Table of contents

With the continued advance in the design and production of small solid state lasers and associated optics, future systems designed to achieve very-high-density storage of digital information are likely to become increasingly dependent on sophisticated optical technology. Non-erasable storage systems that have been accepted for many years (WORM, CD-ROM and so on) show a continuous growth of applications and sales. The first erasable storage systems, produced and marketed to an agreed world standard, are firmly based on magneto-optic recording technology. This review is intended to summarize the systems and materials that have made this technology possible.

The magnetic properties of a process heater tube were determined to evaluate the structural integrity of its use in the oil industry. Carbon diffusion inside the tube during processing of petroleum is a cause of concern in the oil industry because it leads to brittle fracture. Both magnetic hysteresis and magneto-acoustic emission (MAE) parameters were determined for three specimens made from virgin, laboratory-aged and service-exposed materials. It was expected that the coercivity would be a good damage parameter by which to assess the service-exposed material because it increased with carbon concentration. However, the observed insignificant difference in coercivity between service-exposed material and the virgin one ruled out the possibility of using coercivity as a suitable damage parameter. The present study indicated that saturation induction could be used to find the extent of carbon deposition inside the material. The RMS voltage (V_rms) of the MAE signal signal at low field was found to be higher in the laboratory-aged specimen due to greater interparticle distances which allowed domain walls to move longer distances before encountering a pinning centre. At high field, V_rms in the virgin specimen was found to be higher than that in laboratory-aged and service-exposed materials. That was attributed to the presence of higher anisotropy in the virgin sample, originating from the metallurgical processing.

It is proposed that a convenient choice of basis functions for applying the moment method to the analysis of scattering and/or mutual coupling between metallic bodies impinged upon by an electromagnetic field is that of the characteristic modes of a single scatterer. Due to the resonant behaviour of such modes, only a small number of them are required for an accurate representation of the current distribution even in the presence of mutual coupling. Results for the case of two-dimensional strips are reported and compared with those obtained with the conventional, direct application of the moment method.

In a waveguide laser, the laser light is guided in a hollow dielectric waveguide which also serves to confine the discharge. In this paper we study the radial distribution of the resonant field progressive components for such a laser, both theoretically and experimentally. To compute the resonant field distribution, we use a numerical method based on a field expansion in terms of a set of Bessel functions.

Continuous CO₂ lasers have been used for many years to weld a variety of materials. A problem of importance is the question of what is the thickest work piece that can be reliably welded with complete penetration occurring at all times. Empirical results indicate that the size of the keyhole radius at the top of the work piece is almost exactly three times the radius at the bottom in the case of maximum penetration. A mechanism based on the instability inherent in the variable absorption capabilities of the work piece when absorbing energy from a laser beam with an interference pattern which is itself the result of reflection at the wall of the keyhole has been suggested and the implications investigated. A simple model has been studied in which the beam has a uniform intensity at any given cross section; an analysis in terms of geometrical optics and a parallel beam supports the empirical observation. The effect of the curvature of the keyhole wall was studied and found not to make a great difference to the results of the theory. The same techniques were used to investigate the effect of the convergence and subsequent divergence of the laser beam as it passes through the focal plane. The estimates provided by the theory have been compared with experimental results and shown to agree very well with them.

The convective heat transfer coefficients h_el and q_el have been measured in gaseous air, O₂, N₂O, N₂, F-12 and F-22 using a single platinum wire (diameter 0.025 mm) mounted along the axis of a copper cylinder (diameter 53 mm). The measurements were carried out as a function of electric field, pressure, orientation and frequency of the applied electric field in the earth's gravitational field. The convective heat transfer coefficient increases with increasing electric field. A similar trend is noted when the pressure of the gas is increased. The heat transfer coefficient also increases with increasing T_d (the temperature difference between the wire and the surrounding medium). The behaviour of q_el at T_d equivalent to 0 is found to be similar to that for T_d>0. If is found that q_el always increases as the inclination of the cylinder is increased from the horizontal towards the vertical. The coefficient q_el is found to decrease linearly with frequency f.

