Table of contents

The collection of free electrons in an ionization chamber results in a deficit of negative ions. The recombination of the ions in the chamber gas is therefore smaller than maintained by the classical recombination theory, which assumes an equal number of positive and negative ions. Under certain conditions the current induced by the electrons in the outer circuit of the chamber can be measured. If the lifetime and the drift velocity of the free electrons in the chamber gas are known, the collected electron-charge and therefore the deficit of negative ions can be found. The determination of the lifetime and the drift velocity is achieved by analysing the decay of the induced current following a short radiation pulse.

In studying multiple scattering of waves by a half-space of distributed discrete scatterers, the approach of the quasi-crystalline approximation together with hole correction or pair distribution functions has been used extensively. In this approach a system of simultaneous equations must be solved to determine the effective propagation constant and the expansion coefficients of the coherent exciting field. In this paper, we analyse the same problem under Foldy's approximation by using the so-called modified T-matrix approach. Two simple and clear equations are obtained for determining the effective propagation constant and transmission coefficient of the coherent transmitted field when the scatterers are identical spheres. The expressions for the coherent reflected field and incoherent scattered intensity are also given. It is shown that, in the limit of low frequency and sparsely distributed scatterers, our solutions reduce to the well-known results. The numerical results for ka=0.2 also show that our results with the modified T-matrix and Foldy's approximation are better than Foldy's approximation alone, but are still worse than the quasi-crystalline approach with pair distribution functions and Monte Carte simulations because we have not incorporated the pair distribution functions into our modified T-matrix approach. It should be pointed out that the modified T-matrix approach can be generalized to the case of pair distribution functions in which the exciting field is also travelling with the effective propagation constant.

In studying multiple scattering of electromagnetic waves by distributed discrete scatterers, the 'two exterior spaces' T-matrix or the modified T-matrix is needed. In this paper, the modified T-matrix formulae for a scatterer of arbitrary shape are derived, based on Huygen's principle of scattered waves and the method of optimal truncation respectively. Analytical expressions are given for the modified T-matrix elements of a spheroid in the low-frequency limit. Agreement with existing results is shown to be exact to the given order of ka. It is also shown that the results based on Huygen's principle are merely special cases of those based on the method of optimal truncation.

The acceleration of a thin layer mounted on the rear surface of a thick transparent slab target was shown to be feasible with a laser of moderate energy. Both long (about 1 ns) and short (about 25 ps) laser pulse interactions were investigated. Using a single laser beam with 2 J in a 1 ns pulse it was possible to accelerate an Al layer of thickness <or=1 mu m up to a velocity of 2*10⁶ cm s^-1. Atomic beam velocity distributions were measured with a time-of-flight neutral particle detector and a time-gated micro-channel plate neutral particle pin-hole camera. The expanding atomic beam was confined to a very narrow cone, being directed along the normal to the target rear surface and independent of the laser beam axis. Time-resolved imaging of the target rear surface in different spectral bands provided measurements of its temperature and the characteristic scale-length of the atomic beam emission area.

Asymmetric apodization is achieved by adding edge strips or rings to the diffraction slit or circular aperture. It is shown to be equivalent to the superposition of a diffraction field of the main entrance aperture and an interference field of the two edge strips or rings. For further improvement, apodization efficiency is defined as the ratio of integrals of the irradiance square after and before apodization. New designs are proposed to improve side-lobe suppression. The results after adjusting suitable parameters show that the apodization efficiencies are enhanced.

