This thesis investigates the velocity disturbance measurements around a single plate in a rectangular duct during a Flow Induced Acoustic Resonance (FIAR). A recirculating wind tunnel was designed and built specifically for Laser Doppler anemometry measurements of the flow. Detailed acoustic spectra were recorded through a velocity range which included the FIAR 'locked on' range. This revealed variations in the primary acoustic frequency through the range and showed the relevance of the Strouhal frequency bandwidth. The non-resonant and peak amplitude resonant cases were compared, which showed similarities between the mean velocity profiles. Following from this, a thorough examination of the velocity fields associated with the peak resonant case revealed the locations of the acoustic and velocity fluctuations. These locations were compared at four velocities within the FIAR range, covering the resonance lock on, peak, post peak and lock off. Convecting and non convecting velocity disturbances were found in these four cases. The area where the kinetic energy of the flow is periodically transferred to the acoustic pressure field was discovered. The positioning of a round bar downstream causes periodic changes in the resonance response envelope. Acoustic spectra were taken for five selected positions between the bar and plate. The spectra revealed the dominant features of the FIAR velocity range which were then investigated using the knowledge gained from the single plate work. The velocity disturbance waves were also found to occur in the case with the bar. This thesis has shown a new phenomenon which has not been previously addressed, namely the absorption by the acoustic pressure antinode in the Parker beta mode of the velocity disturbance wave leaving the leading edge.

Lasers have been in use to weld materials together for over 30 years, beginning with the ruby lasers. With the recent introduction of compact high average power Nd:YAG lasers with fibre optic delivery, the applications of lasers in welding has considerably increased and is continuing to do so at an expanding rate. A particular example is in the use of fibre optically delivered Nd:YAG laser beams in applications where the flexibility and ease of control of the light beam can be very useful in confined or hazardous areas, for example, inside a nuclear reactor. This thesis has three purposes; firstly, to determine methods through which characterisation of the weld process can be achieved. Two active and three passive systems were designed and built. Of the active systems, it was found that the paramagnetic system was too complicated for accurate use. The laser deflection system required a comparison experiment for interpretation of the weld results, achieved using simulated water "weldpools". The technique gives good information regarding the quality and penetration state of the weld. Of the passive systems, a dual wavelength monitor system and an acoustical monitoring system were built also giving good results regarding weld conditions. Spectroscopic measurements show that plasma conditions in the weld can be neglected to 1 part in 10<SUP>8</SUP>. The second purpose is to use the results from these characterisation techniques to serve as data for a theoretical description of the welding phenomenon and the lack of a plasma serves to make a simple, practical theory possible. Finally, to identify possible systems that can be of use for on-line monitoring and control of a practical laser system for remote welding.

This work investigates Cross-Sectional Scanning Tunnelling Microscopy applied to semiconductor laser structures which are biased whilst they are being scanned. Images are presented as a function of sample bias. Increasing the sample bias removes the built-in band bending across the device and causes the horizontal topographic gradient of the scan to change. The p-type side of the sample is held at ground whilst the n-type side is biased. When tunnelling out of a double quantum well structure the topographic height of the n-type side increases by around 0.2nm at 1V. Tunnelling in to the structure, the height decreases by 0.02nm under low tunnelling current conditions. A tunnelling current model is developed which confirms these changes. Tunnelling in to a buried heterostructure device the apparent topographic height of the n-type side is again found to decrease, by over 2nm. Biased-dependent spectroscopic shifts are also observed with this device which are again confirmed by modelling. In both devices the apparent height of the quantum wells is found to increase by a factor of 2.5 to 4 times, at 1V sample bias. This is caused by the effects of tip-induced band bending being altered by the applied bias. An experimental and modelled example of a superlattice structure which displays contrast enhancement through tip-induced band bending is given. Several other device-specific physical and irreversible changes occur as a result of sample bias. These are well-suited to give characterisation information not available from other techniques.

This thesis is primarily concerned with some laser-induced nonlinear reflection and refraction phenomena in lithium niobate (LiNbO<SUB>3</SUB>) crystals. These are photorefraction and phase conjugation. These phenomena are caused by nonlinear optical processes and the underlying basic theory is presented. A critical review of the theory of phase conjugation by Degenerate Four-Wave Mixing (DFWM) of coherent laser beams is given and an analysis and a simple mathematical description of photorefraction, i.e. light-induced refractive index changes, are presented. Laser beam coupling by two-wave mixing and phase conjugation by four-wave mixing in photorefractive media are discussed in detail. Relevant physical properties of LiNbO<SUB>3</SUB> crystals were measured and are described. An account of the design of experiments and the apparatus used in the present study to investigate photorefraction and phase conjugation in LiNbO<SUB>3</SUB> are presented. The characteristics of the He-Cd laser used in the experimental investigation were measured and are described. Experiments were conducted using two-wave mixing to yield the photorefractive parameters of LiNbO<SUB>3</SUB> which were then used in the interpretation of the experiments in which phase conjugation was observed in DFWM studies. Details of the procedures and the results are given together with estimates obtained for diffusion and photovoltaic field strengths in the conditions used in the present investigation. Photographic evidence of the corrective removal of optical distortion from laser beam propagating through LiNbO<SUB>3</SUB> cyrstal is given.

