This thesis describes the theoretical and experimental investigation into many aspects of circuit and component design for high average power short pulse systems. The primary objective of this work is to develop both circuit design criteria and pulsed components for long life rare gas halide lasers. A pulsed system consists of three major components, energy storage, switches, and load. This investigation considers the type of circuit which uses capacitive energy storage in combination with a closing switch to transfer electrical power to a load. Specific loads are not addressed but the implications of load characteristics that affect circuit/component design and life are considered. The investigation reported, describes the physical and electrical characteristics and analysis of phenomena that adversely affect the performance and life of pulse duty components. In the area of capacitive storage, lifetimes of one particular design was improved by 3 orders of magnitude and a means of detecting the imminent failure of oil filled capacitors was devised and patented. In the area of switching, methods are described by which hydrogen thyratrons can be operated in parallel with equal current sharing without the need for inductive or resistive ballasting. Finally, the design and testing of a 200 watt XeCl laser modulator is presented.

A differential absorption lidar (DIAL) system, based on a xenon chloride exoimer laser, has been developed and applied to measurements of atmospheric sulphur dioxide. This thesis describes the basis of the technique, the construction and operation of the prototype system, and its theoretical and practical sensitivity to sulphur dioxide concentration. The system, comprising the laser, telescope, detector and data handling equipment was developed for robustness in the field, and novel features have been included for this purpose. The Newtonian type telescope was chosen for its small image size and adequate field-of-view. It uses a "Cassegrain" type primary mirror and a visible reflector at the focus, allowing direct viewing of the scattering target. This feature is used in conjunction with direct viewing through the laser cavity, through a dielectric coated mirror, to permit laser/telescope alignment on the target. The laser itself is pumped by transverse discharge after corona/U,V pre-ionization. Laser characteristics have been measured. Those of primary importance to lidar are the energy per pulse, at 5 to 8mJ, the pulse duration of 32ns, the maximum pulse repetition rate, at 20pps, and the number of pulses to half energy, at 18000 per gas fill. The emission spectrum of the laser, with wavelengths at 307.92nm and 308.17nm, has been compared with the absorption spectrum of sulphur dioxide, showing a difference between the respective absorption coefficients. This indicated the suitability of this particular laser to the differential absorption technique. Unique selection between these wavelengths, per pulse, is desirable for optimum sensitivity in DIAU However, a novel method has been devised for modifying the relative wavelength content of one of the pulses, by inserting an absorption cell of sulphur dioxide into the laser optical cavity. The advantages of this over "distinct" wavelength selection are cheapness and robustness. However, theoretical work has shown a subsequent loss of sensitivity to atmospheric sulphur dioxide concentration, by a factor of about 5, A 3-element birefringent filter has been designed in case a more conventional tuning method is required. Specification of the detector and its operating conditions have been closely defined in order to optimise sensitivity to very low levels of backscattered light, whilst reducing the effects of unwanted background and noise. To this end, a solar-blind photomultiplier has been employed in conjunction with a narrow-band interference filter, centred around the laser emission wavelength. A relatively simple signal handling circuit was built to perform the minimum requirement of measuring the intensity of backscattered radiation. A micro-computer is used to control the circuitry in test and data acquisition modes, and to store data, allowing signal averaging and subsequent data analysis. Program algorithms for data analysis were developed from lidar and DIAL theory. A theoretical investigation of atmospheric scattering properties was carried-out to provide scatter coefficients for application in the lidar equation. The validity of the lidar equation was proved when computer-modelled oscilloscope traces of lidar return signals were found to match, closely, experimental traces of return signals from the smoke plume at Methil power station. A theoretical treatment, using the lidar equation, gave expressions for target gas concentrations as a function of return signal intensities. This was done for the case where unique wavelength pulses are transmitted into the atmosphere and was repeated for the case of "mixed wavelength" pulses, applying to the DIAL experiment. Prediction of sensitivity of the system in measuring sulphur dioxide concentration is based on the noise content of the return signals. The analysis has given a detection limit range of about 10ppm.m to 800ppm.m, for the "mixed wavelength" application and an expected range of 2ppm.m to I60ppm.ra if the laser is tuned conventionally, depending on signal strength and number of averaged pulse pairs. Errors expected in measurements of finite sulphur dioxide concentrations are given. The system was applied to measuring sulphur dioxide, emitted under control, into the path of the laser pulse. Measured peaks of about 120ppm were expected and measurements taken successively are in agreement with expected dispersal rates. Fluctuations of measured sulphur dioxide levels, about a mean, are shown to be within the theoretically-evaluated error limits. This close agreement between theory and experiment allows the theoretical detection limits to be treated as realistic.