The previously developed three temperature approximation for the analysis of the processes in the industrial fast-axial-flow CO₂ laser is applied to the general three-dimensional (3D) modelling of the compressible flow in the laser cavity. The 3D geometrical representation and the compressible flow formulation allowed us to describe the realistic flow pattern and the distribution of the laser parameters. Our model predicts the development of the velocity profile along the laser from one which is near parabolic due to turbulent jet impingement, to one which is representative of a turbulent pipe flow. The translational temperature, T, the vibrational temperature of the symmetric stretch mode of CO₂ T₁, (almost equal to the vibrational temperature of the double degenerate bending mode of CO₂, T₂), the vibrational temperature of the asymmetric stretch mode of CO₂, T₃, and the vibrational temperature of N₂, T₄, are shown to increase along the laser axis except under the inlet openings, with T reaching about 360 K, T₁ approximately=T₂ reaching about 400 K, and T₃ and T₄ reaching about 3000 K. The values of T and flow velocities away from the inlet openings coincide within the accuracy of about 10% to those predicted by the 2D model. The average values of T₃ and T₄ (about 2500 K) seem to be in better agreement with experimental observations than those predicted by the 2D model. The predicted population inversion reaches its maximum value of about 5*10²⁰ m^-3 inside the recirculation zones between the inlet openings. We believe that our approach to the modelling of the processes in the CO₂ laser may have a wide range of scientific and industrial applications far beyond the particular type of laser discussed in this paper.

A microwave-excited (2.45 GHz), diffusion-cooled CO₂ laser, capable of operating in a quasi-cw mode, has been developed. By separating the discharge structure from the feeding RF waveguide and using distributed coupling, previous problems with local contracting discharges could be surmounted. For effective diffusion cooling a planar 1.9 mm high discharge channel was used, a structure which easily allows area scaling of laser output power. Two discharge channel designs (metal/glass and all ceramic) were investigated, differing mainly in thermal conductivity and waveguide losses for the 10.6 mu m laser radiation. A conventional magnetron for microwave ovens was used as microwave source. System design and extensive measuring results are presented. With a first prototype non-optimized in gas mixture, optical resonator and waveguide surface quality laser output power above 20 W was obtained with an efficiency of about 10%.

Numerical studies are made on the spatial and temporal profiles of the second harmonic pulse with consideration of both thermal absorption and intensity dependent refractive index for various thermal relaxation times. The two effects deteriorate the second harmonic generation efficiency and also make beam profiles much different from that of the fundamental wave. if the signs of the refractive index gradient differ between the two contributions of different origins, they may counterbalance each other to help reduce deterioration of the second harmonic generation efficiency.

The heating and cooling in a vortex tube is attributed to conversion of kinetic energy into heat and to the reverse process. A two-component model yields the upper limit for the temperature increase on the hot side: (T_h-To)/T₀<or=X( gamma -1)/ gamma , and a lower limit for the temperature reduction on the cold side: T_c>or=T₀(1-X)^{( gamma -1}/ gamma ), where gamma =1.4 for air, and X=(p₀-p_c)/p₀ is the normalized pressure drop between the inlet (p₀) and the cold exhaust port (Pc) Extensive experimental data for a vortex tube of 18 mm inner diameter with working fluid air all fall between these limits. The model predicts that the inlet velocity reaches the speed of sound for X=0.7. No values X>0.7 could be obtained in these experiments, indicating that the flow always remain subsonic.

Neon was excited intensely with an electron beam. The densities of the excited species Ne₂⁺, Ne(3p), Ne(³P₂) and Ne₂(³ Sigma ) were investigated using absorption spectrometry. Varying the density of the primary excitation caused large changes in the temporal development of the excited species Ne₂⁺ and Ne(³P₂), while the density of Ne₂(³ Sigma ) remained constant. The results are reproduced by a kinetic model. The limits for efficient production and maximum attainable density of rare gas excimers with respect to the input energy are discussed.

This paper deals with the modelling of the medium pressure Hg and Hg-Ar positive column (Hg pressure range 5-350 Torr). The aim of this work is to simulate, in a first approximation, the middle phase of the Hg high-pressure lamp warm-up. In this approach, the main assumption is that time evolution of the discharge can be divided into a succession of stationary sub-phases characterized by the Hg partial pressure. Thus, we present here a self-consistent steady-state collisional-radiative model describing the middle sub-phase. This model includes volume recombination of Hg atomic and molecular ions, as well as several atom-atom inelastic scattering mechanisms. Calculations are carried out for both pure Hg and Hg-Ar discharges. Our results, which are in good agreement with experimental data from the literature, confirm that plasma thermalization occurs in the middle start-up phase (electron and gas temperatures become equal during this phase). Furthermore, this simulation shows the importance of different elementary processes, like atom-atom inelastic scattering, for the medium pressure plasma description. In fact, these mechanisms cannot be neglected until LTE conditions are reached.