This paper presents the latest results from calculations of the nonlinear development of instabilities in magneto-plasmadynamic (MPD) thruster flows. Linear instability analysis carried out at the Institut fur Raumfahrtsysteme of the Universitat Stuttgart yielded three basic instabilities for stationary MPD thruster flows: a space charge instability, an acoustic wave instability and an electron drift instability. Nonlinear analysis using the quasi-analytical method (QAM) developed by the authors not only yields these instabilities in the lower orders of the QAM hierarchy but also shows their nonlinear evolution in the higher order development, particularly that of the space charge instability which is actually shown to lead to current chopping. Calculation results for the normalized current density and electric field strength as functions of time for various relative gradients are presented.

The steady state expansion of a low-pressure plasma generated by a given Gaussian ionization profile into a dilute neutral gas is calculated using one-fluid and two-fluid models. The inertia of ions and electrons, collisions and widening geometries are taken into account. The plasma density n(0) at the centre or a so-called escape or drain factor and the effective collision frequency nu occur as intrinsic eigenvalues. Monotonic solutions of the basic equations occur if n(0) is below and nu is above certain threshold values. They involve a small positive space charge density in the bulk and a positive boundary sheath in front of an absorbing wall. Non-monotonic solutions exist if n(0) is great and nu is small. In this case a zone with negative space charge can occur between the plasma core and the sheath if nu does not vanish. Due to the collisions the transition between the two types of solution occurs continuously.

In this paper we present calculations of the spatial distributions of emission in model argon discharges which correspond to the conditions of argon discharges in the GEC rf reference cell but are relevant for other similar rf discharges. The calculations on the basis of the particle in cell (PIC) code show that transitions with high threshold energy are predominantly excited by secondary electrons originating from the instantaneous cathode while the transitions with lower threshold energy are excited predominantly by electrons accelerated by sheath motion. In addition it is shown that the spatial distribution of 811 nm radiation of argon corresponds best to excitation by very low-energy electrons from the metastable state. The different kinetics of excitation of the three groups of transitions mentioned above, in conjunction with different energy dependences of the cross sections and special conditions when there is a significant if not dominant contribution of the gamma process in sustaining the discharge, give rise to the different spatial distributions of emission which are opposed to the intuitively expected distributions and raise questions about the applications of spatial distributions of emission in determining the sheath width and in diagnostic techniques such as actinometry.

Three-dimensional, unsteady behaviour of high-pressure electric arcs in argon is studied by means of numerical simulations. Attention is paid to argon arcs whose steady structure is fairly well understood. Results are reported for the case of a one centimetre long arc, burning in argon and driven by a total current of 200 A and 300 A. The influence of the boundary conditions for the electrical potential on the macroscopic structure of the arc is studied and it is found that the distribution of the current density near the cathode is one of the critical parameters which can significantly modify both the distribution of mean quantities and the stability of the arc. All mean quantities found from three-dimensional calculations are compared with two-dimensional axi-symmetric fields obtained by a previously used code for arc simulations. The results indicate good agreement between the results from three-dimensional calculations, two-dimensional axi-symmetric simulations and results from physical experiments documented in the literature.

This paper reports a study of the origin and propagation of the space stem when a bi-exponential voltage impulse is applied to a 1.3 m point-plane air gap. The tests are carried out by using impulse shapes ranging from 0.25/2000 to 120/2000 mu s. Among these impulse shapes, the optimum with respect to the observation of the space stem formation is the 0.3/2000 mu s one. Some detailed photographs obtained with an image converter allow one to identify the following phenomena: negative streamers, a Trichel zone, a negative corona, a cathodic zone (cathodic stem), positive streamers and space stems. The current measurements allow one to quantify some energetic thresholds. The 5/2000 mu s impulse shape leads to a quasi-continuous space stem propagation in the gap. Thus it is possible to study the role of the positive and negative streamers in the space stem's behaviour, which is defined as their zone of origin. A stem replica mechanism is explained and some associated energetic criteria are discussed.

A technique for observation of the inner structure of objects is proposed. The sensitivity to variations in direction of the primary pseudo-plane beam which occurs on boundaries between media with slight differences in density values is impressive. Two types of contrasts are pointed out: 'area contrast' caused by slight deviation of the primary beam and 'boundary contrast'. Basic principles and experimental results are presented.