The construction and operation of a small active volume, discharge pumped, rare gas halide excimer laser is described. The values of laser output parameters such as pulse duration, pulse energy and overall efficiency presently achieved with such systems are much less that theoretical studies predict. The performance of this laser when pumped using a novel pulsed power modulator design containing nonlinear capacitors to produce a very fast rise time voltage pulse is contrasted with the performance obtained from a conventional driver circuit. The purpose of this was to determine if such circuits could lead to improvements in glow discharge stability and also laser pumping efficiency by quickly achieving the optimum pumping rates predicted by theory. It was concluded that while the rapid establishment of optimum pumping conditions may be beneficial, too fast a rate of rise of discharge current appears to be detrimental to discharge stability, probably due to skin effects. Having established that premature glow discharge collapse is a serious limiting factor in producing long duration excimer laser pulses, a study is carried out of the factors believed to influence discharge stability. While the effects of halogen donors on discharge stability have received most attention in the past the part played by the other constituents of the laser gas mix tends to have been neglected. A theoretical and experimental study of the role of the rare gas partners, xenon, krypton and argon is presented. It is well known that gas mixes using helium as the buffer gas perform less well than with a neon buffer and this is attributed to the driving of discharge instabilities rather than kinetic factors. A comparison of the relative influences of the buffer gases helium, neon and argon on discharge stability is carried out and claims by other workers for improved laser performance using a mixed helium / argon buffer are tested. Finally, in an attempt to inhibit the mechanisms driving glow discharge collapse, the effects of externally applied magnetic fields on discharge stability and laser performance are investigated.

Copper Halide lasers are discussed and the results of experiments reported. It is found that the presence of small quantities of an electron attaching gas (such as bromine) cause discharge instability. Specially designed electrodes which remove excess bromine cure this problem and yield a stable discharge. A 4W copper bromide laser is operated, sealed-off, for 100 hours. This laser has an apertured discharge tube with side-arm reservoirs to control copper bromide vapour pressure, a feature essential to stable operation. The addition of small amounts of hydrogen changes the beam from an annular to a gaussian-like profile. The estimated lifetime of this laser tube is 1,000 hours. Exploiting the theory of metallic walls for discharge confinement, it is found that metal segments shorter than about 1m can support a stable discharge at high pulse repetition rates (5 - 20kHz). On the basis of this, a novel copper halide laser containing cylindrical copper segments is demonstrated. Neon and halogen gases flow through the tube. The reaction between the halogen and the copper segment walls forms copper halide in-situ. Hydrogen bromide, bromine and chlorine have been used. Hydrogen bromide proves to be the most suitable. A one metre long laser tube of this design produces 40W. Pulsed power supplies for metal vapour lasers are discussed. The conventional form of the capacitor-transfer circuit has the peaking capacitor value around one half of the storage capacitor value. It is found that equal capacitor values produce the best results for the gold vapour laser system we describe. The reliability of a gold vapour laser is improved by replacing the hollow anode thyratron (which has a high latch rate) with a solid anode thyratron. The replacement thyratron, in combination with a saturating charging choke, significantly reduces the latch rate. The laser is used for studies of photodynamic therapy of cancer in a local hospital.