A common flowing post-discharge of an Ar-N₂ plasma and an Ar-H₂-CH₄ plasma has been used to produce nitrogen and carbon atoms. The concentrations of the nitrogen and carbon atoms have been determined by NO titration and from measurement of the vibrational band intensities of the spectra emitted by the N+N and C+N chemiluminescent reactions. For plasmas created with microwave powers of 40-100 W and for total gas densities of 5*10¹⁷ cm^-3 typical values for the atom densities of 10¹⁵ cm^-3 for nitrogen and 10¹¹-10¹³ cm^-3 for carbon are obtained after a residence time of 25 ms in the post-discharge.

Measurements of ion energy distribution functions were made at the grounded electrode of a parallel plate radiofrequency discharge by means of a gridded energy analyser in discharges in Ar, He, and O₂ at pressures of the order of 0.001-0.1 mbar. In this regime the transition from the collisionless space charge sheath (in front of the electrode) to a sheath dominated by collisions, mainly charge exchange collisions, appears. Although the Debye length is small compared to the mean free path for charge exchange, for high sheath voltages the sheath thickness increases and may exceed the mean free path. Measured distributions in Ar are compared to energy distribution functions calculated in a self-consistent sheath model, including charge exchange collisions. The agreement of theory and experiment is very good.

Investigations of the near-cathode region of the atmospheric pressure, 200 A arc burning in pure argon have been performed using spectroscopic techniques. Several slices at different distances from the cathode tip have been observed side-on. In order to perform diagnostic measurements of the plasma, the profiles of the Ar I and Ar II lines have been recorded. Phenomena inexplicable in terms of the model based on assumption of a local thermodynamic equilibrium state and constant pressure in the plasma have been observed and are shown on an Olsen-Richter diagram.

We present the current and voltage dynamics of a cathode sheath when a fast downward step of negative voltage is superimposed on the constant cathode bias. Owing to the very different reaction times of electrons and ions, the early interval of the dynamic can be used to measure the stationary cathode sheath width. The method is applied to an abnormal glow discharge in oxygen.

An analytical model based on the solution of the Boltzmann equation and the theory of ion avalanches has been developed for the energy distributions of ions and neutrals created in the cathode fall of a gas mixture glow discharge. The model allows the determination of the energy distributions and flux densities of all possible species, including multiply charged ions. The use of the model for evaluation of real gas mixture direct current systems is discussed, and it is shown that calculations of distribution functions achievable from the model in a few seconds agree remarkably well with the results of other models based on Monte Carlo simulations and with the limited experimental results available.

A numerical investigation of recombining process in a high-temperature plasma flow in a quasi-steady state (electron temperature about 10 eV, electron density about 10¹⁴ cm^-3 and flow velocity about 10⁶ cm s^-1) was performed under gas contact cooling conditions using the collisional-radiative model, in which gas cooling is mainly due to surrounding gas particles freely coming into the plasma column. The calculation has shown that the electron temperature relaxes in accord with experimental results and also that the occurrence of the recombining region and the inverted populations almost agree with the experimental ones. Atomic processes responsible for gas cooling are discussed.

The motion of arc spots on electrodes in the presence of a magnetic field is analysed by considering the concentration of current in the arc spot as the final state in the growth of an instability. Linear perturbation theory is used to predict the properties of unstable waves in the plasma above an electrode, and the results are assumed to apply to the arc spot. It is found that various features of the experimentally observed behaviour are predicted by the linearized treatment, including retrograde motion. A simple physical interpretation of retrograde motion follows from the model: retrograde motion is caused by the Hall effect and a voltage-current characteristic with negative slope at the electrode sheath.

The temperatures of the cathode surface and of the plasma for an atmospheric-pressure high-current free-burning argon arc have been measured for a range of cone angles for cathodes of thoriated tungsten. These measurements have shown that both the surface temperature of the cathode and the temperature of the plasma depend strongly on the cathode shape. For cathodes with conical shape, plasma temperatures were found to be a maximum for a cathode cone angle of 60 degrees .