The pronounced glass-forming tendencies of alloys of Se and Ge with Sb are discussed topologically in terms of the chemical bonds expected to be present in these materials. Using a simple consideration based on coordination numbers (m), and bond energies, the average number of near neighbours of each type expected to surround atom has been estimated. These average numbers of bonds have been used to estimate the cohesive energies (CE) of these glasses, assuming simple additivity of bond energies. A trial also has been made to explain the previously published data of threshold voltage (V_th), optical properties (E₀₄) and glass transition temperature (T_g) of these materials; on the basis of our calculations the correlation between T_g and E₀₄ has been interpreted in terms of a proposed fluidity equation for covalent liquids with 2.3<or=m<or=2.49.

X-ray diffraction from simple monocrystalline and epitaxial multilayered surface gratings will be described theoretically by use of a distorted wave Born approximation of second order. The intensity patterns of simple and multilayered gratings have been simulated by this treatment and compared with the kinematical approximation. The gratings act simultaneously as reflection and transmission gratings. This gives rise to Umweganregung also between the grating and the dynamical diffracting planar structure including the substrate.

This work deals with the analysis of the electric impedance of piezoelectric ceramics embedded in viscoelastic materials. By using a theoretical approach, we show that this parameter is a function of the viscoelastic properties of the medium surrounding the piezoelectric ceramic. Then a method for extracting the viscoelastic properties of the outside medium from the piezoelectric ceramic electric impedance is described. Finally, this technique is used for monitoring the polymerization of epoxy-amine resins.

Carbon nitride thin films with high nitrogen content have been grown on various substrates using low-power radio frequency reactive sputtering of graphite in pure nitrogen plasma. A quantitative compositional analysis using Rutherford backscattering spectrometry shows that, apart from a low concentration of oxygen (about 4 at%), the films contain about equal amounts of carbon and nitrogen, the highest C:N ratio yet observed in sputtered thin films. Electron energy loss spectroscopy and Raman spectroscopy measurements reveal a graphite-like structure.

We have measured the spectral momentum densities of thin foils of diamond-like carbon using (e,2e) spectroscopy. Transmission electron energy loss spectra and (e,2e) spectra were measured before and after annealing a thin foil at around 900 degrees C and before and after thinning the foil using reactive ion etching in an argon-oxygen plasma. The valence band spectral momentum densities are compared with spherically averaged graphite and diamond band theory calculations. After annealing the surface sensitive (e,2e) data are closer to the graphite theory for the foil. Before annealing and also after plasma etching the (e,2e) data compare more favourably with the diamond theory. Bulk-sensitive transmission energy loss spectra for the annealed sample show a weak graphitic plasmon at around 6 eV energy loss which disappears after subsequent plasma etching. These measurements show that the diamond-like carbon films become graphitic only at the surface after annealing and that the graphitic surface layer can be easily removed by reactive ion etching.

Porous silicon layers formed on p⁺ substrates were investigated by secondary ion mass spectrometry technique. It has been found for the first time that the dopant impurity (boron) does not escape from the porous silicon (PS) layer during the anodization. Analysis of the boron excess in PS in relation to the crystalline substrate provides a means by which to study the porosity depth distribution. The resistivity of the PS layer has been observed to depend strongly on depth: in particular, in the high-porosity layers (50-65%) a relatively thin insulating sublayer (about 300 nm) is found to form at the surface. After annealing of the PS layers in an oxidizing atmosphere, the high-resistivity regions disappear.

We have studied the charge transfer to polypropylene films containing varying concentrations of amide (donor) and fluorine (acceptor) groups grafted onto the polymer backbone. Pure polypropylene charges very weakly but the addition of amide causes the charging to become strongly positive, and fluorine causes it to become strongly negative. The magnitude of charge density is similar in the two cases but is small compared to the density of donor or acceptor groups. Moreover, the charge transfer is independent of donor or acceptor density over a wide range of composition, indicating that charge transfer is limited by a feedback mechanism (such as back-tunnelling) during the separation of the metal and polymer surfaces. When gradually increasing amounts of fluorine or chlorine are added to a nylon surface (so that donors and acceptors are simultaneously present) the magnitude of the charge transfer is in general similar to that found for polypropylene with amide or fluorine groups, but depends on the relative concentrations of donors and acceptors. We show that this observation is also compatible with feedback mechanisms such as back-tunnelling. However, the precise nature of the feedback mechanism has not been ascertained.