The commercial applications and types of photochromic are many and varied. In this work spiroxazines, spiropyrans and fulgides, of the type used in optical lenses, were examined using a variety of analytic techniques. Initially the feasibility of using supercritical fluid extraction (SFE) for the extraction of photochromics, and some UV stabilisers, from lens matrices was investigated for the first time. Packed column supercritical fluid chromatography combined with mass spectrometry (pSFC/MS), using both a moving belt and a thermospray interface, was assessed as a method for the separation and analysis of standard photochromics, prior to analysis of unknown photochromic lens extracts. Standard mixtures of the photochromics were successfully separated by pSFC and the mass spectrometric fragmentation patterns were recorded. The unknown photochromics, extracted by SFE, were separated by pSFC and their structures were then suggested from the mass spectra. A preliminary investigation was also carried out into the use of electrospray/mass spectrometry for the analysis of standard photochromics and also a nickel quencher (UV stabiliser). The lens extracts were also analysed by GC/MS and GC/Fourier-transform infrared, which were found to be complementary analytical techniques to pSFC/MS. Part of the work in this thesis concerned the development of a liquid chromatography/mass spectrometry particle beam interface for use with pSFC/MS. After physical adjustment of the interface, the operating parameters and sensitivity of the technique were investigated. pSFC/MS with the particle beam interface was then successfully used for the first time for the separation and analysis of standard mixtures of tocopherols, estrogens and benzodiazepines. Timbo powder (Derris root) was then extracted by SFE and the resulting extract was analysed using the developed technique.

A problem inherent when using the finite element method to model an open bounded solution domain, such as required for electromagnetic scattering, is the termination of the finite element mesh. The terminating boundary must be constructed so that any scattered waves incident on it, are absorbed, thus eliminating any spurious reflections. A significant amount of work has been carried out in this field to ascertain the most efficient absorbing boundary condition; this work has been extended within this thesis to include the development of a new general order wave envelope infinite element. The implementation of these new general order infinite elements allowed the mesh boundary to be terminated closer to the scattering body, when compared with previously presented infinite elements, thus reducing the size of the solution domain. Results have been shown to compare these new general order infinite elements with the Bayliss Turkel ABC, they were however found to be, at best, comparable to this absorbing boundary condition. In addition to the problem of normally incident plane wave excitation, the problem of obliquely incidence excitation has been investigated. The new general order infinite elements have been formulated for this oblique incidence problem and they were again shown to give considerable improvement over previously presented infinite elements. The reduction of electromagnetic backscatter has important implications in the field of radar, to this end a number of radar absorbing materials have been developed. This text concentrates on the Jaumann absorber as a method of radar absorption. It is necessary to model these radar absorbers, either analytically or numerically, to this end a new edge element has been developed which allows the more efficient modelling of infinitely thin reactive sheets, to be used in the construction of a Jaumann absorber. Both the numerical and the analytical modelling of Jaumann absorbers were investigated and the analytical results were used in a formal sensitivity analysis. This analysis was carried out via the Taguchi method of parameter design, which constituted an investigation into the stability of both planar and cylindrical Jaumann absorbers. It was found that the absorbers were insensitive to changes in the spacer thickness' and sheet resistance and to their radius of curvature. In addition, the sheet resistances were found to be entirely independent of each other for the planar absorber and could therefore be individually optimised.

This thesis is primarily concerned with the design, construction and characterization of a metal vapour laser capable of simultaneously generating three primary colours, blue, green, and red, thereby constituting a "white light" laser. The physical processes responsible for the simultaneous establishment of population inversions between eight energy levels of the cadmium ions which lead to the generation of coherent blue, green and red spectral lines are examined in detail. The essential basis of the white light laser lies in the use of a negative glow discharge in a mixture of helium gas and cadmium vapour in a hollow cathode-pin anode electrode configuration, and the energy distribution and concentrations of electrons, helium and cadmium atomic and molecular ions available in this arrangement. Following a critical review of previous relevant work on metal vapour lasers, which discloses the "white light" generating possibility of the He-Cd+ glow discharge, a discussion of glow discharges is presented with emphasis on the negative glow discharge. An account of apparatus built and the techniques used to investigate the optical and electrical characteristics of He-Cd negative glow discharges is given and the data obtained is presented. This is used together with the analysis of negative glow discharges as the basis for the development of a white light laser. The successful achievement of simultaneous blue, green and red achieved. A series of measurements of the decay of spontaneous emission red boherent beams and the operational characteristics of this laser are presented in detail. Up to 40 mW of white light were of the blue, green and red spectral line intensities in He-Cd+ negative glows is described and the decay rates related to the helium atomic and molecular and cadmium ionic concentrations. From these data the collisional processes responsible for the creation of population inversions are deduced to be direct electronic excitation and Penning effect, in the proportion of 60% to 40% for the 441.6 nm blue line, helium atomic ion charge transfer excitation for the 635.5 and 636.0 nm red lines. As for the 533.7 and 537.8 nm green lines the major excitation processes involve cascading due to stimulated and spontaneous decay of the red line upper level and charge exchange from molecular He2+ ions following atomic He+ ion destruction in a three-body collision with ground state helium atoms. This last process becomes increasingly important as the pressure is increased. Finally, new improved white light laser designs are proposed and suggestion for further work to clarify further details on the inversion mechanisms are made.