This paper proposes a method that presents simply the relationship between the space-charge voltage drop, the electron temperature at the plasma boundary and the cathode temperature for high-intensity arcs of thermionic cathodes. According to this method, the cathode space-charge voltage drop V_s is estimated. The results are compared with indirect measurement and numerically analysed results and good agreement is obtained. The results have shown that V_s ranges from 2 to 4.5 V for the Th-W thermionic cathode of argon arcs at atmospheric pressure. To a great extent V_s is decided by the boundary electron temperature, which means that the boundary electron temperature plays an important role in controlling the arc cathode energy balance. It has also been found that, under the conditions concerned in this paper, the kinetic energy transfer of electrons emitted from the cathode by elastic and inelastic collisions with heavy particles at the plasma boundary is negligibly small due to the very low collision probabilities and/or the very small mass ratio, while energy transfer by electron-electron collision is dominant. This may be the reason why the cathode space-charge voltage drop is below the ionization voltage of arc atmosphere.

The decay of picosecond-laser-induced thermal gratings in rather thin liquid crystal films is investigated. It is shown that, in contrast to the case samples several tens to hundreds of micrometres thick, the relaxation of thermal gratings in liquid crystals of about 2 mu m thickness, stacked between two semi-infinite glass substrates, is mainly determined by transverse heat transport processes through the cell surfaces instead of longitudinal diffusion in the direction of the grating wavevector. Experimental observations are compared with theoretical considerations and numerical solutions of the heat flow equation.

The Rutherford backscattering/channelling technique of 2.1 MeV He ions has been used to study the damage distribution in Si irradiated with 1.0 MeV Ti ions to a dose of 5*10¹⁴ ions cm^-2 under 7 degrees and 60 degrees incidence. A dechannelling analysis of multiple scattering has been made. The longitudinal damage straggling and lateral damage spread are estimated. The values obtained are compared with transport of ions in matter. The results show that the differences between experimental and calculated values for longitudinal damage straggling and the lateral damage spread are 25% and 28%, respectively. Also, it is observed that the damage distribution shape under 60 degrees irradiation is found in good qualitative agreement with an earlier simulation.

The decay of the surface potential on a charged insulating film, which is inhomogeneous across its thickness, is theoretically analysed by considering the injection of surface charges into the bulk and their subsequent transport through the film towards the grounded substrate. The present analysis models xerographic discharge that involves unipolar transport. It is assumed that during transport the carriers interact with a monoenergetic set of traps from which there may be release. A second-order partial differential equation is derived for the electric displacement D(x,t) which describes the time evolution of D at any point in the sample. This partial differential equation is numerically solved for the xerographic time-of-flight (XTOF) condition which requires the injection of a small fraction of the surface charges into the film by pulsed photoinjection. The sample is assumed to be composed of a double-layer photoreceptor type device where the first layer has a lower drift mobility than the second layer. The drift mobility, trapping and release times as well as the dielectric constant vary across the interface from one layer to the other. The numerical calculations provide insight into the interpretation of XTOF signals obtained from double-layer devices and demonstrate that the main peak in the photocurrent is due to prolonged injection into the second layer as a result of dispersion in the first layer.

The peculiarities of the photocapacitance effect in dry processed Cu₂S-CdS heterojunctions have been investigated. The light flux dependence of capacitance is analysed. The main effect of low-level illumination is the narrowing of the 'intrinsic' layer that is formed as a consequence of Cu diffusion from the Cu₂S layer. It was found that, in photocapacitance effects, a major role is played by the intermediate recombination centres present in the near-interface region.

Temperature-induced phase transitions in the 'cluster' magnetic bubble lattices on thin ferrite-garnet films were investigated. The term 'cluster' magnetic bubble lattice is used to denote a structure containing a large number of disoriented hexagonal lattice clusters. To explain discontinuous and continuous transitions from the cluster lattice to the structure consisting of two phases (stripes and lattices), the thermodynamic method was used. Transition temperatures were calculated and showed good agreement with experimental results.

A kinetic model of time-dependent dielectric breakdown for polymers is presented. The micromechanism of thermally activated bond-breakdown is developed for polymers to account for the dynamical process of dielectric breakdown. We first prove that the thermally activated polymer bond-breakdown processes can explain not only the burst of a conducting pathway nucleated in any defect and the following successive propagation, but also the time-dependent dielectric breakdown strength of polymers. The formulae for conducting microcrack growth rate and time to failure are derived and applied to the experimental data for polyethylene terephthalate films.

