Table of contents

The effect of the laser light on the discharge development characteristics in the active medium of an ArF excimer laser is investigated using a one-dimensional fluid model including laser-light-induced photo-ionization and photodetachment processes. These processes are found to have a significant effect at times after the maximum in the laser output signal. For example, the laser-light-driven interactions increase the bulk electron number density by a factor of two and the electric field adjacent to the cathode by a factor of 1.5. Near the anode, the field changes from slightly negative at early times to extremely positive near the end of the laser pulse, resulting in electron drift in the direction of the bulk plasma. This tendency of the field to change sign at the anode is not observed when photon processes are neglected. This strong field enhancement suggests the generation of a filamental instability at the electrodes, as is indicated by experiments.

We have developed an interactive numerical model, which describes the operational characteristics of multiple diode-pumped laser systems that are Q-switched or Q-switched/cavity-dumped. These are the first computer-generated results obtained using rate equations that incorporate the 'inversion reduction parameter' gamma . The quantity gamma can partially take into account effects of relaxation and thermally driven transfer, which occurs between laser level multiplets. A value of gamma near unity is found for the Nd:YAG lasers, implying considerable thermal relaxation of the lower lasing level, and it may also imply some thermal refreshing of the upper lasing level. A value of gamma =2 is found to fit the Ti:sapphire results, indicating that the lower lasing level is not relaxed during laser pulse development. The interactive nature of the model allows for variation of all the significant parameters required to generate a Q-switched or cavity-dumped pulse. Input information is expressed in terms of standard variables such as stored energy, optical and dissipative losses and stimulated emission and pump absorption cross sections. The effects of additional parameters, including the Gaussian beam profile of the cavity, the induced losses associated with the rise and fall times of the Q-switch and cavity-dump Pockels cell, and the induced thermal lensing effects produced by the pump rate in the gain medium are taken into account. The width and energy of the cavity-dumped laser pulse can also be studied in the model by varying the cavity-dump voltage switch transition time.

We have analysed the phase-matching features of (001)Ba₂NaNb₅O₁₅/(001)KTiOPO₄, (100)Ba₂NaNb₅O₁₅/(100)KTiOPO₄ and (100)Ba₂NaNb₅O₁₅/(001)KTiOPO₄ waveguides for second-harmonic generation. Theoretical results show the existence of an optimum waveguide design, which is insensitive to inhomogeneities in the waveguide thickness due to 'non-critical' phase-matching in some waveguide geometry. Using a coupled-mode theory, the overlap integral in second-harmonic generation in the film waveguide has been analysed. Some proposals are offered for increasing the overlap integral of the fundamental wave and the harmonic wave. Application of such a waveguide design can significantly ease the fabrication tolerance, leading to second-harmonic generation devices with long interaction lengths and practical conversion efficiencies.

Ion energy spectra of a laser-produced Ta plasma have been investigated as a function of the flight distance from the focus. The laser (Nd:YAG, 20 ns, 210 mJ) is incident obliquely (45 degrees ) and focused to an intensity of about 10¹¹ W cm^-2. The changes in the ion distributions have been analysed for the Ta⁺ to Ta⁶⁺ ions in an expansion range 64-220 cm. With increasing distance from the target, a weak but monotonic decrease is observed for the total number ions, which is essentially due to the decrease in number of the more highly charged species. For the Ta⁺ and Ta²⁺ ions the net changes approximately cancel. A more sophisticated picture of the recombination dynamics is obtained, however, if the changes within individual groups of ions expanding with different velocities are compared. Here, in the same spectrum, both increasing and decreasing ion numbers can be observed. This can be interpreted as direct evidence of recombination and its dependence on temperature, density and charge.