We have studied the determination of the activation energy of thermally stimulated luminescence (TSL) peaks of arbitrary order of kinetics by using a method based on their points of inflection. The method has been applied to both numerical and experimental TSL peaks.

Pyroelectrically induced electron emission (EE) is studied from LiNbO₃ crystals with a layered domain structure fabricated under various diffusion treatments. It is observed that the appearance of a thin oppositely polarized surface layer at the +C-surface suppresses EE. For bidomain configuration with equal thicknesses of the inverted and the original domains EE occurs from the boundary between two domains. It is proposed that a potential barrier for emitted electrons depends strongly on the spontaneous polarization direction at a free polar surface.

Ionized cluster beam deposition uses a beam of ionized, accelerated atom clusters to grow thin films. Late in 1991, following nearly 20 years of development, synthesis of large clusters of zinc by homogeneous nucleation was achieved through significant changes in two source parameters. Crucible pressure was increased from 2 Torr to over 1000 Torr and the nozzle was changed from cylindrical (1 mm*1 mm) to converging-diverging (18 mm long and 0.4 mm in diameter at the throat). Such high vapour pressure is presently impractical for materials of industrial interest such as gold, silver, copper and aluminium. Recent results using gold with the converging-diverging nozzle at a pressure of less than 1 Torr showed no significant synthesis of large clusters. The work presented here describes results using silver at an intermediate pressure of around 100 Torr. The deposition rate was intermediate (2.2 nm s^-1) between that for gold (0.02 nm s^-1) and that for zinc (4.3 nm s^-1). Significant numbers of large clusters were not detected. The purity of the silver influenced the appearance of the beam. X-ray and roughness analyses of deposited films are presented.

Textbooks on lubrication show that a centrally pivoted hydrodynamic plane slider bearing can carry no load, and imply that in practical bearings the pivot must be offset. In practice offset pivots are the exception. This is attributed in the technical literature to a complex combination of thermal and elastic distortions. We show that operation is in principle possible purely as the result of the hydrodynamic pressures bending the pad to a desirable shape. Calculations for a wide pad suggest that successful operation requires only that a parameter p/⁴/(E/h_min) lie within the very wide range 20-330.

We propose and study a new technique for estimating the weight of very heavy objects. This new technique is based on measurements made by a device, a gravity gradiometer, that is capable of measuring subtle changes in the gravitational field produced by an object with respect to distance. The physical observables, known as gravity gradients, depends upon the mass density distribution of an object and the relative distance between the object and the measurement device. We show that, for very heavy objects, gravity gradient measurements can be used to make accurate weight estimates. This technology could be potentially important in the eventual implementation of arms control agreements or for application in the transportation industry.

Domain reversal in LiTaO₃ with period 6.5 mu m and 1:3 duty cycle has been obtained by using a partially switching technique in a 0.25 mm thick sample. A CW second-order quasi-phase-matched second-harmonic-generation experiment yielded a 12 mu W, 407 nm blue light output for 200 mW, 814 nm fundamental input. The technique can be used for the fabrication of the periodic poled LiTaO₃ for bulk violet and ultraviolet second-harmonic generation and for the fabrication of other ferroelectric microstructure materials.

We inserted pieces of metal foils (Al and Au) into a metal Berthelot tube together with water or xylene to investigate whether corrosion of metals limits the achievement of negative pressure. For the test using Al foil with water, the negative pressure rose to -30 bar but diminished almost to zero after 250 cycles, whereas those for all the others increased with the number of temperature cycles. Corrosive reaction of Al with water yielded H₂ gas and the resulting increase in the gas concentration in the water enhanced the nucleation probability of cavitation, not on the metal surface but in the solution. In order to attain high negative pressures for water in a metal tube, we should take into account a possible restriction of this achievement of negative pressure by corrosion of metals in contact with water, even though the metals used were well de-gassed beforehand.