A new approach based on single frequency measurement is suggested to estimate the double relaxation times, tau ₁ and tau ₂ of some highly non-spherical polar liquids in solvents benzene and carbon tetrachloride. The smaller relaxation time tau ₁ refers to the smallest flexible part attached to the parent molecule while the larger one tau ₂ is due to end-over-end rotation of the polar molecule. The weighted contributions c₁ and c₂ towards relaxations are calculated from Frohlich's equations as well as by a new technique adopted here. The dipole moments mu ₁ and mu ₂ are also computed from the slope beta of the ultra-high-frequency conductivity K_ij against the weight fraction omega _j for the compounds in terms of tau ₁ and tau ₁. The close agreement of mu ₂ values thus computed with those of existing methods immediately indicates that the approach suggested is correct.

An ultra-sensitive photomultiplier and large-area light collection optics were used with a high-sensitivity charge-coupled device camera to detect and image the long-term light emission behaviour during electrical tree initiation in pin-plane CT200 epoxy resin samples under constant alternating current voltage stress. The observed light emission is interpreted in terms of electroluminescence due to charge injection/extraction at the pin-tip followed by slow material degradation and eventual failure of the metal pin-resin interface. Interface failure is a prelude to formation of microchannels that result in tree initiation. High-resolution charge-coupled device imaging of the electroluminescence reveals that the maximum light emission occurs at the pin-tip and is still observable 10 mu m into the resin. Under long-term continuous electrical stress, the light emission is found to spread principally along the cone of the pin rather than into the resin.

Poly(vinylidene fluoride/trifluoroethylene) copolymers of different molar ratios in the range between 60:40 and 80:20 have been studied by thermally stimulated processes, namely thermally stimulated depolarization current, thermally stimulated current and differential scanning calorimetry, and by dielectric constant measurements. The results obtained show unequivocally that a ferroelectric-to-ferroelectric phase transition exists for 70:30 and 75:25 copolymers, in addition to the well-known ferroelectric-to-paraelectric transition, which has already been observed in all copolymers.

A method of evaluation of the degree of inhomogeneity of the spatial distribution of matter is presented. It may be used to evaluate the most common form of spatial inhomogeneities in which fine particles, their aggregates or metallic islands are present in an otherwise homogeneous medium. Three micrographs of gold films having a variety of structures were examined with this method. All of them show deviations from randomness towards an inhomogeneous spatial distribution of islands.

The optical properties of amorphous GeSe₂, GeSe₄, GeSe₂Tl and GeSe₄Tl films, prepared by thermal deposition from their bulk glasses, have been studied. The respective effect of adding thallium to a-GeSe₂ and a-GeSe₄ results in increasing the density, d, from 4.16 to 6.2 g cm^-3 and from 4.34 to 5.72 g cm^-3, the refractive index, n, from 2.42 to 2.78 and from 2.43 to 2.62, whereas decreasing the glass transition temperature, T_g from 395 to 192 degrees C and from 165 to 134 degrees C, and the optical energy gap, E_g, from 2.07 to 1.54 eV and from 2.02 to 1.48 eV. The obtained results are discussed in the light of the proposed structural models of amorphous Ge-Se. The decrease in the energy gap with the introduction of thallium involves a change in the microstructure accompanied by a decrease in the chemical ordering of the stoichiometric compound a-GeSe₂ and an increase in the amount of chemical disorder in the composition of a-GeSe₄. The pronounced decrease of the gap (about 25%) is attributed to an alloying effect rather than to a hypothetical increase of the gap state density.

The distribution function of electrons in metal affected by ultra short laser pulse radiation is obtained. The effect of the deflection of the distribution function from the equilibrium one is analysed by an example of photoelectron emission and 'hot' luminescence.