Spatially resolved photodetachment measurements have been carried out in a pulsed low-pressure multicusp discharge. Previous H^- extraction measurements and photodetachment measurements of the H^- density have shown large increases in the negative ion density in the post discharge of a pulsed plasma. However these increases are shown here to be strongly dependent upon the spatial position of the measurements. The post discharge increases in the negative ion density are explained by the rapid reduction of the fast electron density and the cooling of the electron temperature after switch off. The spatial dependence of the H^- density is attributed to the variation of plasma species and temperatures throughout the source due to the magnetic fields of the confining magnets. It is also shown that the rise time of negative ions in the post discharge is strongly dependent upon position from the filaments. In the region close to the filaments the negative ion rise time in the post discharge is 10 to 20 mu s but away from this region the rise time reaches values of over 50 mu s, which indicates that transport properties of the negative ions play an important role in the temporal behaviour at positions away from the central region where the filaments are located.

Spectroscopic measurements were made of the population of the excited states in a low-voltage, low-temperature, open caesium plasma diode. Such a device is under certain conditions seen to run in a special mode that is essentially different from the ordinary plasma mode. This mode, which has been called the Rydberg current mode, exhibits many interesting features, such as a very strong electron emission from the cold to the hot electrode. The objective of the present study was to investigate and compare the plasma conditions of the two modes. The spectral line intensities and the excitation temperatures of the upper states were found to differ considerably although the current, the voltage and the electrode temperatures were comparable.

We study the influence of a matchbox on the performance of an RF discharge. As a case study we investigate a low-pressure asymmetric discharge using a PIC-MC code. We find that the presence of an inductivity in the matchbox smoothes the electron current in the discharge. As a consequence, the electron temperature is raised and hence the ion density. The reason hereto lies in the fact that the inductivity damps out current peaks in the discharge.

Time-resolved mass spectrometry of negative ions and time-resolved Langmuir probe measurements are carried out in a pulsed hydrogen discharge. Both diagnostics show a strong increase of the H^- density in the post-discharge. The measured transient shape of the H^- density in the hydrogen post-discharge is compared with an advanced theoretical model. This comparison shows that the H^- slope is mainly influenced by the density distribution of vibrationally excited molecules and the transient drop of the electron temperature in the post-discharge. In accordance with our model, no influence of positive ions and atomic hydrogen on the H^- density is observed up to plasma densities of 10¹¹ cm^-3. Measurements of the maximum H^- density in the post-discharge in dependence on the duration of the single pulse indicate the density evolution of vibrationally excited molecules during the discharge. At plasma densities between 10¹⁰ and 10¹¹ cm^-3 a saturation of the density distribution of the vibrationally excited molecules is observed after 200-500 mu s. By variation of the pause duration with a fixed short discharge pulse (40 mu s) the lifetime of the vibrational excited molecules in the post-discharge is found to be approximately 1-4 ms.

Radio-frequency discharges at 13.56 MHz in pure H₂ and CH₄-SiH₄ mixtures highly diluted in H₂ are investigated by spatially and spatiotemporally resolved emission spectroscopy. Spatial emission profiles of the excited species exhibit double layers near the electrodes in pure hydrogen and hydrogen-controlled discharges. Spatiotemporal analyses give complementary information showing that the emitting layers, which result from atomic and molecular excitations, occur at different times in the radio-frequency cycle. The mechanisms involved in the double layer formation are interpreted by simplified numerical models based on both a fluid model and a particle-in-cell-Monte Carlo model. The results of the models are in good agreement with the spatiotemporal representations of the excitation rate obtained experimentally. The formation of the double layers is analysed as resulting from the high drift velocity of the hydrogen ions. This behaviour seems to be characteristic of radio frequency discharges in hydrogen.

The radiative transfer in SF₆ and SF₆-Cu arcs was calculated using the method of partial characteristics. The partial sources and sinks of radiation were tabulated for a temperature range 300-30 000 K and a pressure range 0.1-1 MPa. The method was applied to one- and three-dimensional calculations of radiative transfer and in arc plasma models. In circuit breakers, radiation absorption increases the upstream pressure either by direct heating or by accentuating the clogging effect in the nozzle throat.

