Table of contents

The three-dimensional, time-dependent temperature distribution in a moving solid of finite thickness due to a laser beam with a Gaussian power distribution on its surface is investigated. The effects due to latent heat and the keyhole are neglected. The model is determined by Peclet number Pe=r₀U/2 kappa , a non-dimensional melting temperature Theta _m= lambda r₀ pi ³2/(T_m-T₀)/P and a non-dimensional thickness xi ₀=h/r₀. An upper limit for the time needed to establish the steady state is 0.1 s in the case of iron for all travel speeds. Accounting for the effects due to the finite thickness of the specimen is essential for the thin metal sheets used in high-speed laser welding. Asymptotic solutions for high Pe are provided. The resulting weld pool for high Pe are long, narrow and shallow; the weld pool may be approximated by a cylinder. For a given value of the power, the weld pool length depends only slightly on Pe, and consequently a simple approximate formula for the dependence on laser power P is possible and is presented.

Ion cyclotron modes excited in a low density, cylindrical plasma cavity using an external inductive antenna are investigated theoretically and experimentally. These modes, which have a long parallel wavelength, exhibit a strong electrostatic character and are only weakly coupled to the antenna fields. Comparison is made of the measured wavefields with those predicted by a theory that includes the influence of electron dynamics. It is thus shown that not only the electron inertia but also electron Landau damping plays an important role in determining the cavity mode structure.

Recently, attention has been focused upon laser plasma sources of thin-film diamond. The paper reports a model of an expanding carbon plasma ablated from graphite feedstock by a focused laser beam. The model includes as essential ingredients adiabatic cooling of plasma electrons and electron-ion recombination through both collisional and radiative processes. It is shown that, under experimental conditions that have been proven relevant for producing hard amorphic diamond films characterized by a high percentage of sp³ bonds, the plasma is composed of multiple-charged carbon ions with kinetic energies of the order of 1 keV. This ionization can readily survive plasma transport to substrates positioned a few centimetres from the graphite target.

A method combining laser-induced fluorescence and injection of a neutral beam was employed to determine a time-varying electron-density in a conical theta-pinch plasma. The method consists of measuring the electron excitation rate relative to the spontaneous emission coefficient. An expression for the electron density in terms of the measured fluorescence and collision-induced emission signals has been formulated, taking into account the appreciable population of the excited level, as well as incomplete laser saturation. Neutral atomic beams (Li and Mg) were produced using laser ablation. A low-density hydrogen plasma was produced using a conical theta-pinch to simulate tokamak edge-plasma conditions. Using a Langmuir double-probe and emission spectroscopic methods, a time-varying electron density and an electron temperature. Measuring the electron excitation of the atomic beams with laser-induced fluorescence (LIF), the rapidly varying electron density could be deduced using either the Li 670 nm or the Mg 285 nm transition.

Equations describing the longitudinal electron diffusion in gases with Coulomb collisions in an electric field are derived. It is shown that the presence of electron-electron interactions leads to the renormalization of the longitudinal diffusion coefficient which is calculated in Ar, He, and N₂. The numerical results are used in the investigation of the thermocurrent instability threshold in the glow discharge in He. The thermocurrent instability is retarded at high ionization levels.

The experimental study of highly non-equilibrium neon plasma has been carried out when the 2p⁵3s excited state atom density essentially exceeds the electron concentration. The initial electron density N_eo was between 10⁹ and 10¹⁰ cm^-3, the density of the ground-state atoms N was within the limits 10¹⁷-10¹⁸ cm^-3 and the reduced electric field E/N varied between 5 and 25 Td. The plasma was created as a result of an interaction between the stationary background and the pulsed electric fields. The excited atom and electron density growth induced by the stepwise electric field variations were found to be of an explosive character due to the accumulation of atoms in the lowest-lying excited states. The binary chemi-ionization reactions of the excited atoms were shown to play the principal role in the kinetics of the excited atoms and electrons. Due to the generation of fast electrons during the chemi-ionization processes, the electron energy distribution function was found to be a highly non-equilibrium one.

Elementary processes in the atmosphere involving ozone, which lead to a reduction of the electric strength are considered. It is shown that elementary processes in the atmosphere under the action of an electric field lead to ozone accumulation which, in turn, causes an increase of the electron density. Electrical breakdown of atmospheric air is considered to be a result of the onset of kinetic instability. The threshold condition for this instability connects the background ozone number density and the electric field strength. This connection is derived on the basis of a simplified model which includes the processes of formation and destruction of both negative oxygen ions and ozone molecules, as well as the ionization of molecules by electron impact and mutual ion recombination. Evidently, the processes considered contribute to a reduction in the electrical strength of the atmosphere during thunderstorms.