Based on the Brinkman-extended Darcy model, the main objective of this paper is to predict theoretically the effects of viscous shear stresses on the static and dynamic characteristics of long porous journal bearings. The static and dynamic characteristics obtained from the model, as the value of permeability parameter (D approaches zero,give good agreement with those of a solid bearing and hence provide support for the present predictions. The results show that the influences of viscous shear stresses on the static performance and the stability limits of porous journal-bearing systems are apparent and not negligible. In comparison with those of the slip-flow model and the Darcy model, the viscous shear effects of our model significantly provide an increase in the load capacity and static attitude angle as well as a reduction in the friction parameter of long porous journal bearings. Moreover, the long bearing system using our model under small disturbances is more stable than those of the slip-flow and Darcy models; and the quantitative effects on the stability criterion are more pronounced for the low-upsilon and high-epsilon_s. cases.

It is difficult to interpret acoustic emissions generated within stressed fibre composites. To obtain a better understanding of the characteristic features of acoustic emission at source in carbon fibre reinforced plastics, a number of experiments have been performed to measure the emission from the fracture of single carbon fibres alone and of single fibres in microcomposites. An exponentially shaped steel cone was used as a waveguide to couple a single fibre to a transducer and tensile loading was achieved by slowly adding water to a container suspended from the sample. The microcomposites consisted of a single carbon fibre surrounded by a small number of glass fibres, all bonded by acrylic or epoxy resin. Measurements were made on five different types of carbon fibre. For freely mounted single fibres the amplitude of the acoustic emission produced at fracture was high, and it increased approximately linearly with strain energy in the fibre prior to fracture. A similar carbon fibre in a microcomposite suffered a succession of failures, and the amplitudes of the emission were lower, in qualitative agreement with the strain energy being lower at fracture. Results are also given for the acoustic emission produced during torsion tests on a microcomposite. In these samples the fibres remained intact and emissions were produced by debonding and/or matrix cracking.

The absolute densities of the nitrogen metastable molecules N₂(A³ Sigma _u⁺, v=0) were measured as a function of time using a high resolution laser absorption within the first positive system (B³ Pi _g(v^'=2) from A³ Sigma _u⁺(v^"=0)) in a stationary afterglow of the pure nitrogen pulsed electric discharge. The experiments were performed at various pressures (0.25-1.25 Torr), discharge currents (0.4-1.3 A) and pulse lengths (5-30 mu s). The lifetimes and populations are in the range 100-470 mu s and (0.3-2.7)*10¹² cm^-3 respectively. The postulated kinetic model describes the measured lifetimes within an experimental error. The model takes into consideration the loss of N₂(A) metastables due to energy pooling, quenching by nitrogen atoms and diffusion to the walls. Calculations of dissociation rates and nitrogen atom densities are based on the vibrational excitation of the ground electronic state N₂(X).

A theoretical model for growth of porous anodic films during electrochemical treatment of metallic and semiconductor materials is developed. It is found that the characteristic parameter determining morphology of porous anodic films is the rate of electric-field-enhanced dissolution of the electrode material at pore bases. Depending on its value, pores with different morphologies (needleor bottle-like, cylindrical, branched, and so on) may be formed. Particular cases of pore nucleation and steady-state growth are considered, taking into account features of the electric field distribution at the pore bases, the density of pores and anodization process variables. The suitability of the model for interpreting the experimental data on porous anodic film growth is demonstrated in the cases of fractal pore formation and growth of porous silicon layers.

A relationship relating aerodynamic diameter to volume equivalent diameter has been developed, which enables the fractal dimension of aerosol aggregates to be determined. The fractal dimension for carbonaceous smoke, under different formation conditions, ranged from 1.40 to 1.96, whilst for magnesium oxide smoke a figure of 1.08 was found. This is in agreement with observations based on scanning electron microscope examination, in which carbonaceous smoke clusters varied in degree of compaction and magnesium oxide smoke clusters were found to be chain-like. The results suggest that the fractal dimension is dependent on cluster material but independent of cluster size and show that smoke clusters forming under similar conditions have the same fractal dimension.

A new mechanism is proposed which accounts for the formation of a positively charged ion sheath in RF inductive discharges, and which allows the quantitative prediction of the existence of high plasma-to-wall potential in such discharges. In this paper we consider electron oscillations in crossed magnetic and magnetic-field-induced electric fields for both collisionless and collisional plasmas, under conditions relevant to plasma processing and gas discharge lamp applications.