High-speed emission pictures of arc cathode spots in vacuum have been obtained in a spectrally narrowed range. It has been found that the total radiation was delayed with respect to the arc current by 8.5 ns for Ti cathodes because of the limited transition probability of excited states. This effect sets lower bounds to spatial and temporal resolution. By selecting a spectral range with prevailing continuum radiation, the resolution could be improved down to 5 mu m and 2 ns, respectively. In this way a typical size of the spot plasma of about 10 mu m was observed for nanosecond discharges as well as for durations up to 100 mu s. The spots showed random displacements with a diffusion constant D=4*10^-4 m² s^-1. The spot brightness was subjected to periodical fluctuations with discrete intervals T_n, n=0-3. The measured values obey the relation T_n/T₀=(D/4 kappa )ⁿ, with kappa being the thermal diffusivity of the cathode material. The smallest time T₀ is assumed to be the elementary spot lifetime tau _s. It shows a distribution according to a 'survival law' with an average value of 14.2 ns for Ti. These results agree with studies of laser absorption pictures and surface craters. They are in line with explosive-spot models. The occurrence of fluctuations at discrete intervals T_n is explained by heat accumulation at the cathode surface during the random walking of the spots.

Experimental results on excitation of a nitrogen laser by means of a nanosecond microwave discharge in a wave beam are presented. The possibility of producing an atmosphere laser with remote microwave pumping is demonstrated. The numerical model for the nitrogen laser excited by a nanosecond discharge in a cylindrical TE wave in a tube and in free space is constructed. It is shown that, by changing the pressure of the laser mixture, the diameter of the gas discharge tube and the value of incident microwave power one can efficiently control the discharge parameters and achieve total absorption of microwave radiation and high efficiency of laser generation.

We consider the problem of an elastic layer perfectly bonded to an elastically dissimilar substrate, when an edge dislocation, of arbitrary orientation, is present either in the layer or in the substrate. The solution given includes the complete stress field, which is needed for the solution of crack problems, together with the Peach-Koehler force.

The effects of electron beam irradiation on highly oriented poly(di-methyl silane) film are studied. The highly oriented films, in which the silicon backbone of poly(di-methyl silane) is perpendicular to the substrate surface, are prepared by means of a vacuum-evaporation technique. The orientation of the polysilane is confirmed by ultraviolet absorption and X-ray diffraction measurements. By electron beam irradiation, the polysilanes in the film are bonded with the nearest neighbour chains by forming C-O-C, Si-O-C, and/or Si-O-Si groups, which are investigated using Fourier transform infrared spectroscopy. The irradiated part of the film is hardened, and remains on the substrate after etching by concentrated H₂SO₄. whereas the other part is completely removed. By using such a process, a negative-tone pattern having sufficient resistance against acid can be obtained. The possibility of the application of this effect is also discussed on the basis of the microscopic structure of the present film.

Supercooling of crystalline bismuth inclusions in aluminium crystals has been observed and studied with different techniques: X-ray diffraction, in situ Rutherford backscattering/channelling spectrometry and transmission electron microscopy. The results of the measurements with different experimental methods (and on different samples) agree remarkably well. The inclusions melt at temperatures at or below the bismuth bulk melting point, and the solid/liquid phase transition exhibits a hysteresis of 100-150 K. Average inclusion sizes ranged from a few nm to some tens of nm. The X-ray diffraction melting data are discussed in relation to different existing models for the melting temperature of an inclusion as a function of its size. From this an approximate size distribution for the inclusions is derived.

A method for the correction of the size of spherical, disc-like or ellipsoidal particles in foils of finite thickness has been derived. This method corrects for errors due to spheres (or ellipsoids or discs) centred outside the foil that are observed in truncated form; this method can be used where overlaps or near overlaps can be clearly observed. Some experimental results for a partially stabilized zirconia system are evaluated using this method.

The results of studying GaAs samples with built-in pi - nu junctions as the base for the construction of radiation-resistant coordinate-sensitive detectors are presented. The GaAs samples have been exposed to the beam of the linear proton accelerator, using an Al target for neutron production. The I-V characteristics of GaAs samples have been analysed to investigate the change in their properties. The study of the radiation resistance of the GaAs samples has shown that their main characteristics (charge collection efficiency, signal-to-noise ratio) are degraded by less than 20% at the integral neutron fluence of 1.2*10¹⁵ cm^-2.