The optical perturbation of discharge current voltage characteristics (optogalvanic effect or OGE) has been investigated for CO2 and CO laser discharges. A quantitative power perturbation model is constructed and a series of experiments show, a close agreement for changes of all the major operating parameters of the CO2 laser. The theory consists of an evaluation of the microscopic kinetic relaxation processes leading to the changes in thermal balance of the discharge that occur due to the absorption and amplification of the resonant laser radiation. A generation of compact and efficient cw CO2 lasers has been developed which produces higher stable output powers per unit length than previously reported, and these have been actively stabilized by OGE to provide a high degree of frequency stability (<50 kHz) and amplitude stability (< 0.5%) which is a six orders and two orders of magnitude improvement over the passive resonator capability, respectively. New optogalvanic effects have been discovered both at high frequencies (up to 100 kHz) and for sequence (00°2) laser transitions in CO2 and also in the cw CO laser. Preliminary investigation of optogalvanic detection of standing wave saturation resonances in low pressure discharged CO2 have been carried out and some analysis of discharge noise has been necessary to evaluate the ability of such a narrow band detector for laser stabilization. The empirical evidence provided by the temporal response of OGE combined with the gas composition dependence (including N2 free mixtures) proves conclusively that no major ionization mechanism described so far can be responsible for the effect. A thermal explanation of the effect due to modified kinetic cooling of the laser gas has been developed from existing qualitative explanations. This proposed "gas temperature" power perturbation model provides for the first time an accurate (~20%) prediction of perturbations.(amplitude, phase, and frequency) due to the resonant interaction of a CO2 laser beam over a wide range (up to 4 orders of magnitude) of detailed parametric changes, with a CO2 or laser mixture discharge.

The work described in this thesis covers various aspects of pulsed copper and gold vapour lasers. The work is divided into four main parts : a computer model of the kinetics of the copper vapour laser discharge; construction and characterization of a copper vapour laser and a gold vapour laser system (to be used for photodynamic cancer treatment); analysis of the thermal processes occurring in the various forms of thermal insulation used in these lasers; and studies of the use of metal walls to confine a discharge plasma. The results of this work were combined in the design of the first copper vapour laser to use metal rather than an electrically insulating ceramic material for confinement of the discharge plasma. Laser action in copper vapour has been achieved in a number of metal-walled designs, with continuous lengths of metal ranging from 30 mm, in a segmented design, to 400 mm, where the discharge plasma was confined by two molybdenum tubes of this length. A theoretical explanation of the behaviour of plasmas in metal-walled discharge vessels is described.

A high resolution study of magnetic-field-induced second harmonic generation in sodium vapour was performed using a single-frequency continuous-wave dye laser. Resonant enhancement was obtained by tuning the laser to the frequency of the 3S-40 two-photon transition of the sodium atoms. Coherent parametric emission of the second harmonic radiation ocurred in the presence of a symmetry-breaking magnetic field by means of a coherent electric-quadrupole emission at twice the laser frequency. A theoretical model of the second harmonic generation (SHG) was developed, and includes the roles played by atomic selection rules, Zeeman splitting and eigen function mixing in a magnetic field, and the effects of homogeneous and inhomogeneous broadening. The use of a single-frequency laser and an atomic nonlinear medium allowed an experimental investigation of a SHG process in unprecedented detail. This provided astringent test of the theoretical model developed to predict the properties of the nonlinear interaction in terms of fundamental atomic parameters. The excellent agreement obtained between theoretical and experimental results vindicated the theoretical methods used in the modelling.

A transverse gas flow TEA C0₂ laser has been constructed and the use of a perforated hollow rod to serve simultaneously as the cathode of the discharge and the gas inlet vessel was successfully demonstrated. The electrical efficiency of the laser is typically 3% with peak powers of 40 kw being emitted on the P(20), P(18) and P(16) lines of the C0₂ spectrum. A parametric study of peak power, gas pressure, gas composition, time delay of the laser pulse and the interdependence of these quantities was undertaken. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A technique for measuring fluid velocities by means of neutrally-buoyant, phophorescent particles was investigated in a small-scale water jet facility. A nitrogen laser briefly illuminated the flow, exciting only those particles resident within the pulsed beam. The particles luminesce for a short while following excitation, during which time they also move with the flow. This creates a visible particle streak, the intensity of which decays along the direction of motion. A strobe illuminates the particles again a known time following the laser pulse. The magnitude and direction of a particle's velocity in the plane of view are deduced from an image of it streak captured by a video camera and recorded by a digital image processing system.

No description available.