using a mathematical model and the results are compared with data obtained from atomic absorption spectroscopy studies of the same sputtered metal atom distribution. The smoothing of the distribution function calculated from the model using an experimentally derived instrumental function is detailed. The correlation between the calculated distribution profile and two reconstructed data sets is presented and this is related to experimental studies of the distribution profile. Details of the voltage-current characteristics of the sputtering cells for a number of different hollow-cathode bore geometries are also presented. These and the observed variation in sputtering damage along the length of the various hollow-cathode bores are shown to be related to the discharge properties exhibited by the sputtering cells used in these investigations.

Positive and negative dc corona discharges in CO-air and -air mixtures were applied. Natural humid air was used. The step by step development with time of the formation of gas products after the action of the corona discharge was measured in situ. The discharge tube was situated in an IR gas cell. The IR absorption spectra were scanned from the area of the inter-electrode distance in successive time steps of the action of the discharge (about 1 min).

Measurements were performed for three combinations of electrode materials, namely Mo-stainless steel, Mo-brass and Cu-brass. Reflection IR absorption spectra from the surfaces of the electrodes used were scanned after the action of the discharge. The influence of the electrode material on the development with time of the reaction products was observed. Polymer-metal complexes with possible catalytic activity are formed on the surfaces of electrodes. From measurements it resulted that the discharge processes consist of simultaneously acting volume processes of plasmochemical nature (probably initiated by electrons) and electrocatalytic surface processes on electrodes (probably initiated by photons).

The estimate of overall moduli is studied using a perturbative technique developed by Kröner (1967) and applied to a permeability problem. Correction terms to allow for the discrete character of a granular medium and the non-linearity of the flow are introduced. The combination of heterogeneity and non-linearity is shown to lead to the formation of direction-dependent features. It is shown how these can be predicted by theoretical means and an experiment in which these features have been observed is described.

Diffuse scattering of x-rays is applied to studies of microdefects in annealed highly doped crystals of GaAs:Te. X-ray diffraction reciprocal space mapping was performed by high-resolution x-ray diffractometry in the triple-axis mode. The most characteristic feature of the experimental maps is the lack of the zero-intensity lines for all high-symmetry reflections. Owing to this aspect of measured maps it was possible to determine the type of microdefects. The computer simulations performed for point defects with orthorhombic symmetry in annealed crystals of GaAs fit very well to the patterns of measured iso-intensity contours. The experimental data obtained so far give the possibility of determining the displacement field outside the defect core but are not sufficient to obtain information about the interior of the defect. The mean radius of microdefects was estimated to be smaller than 0.2 m.

Reconstruction from a finite set of projection data is known to be a highly underdetermined and ill-posed inversion problem. An application of the maximum entropy (ME) approach to this problem is considered here. The sense in which a ME solution can be regarded as an optimum estimator is analysed. Numerical examples of a two-dimensional reconstruction of test models chosen to simulate typical situations in diagnostic experiments are given.

Anisotropic and transient diffusion in spatially periodic and heterogeneous media is formulated. Based on the formulation, a method of numerical calculation is proposed as a useful tool in the design and analysis of heterogeneous materials. This method is applied to several binary systems including a honeycomb structure and a square grid with ellipsoidal filler. Calculation results strongly suggest that transient diffusive behaviour, which can be observed in some experimental systems, should be important to elucidate the internal structure of a heterogeneous medium. The time scale of this transient property of thermal diffusion in practical composite materials is estimated using a physical constant of polyethylene; it proved to be of the order of magnitude which is quantitatively detectable by experimental means.

Sorption kinetics in excess toluene and electron paramagnetic resonance (EPR) experiments on carbon black polymer composites show that these systems exhibit remarkable behaviours. Two types of carbon blacks, having very different primary structures, were used. The swelling ratio expressed as the mass of absorbed solvent divided by the mass of the dry sample in excess toluene has been measured over a wide range of carbon black concentration. The absorbency of these materials is strongly affected by the surface area of the carbon black and is related to the characteristics of the microstructure (porosity and tortuosity). The EPR spectra depend significantly on the structure of the carbon black aggregates. The peak-to-peak amplitudes and the line widths of the EPR spectra are suggested to be significantly affected by the surface area and the morphology of the carbon black. For Y50A samples (low-surface-area carbon black), the experimental observations are explained within the context of a heterogeneous gel: finely dispersed aggregates of submicrometre size dispersed in the polymeric matrix may be considered as additional cross links. For Raven 7000 samples (high-surface-area carbon black), it is suggested that much of the observed behaviour at low concentration can be explained in terms of the agglomerated structure increasing the tortuosity of the material and again by the heterogeneous gel picture for high concentrations.

Within this paper we propose a versatile Brillouin technique which allows an easy determination of acoustic and opto-acoustic properties of thin films. This new technique is especially suitable for studying acoustic dispersion behaviour at hypersonic frequencies. Typical applications in the field of polymers have been used to demonstrate the efficiency of the proposed scattering technique.

To make near-field detection cathodoluminescence investigations with a resolution in the nanometre regime feasible, a scanning near-field optical microscope (SNOM) has been implemented in the chamber of a scanning electron microscope. Two different modes of operation which can yield complementary information on the specimen can be carried out in our set-up. In the first mode, the SNOM probe position remains unchanged while the primary electron beam is scanned in raster fashion over the sample surface. In the second mode, the sample is homogeneously irradiated by the electron beam while being raster scanned underneath the SNOM probe. Whereas in the first mode specimen properties such as diffusion lengths are accessible, the achievable spatial resolution in the second mode depends only on the size of the aperture at the apex of the SNOM probe. A spatial resolution of well below 50 nm has been achieved on bulk YAG specimens in the second mode using commercially available probes.

We report experiments in which liquid mercury is subjected to tension by dynamic stressing, involving pulses of ca s duration, and stressing rates of approximately 1 bar . Under these conditions, the tensile strength of liquid mercury is found to be ca 2500 bar. These results are markedly lower than those obtained in experiments involving shorter pulse durations, and support the conclusion of other workers that in the dynamic stressing of liquids by tension, the higher the rate of stressing, the higher the measured value of tensile strength.