The electronic properties of slightly hydrogenated silicon nitride prepared by plasma-enhanced chemical vapour deposition are studied by electron spin resonance and photoluminescence. We show that the detection of the nitrogen dangling bond, after thermal annealing followed by an ultraviolet irradiation, critically depends on the composition, as has already been observed in strongly hydrogenated materials. Moreover, correlations are established between the appearance of the dangling bond and the photoluminescence efficiency, suggesting that an electronic state associated with a nitrogen dangling bond could be a radiative centre.

The reflectance and transmittance of a multilayer thin film assembly with columnar-structure-induced anisotropy were measured. A simple method based on circuit theory was applied to analyse the thin film assembly. Experiments were carried out on several SiO₂ and TiO₂ films deposited onto BK7 glass substrates and on TiO₂ films deposited onto SiO₂ underlayers. The refractive indices, orientation of columns, and thicknesses of these films were recovered from the measurements.

A kind of field-enhanced a-Si:H photoemitter with a sandwich structure of the form of SnO₂-n^--p a-Si:H-Ag:Cs:O, a low work function surface and an increased photoelectron yield due to the charge-intensifying effect has been designed. First experiments have shown that its quantum yield is 3.9% at wavelength 0.56 mu m and its cut-off wavelength is 0.82 mu m. In this paper, an expression for the quantum yield is deduced from the five-step photoemission process. Then the difference between the experimental results and expected values of the quantum yield is discussed and the way to further increase it is explored. Also, the fact that it is quite possible for the quantum yield to become higher than unity within the visible spectral area is indicated.

The thermal conductivity of evacuated pure and opacified silica aerogel powders was measured in a guarded hot plate apparatus for temperatures from 10 K up to 275 K and external loads from 1.5*10⁴ Pa to 10⁵ Pa. Infrared optical measurements of the extinction give the possibility to separate contributions from solid thermal conduction and radiative thermal transport. Measured thermal conductivities range from 29 to 80*10^-6 W m^-1 K^-1 at a temperature of 15 K and from 0.9 to 3.1*10^-3 W m^-1 K^-1 at a temperature of 250 K. The solid thermal conductivities lambda _powder of the powders were compared with those of the monolithic silica aerogel ( lambda _monolith) the powder grains consist of. The ratio g'= lambda _monolith/ lambda _powder was found to be temperature independent in the investigated temperature range. Thus g' can be interpreted as a geometrically induced reduction factor. The mean thermal conductivities averaged over the application temperatures range from 0.7 to 2.2*10^-3 W m^-1 K^-1 for boundary temperatures of 77.6 and 290 K and from 0.5 to 1.8*10^-3 W m^-1 K^-1 for boundary temperatures of 20.4 and 290 K. The investigated silica aerogel powders show a large improvement potential for cryogenic thermal powder insulations. They are well suited to substitute expanded Perlite as cryo insulation.

Despite the greatly superior performance in terms of hardness modulus change, the use of electrorheological fluids under compressive stress has been largely neglected in favour of implementations relying on shear-stress effects alone. This paper shows how electrorheological fluids, in addition to Bingham plasticity, also exhibit what appears to be rheopectic behaviour when subjected to compressive stress.

At high frequencies (1 GHz), it is difficult to maintain a sufficiently high probe impedance to allow measurements of voltage differences using a double probe. A new type of double probe with terminated coaxial cables (50 Omega ) is proposed and tested. The low impedance implies that the two probe wires are essentially short-circuited and only the probe current is measured. The relation between the displacement current due to surface charges and the electric field is derived and tested in a known field. The current measured in a plasma can be identified with the displacement current when the current due to plasma electrons is damped by an appropriate DC bias. Perturbations of the plasma by the probe field can also be checked by varying the DC potential of the probe.

Sputter deposition of Cr, with oblique incidence of the species towards a substrate, has been used to produce films with angular selective optical transmittance. Multiparameter fits, embodying the Bruggeman effective medium theory applied to an anisotropic medium of Cr and voids, were able to reproduce the optical data.