The study is concerned with the characterization of a HCD HeCd<SUP>+</SUP> laser design and its spectroscopic investigation. The design, construction and operation of the laser are described, and in particular the advantages of the hollow cathode discharge for achieving the population inversion of the laser transitions are discussed. Simultaneous C.W. oscillation on seven transitions of the HeCd<SUP>+</SUP> laser tube has been achieved, i.e. at 441.6 nm (blue), 533.7 nm and 537.8 nm (green), 635.5 nm and 636.0 nm (red), 723.7 nm and 728.4 nm (infra red). The simultaneous operation of blue, green and red lines generates 'white light'. Thus this laser has potential applications in a variety fields like e.g. in colour scanning, colour printing, holografic film recording and storage, and monitoring of compound semiconductor growth. A spectroscopic study of a pure helium discharge has been conducted and the major results for pressure and discharge current dependence are discussed to establish missing links in the understanding population mechanisms for the upper level of the different transitions in the HeCd<SUP>+</SUP> laser. The green laser transitions had generally been assumed to be generated via radiative cascade through the red laser lines. However, the existence of the green laser lines at high pressures gave an argument that the population inversion of the green upper levels is produced by collisional processes between the Cd atoms and molecular helium ions, He<SUB>2</SUB><SUP>+</SUP>. The observation of a number of He<SUB>2</SUB><SUP>*</SUP> bands, at the expense of atomic lines, confirms the important role of molecular helium. The most striking feature in this study was the observation of the banded emission extending up to 40 nm around the laser lines; the observed bands have to be ascribed to emission from the (He..Cd)<SUP>+</SUP>. Finally, preliminary experiments based on the exploitation of the optogalvanic effect were carried out in order to measure gain and losses of the transitions in the HeCd<SUP>+</SUP> and the molecular emission of (He..Cd)<SUP>+</SUP>.

The CERN Laser Ion Source (LIS) aims to provide a short pulse (˜5 μs), high current (˜ 10mA) and high charge state heavy ion beam (Pb<SUP>25+</SUP>) for acceleration by a LINAC and injection into the Proton Synchrotron Booster (PSB). The laser beam time profile was measured using detectors with time response of the order of 1 ns. Cross correlating of the laser pulse form and the ion beam current one fails to find any significant relation. The laser spatial profile was measured in the focal plane by a Spiricon pyroelectric camera in conjunction with a long focal length lens. In the Master Oscillator and Power Amplifier laser configuration, the beam was found to be astigmatic and exhibit a large pointing instability. The free-running laser produced a beam in good agreement with the simulation of beam propagation along the 30 m path extension and was also astigmatic. As for the time profile, no firm correlation of the laser spatial profile fluctuation and the ion beam instability could be found. Within the framework of this thesis, a critical study has been performed on whether shot-to-shot instabilities are being caused by fluctuation in the laser beam parameters (time profile, spatial distribution and energy) as well as the extraction of the ions from the expanded laser plasma to form an ion beam. In addition, a technique for the calculation of absolute ion numbers was derived. The extraction of the ions from the laser ion source plasma was experimentally studied using Faraday cup collectors, and a compact single shot emittance measurement system. The extraction of the ions was found to be correctly modelled by the Child-Langmuir equations for charge extraction, with some modification necessary to account for the initial significant ion velocity (˜10<SUP>5</SUP> ms<SUP>-1</SUP>) found after plasma expansion. The equivalent proton perveance applicable to the extraction geometry used, was found by systemic measurements of the ion transmission to a Faraday cup as a function of the applied source voltage. Absolute ion numbers were calculated from measurements using an electrostatic ion analyser and a Faraday cup ion collector. From these measurements it was possible to deduce the transmission losses through the Low Energy Beam transport line between the LIS and an RFQ; they were found to be as high as 80%. In summary, the measurements detailed here allow the prediction of the source parameters required for extraction of a higher current, higher charge state beam required for a final LIS implementation capable of supplying the ion beam for the Large Hadron Collider (LHC). In addition, simulations of the beam extraction using a time dependent macro-particle and static ray-tracing software package are described which provide a reasonably good modelling of the beam extraction.

No description available.