The light intensity and its distribution were measured using an optical system with a high time resolution. The light intensity, as well as the arc voltage, have high-frequency oscillation, and the waveform of the light intensity at the arc's centre is almost the same as that of the arc voltage. The fluctuation of the light intensity is the result of the oscillation (the movement of constricting and spreading) of the arc column. By adding an oscillating current to a main current with an approximate constant, the dynamic characteristics of a vacuum arc were measured. The dynamic constriction of a high-current vacuum arc may show an oscillating arc column due to the interaction between the current flow and mass flow. The oscillating arc column may trigger anode spot formation. Moreover, the oscillating arc column appears when the peak current is in excess of a certain threshold current which is largely dependent on the axial magnetic field.

A theoretical model is proposed which describes the solid state amorphization of metallic materials under thermal treatment and mechanical alloying to be a process related to a special type of diffusion-induced grain boundary migration. Solid state amorphizing transformations in metallic materials under mechanical treatment (in particular, mechanical alloying or high-strain deformation) are thought of as being related to the spreading of fragment boundaries, deformation-induced grain boundaries.

The authors present an explicit three-dimensional theoretical model of modulated photothermal deformation in solids which applies directly to the square-wave intensity modulation case. They also show the dependence of the signal amplitude on the modulation frequency and discuss the effects of the thermal diffusivity and the thickness of the solid sample, together with the pump-beam radius, on the frequency dependence. The experimental verification of the theoretical predictions shows that their theoretical model provides an accurately quantitative tool for the study of the thermophysical properties of materials.

Study of the heat-transport properties of actinide-based mixed oxides is of importance in nuclear technology. This paper describes laser flash technique studies of the thermal diffusivity and thermal conductivity carried out on thorium-lanthanum mixed oxide solid solutions covering a temperature range from 573 to 1573 K and a compositional range from 0 to 30 mol.% LaO_1.5. The thermal diffusivity and thermal conductivity of the solid solutions were found to decrease with increase in lanthanum oxide content or temperature. From the temperature dependence of the thermal conductivity it was noted that heat transport in these materials is by lattice conduction. The variation of the two resistivity terms, namely the defect thermal resistivity and the intrinsic thermal resistivity, as well as their ratio as a function of lanthanum oxide content was examined. Employing the thermal conductivity data, phonon mean-free paths were also obtained as a function of composition and temperature.

Relatively thick (13.5 mu m) Bi substituted ferrite garnet single-crystal films exhibiting wary magnetic domain structure are prepared. It is found that domain wall waviness affects the magneto-optic diffraction (MOD) pattern resulting in the appearance of the additional diffraction maxima characteristic of two-dimensional diffraction gratings. The observed MOD parameters are adequately described by the Raman-Nath thin layer approximation to the theory of diffraction.

Anodic nitridation of a germanium wafer was carried out successfully in a plasma of ammonia:nitrogen excited by a high-frequency (500 kHz) electromagnetic field. The transmittances of the plasma-nitrided (PN) films on Ge(111) were measured by a Nicolet 170 SX Fourier transform infrared spectrometer and an IR-450S type infrared spectrophotometer. The infrared absorption bands of the beta -phase PN film on Ge are at 910 cm^-1 (w) and 780 cm^-1 (s) in the range of 4000-400 cm^-1. The absorption bands of the two-phase ( alpha + beta ) growing PN film are at 910, 780 and 730 cm^-1.

The apex dimensions and length are calculated as a function of current for the ion-emitting jet in liquid-metal ion sources (LMIS). The results agree well with observations. Since the final expressions are analytical, they give more insight into the fundamental mechanisms involved than do numerical calculations. Some implications of the model are discussed concerning focused ion beam (FIB) systems employing LMIS.

The microhardness H_v of xZnO.(50-x)PbO.50B₂O₃ glasses has been studied as a function of ZnO content and gamma dose. H_v shows a maximum at about 15 mol.% ZnO. ¹¹B NMR suggests the formation of non-bridging oxygen ions in the borate network as ZnO is added in the PbO.B₂O₃ glass. Both the density and the molar volume decrease with the increase of ZnO. Irradiation with gamma rays lowers the microhardness, whereas heat treatment causes its increase. Infrared investigations indicate that radiation induced defects can activate phase separation processes when the irradiated glass is heat treated.

The conversion efficiency of laser energy into kinetic ion energy in a laser produced plasma has been investigated for two quite different targets: graphite and tantalum. The laser energy (intensity) varied from several mJ to 200 mJ (10⁹ to 7*10¹⁰ W cm^-2) which is appropriate to many applications of a laser produced ion source. The conversion efficiency as a function of the laser energy was directly determined by differential measurements of the charge, kinetic energy and angular emission distribution of the plasma ions in absolute units. Whilst for the Ta target a nearly constant efficiency of about 30% was observed, the graphite result shows an unexpectedly strong enhancement of the transfer efficiency of up to 80% in the laser intensity range around 1.5*10¹⁰ W cm^-2. It is assumed that the results are related to the difference in the surface roughness of the targets.

A computer simulation of electron diffraction pattern and strain levels of various crystalline and packed bodies is developed. A flow chart of the program is given and it is found that the simulation result curves coincide with the experimental curves of interference function and diffraction intensity.