Thesis (M.Sc. (Hons.)) -- University of Western Sydney, 2002. / "A thesis submitted as part of the requirements for the degree of Master of Science (Honours)" Bibliography : leaves 175-181.

This thesis describes a detailed investigation into the process of laser-wakefield acceleration (LWFA) for the generation of high-energy electron beams using the hydrogen-filled capillary discharge waveguide. In only the second experiment to be performed using the newly commissioned Astra-Gemini laser at the Rutherford Appleton Laboratory, electron beams were accelerated to energies greater than 0.5 GeV by laser pulses of energy 2.5J and peak power of 30T\~T. The injec- tion and acceleration of electron beams was seen to depend on the state of the plasma channel for axial electron densities less than 2.5 x 1018 cm -3. With the aid of simulations performed using the code WAKE it was found that the plasma channel allows the laser pulse to maintain its self-focussed spot size along the length of the capillary even below the critical power for self-guiding. It was found that the threshold laser energy required for the production of elec- tron beams was reduced by the use of an aperture placed early in the laser system. This was attributed to the increased energy contained in the central part of the focal spot of the laser. A short paper on this work was published in Physical Review Special Topics - Accelerators and Beams and a longer paper was published in the New Journal of Physics. Transverse interferometry was used to measure the electron density of the plasma channel used in the Astra-Gemini experiments. An imaging system was devised which used cylindrical optics to increase the field of view of the capillary longitudinally, whilst maintaining the trans- verse resolution. The measured properties were consistent with previous measurements made by Gonsalves et al. [J]. The observed longitudinal variations in the plasma channel parameters were not found to be significant enough to affect the injection process.

This thesis presents an investigation into inertial confinement fusion through mathematical models and computer simulations. Salient features affecting fusion are identified, in both energy absorption and fusion gains. Mathematical tools are applied to a directed investigation into plasma structure. Parameters such as these involved in electromagnetic energy absorption are identified first, and the next step is to model the immediate response of the plasma to this energy input, with a view to how this may be advantageous to initiating fusion. Models are developed that best suit plasma behaviour. The parameters are presented graphically against time and distance into a small plasma fuel pellet. It is noted how field density and ions form undulations through the plasma. Types of plasma fuels are discussed with regards to their key parameters. Computations are performed using the laser driven inertial energy option based on volume ignition with the natural adiabatic self-similarity compression and expansion hydrodynamics. The relative merits of each fuel are discussed against the parameters of density, volume and energy input versus fusion gains. / Master of Science (Hons)

An experimental study of the interaction between 10.59 μm C0₂-laser radiation and the plasma produced by that radiation on plane, massive targets in a vacuum, is described. The laser used was a TEA double discharge device, giving an in vacuo irradiance in the regime of 10¹² W/cm² with 50 ns FWHM gain switched pulses. The laser oscillator cavity was operated in both stable and unstable configurations. Particular emphasis was placed on the full diagnosis of the scattered light and experiments were, therefore, designed to investigate the spatial, temporal and spectral characteristics of this unabsorbed fraction. The irradiation conditions under which the experiments were performed were investigated by analysis of the far field energy distribution. High absorption of the incident radiation was inferred from the low reflectivity level, the fraction of the incident radiation scattered out- side the solid angle of the focusing lens being accounted for, using a novel collecting mirror, or by the use of calibrated burn paper. Thus a total reflection was measured for the first time. A simple model was developed to enable the classical absorption contribution to the total absorption to be estimated, and results indicated that inverse bremsstrahlung absorption would be an important process for the conditions pertinent to the interac- tion. The contribution of anomalous absorption and scattering processes was estimated. Predicted cold electron temperatures and density scale lengths from the model, agreed well with those determined experimentally without the need to invoke flux inhibition. The spectral composition of the backscattered radiation in the region of the fundamental was investigated, and measured spectra were successfully interpreted by considering the stimulated Brillouin scattering process, and the Doppler effect of the expanding plasma corona. In particular, the depen- dence of the parametric growth rate on electron density was investigated by numerical solution of the ion acoustic wave dispersion relation. The angular distribution of the backscattered radiation was determined using annular apertures in the backscattered beam and the distribution found to be non-isotropic. Interpretation was based upon the resonance absorption process in a non-planar plasma geometry and invoked the retro reflecting nature of the stimulated Brillouin scatter mechanism. To investigate the effect of wave refraction in the underdense plasma (exhibiting a continuous density profile), a compact and readily used ray-tracing routine was developed and applied to plasmas with planar, rippled and simple concave or convex iso-density contours.

Thesis (Ph. D.)--University of Nevada, Reno, 2007. / "December, 2007." Includes bibliographical references (leaves 244-254). Online version available on the World Wide Web.

Thesis (M.S.)--University of Nevada, Reno, 2007. / "August 2007." Includes bibliographical references (leaves 80-83). Online version available on the World Wide Web.

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 125-159). Available also in a digital version from Dissertation Abstracts.

Thesis (PH.D.)--Michigan State University. Mechanical Engineering, 2008. / Title from PDF t.p. (viewed on Aug. 28, 2009) Includes bibliographical references (p. 154-167). Also issued in print.

A CO₂ laser system delivering a 12 J pulse with a FWHM of 2 ns on target was developed to serve as a driver for studies of laser plasma interactions within a cylindrical cavity. The system consisted of a hybrid oscillator, followed by an amplifier chain, and it achieved its design goals of delivering an intense CO₂ pulse, Gaussian in time and space, with a high contrast ratio on a reliable basis. The targets in which the plasma was produced consisted of small rectangular plates of lucite, with holes drilled through one of the long axes. The holes were 350 μm to 600 μm in diameter, and 10 mm in length. These dimensions allowed the laser beam, focused at the entrance of the hole, to produce sufficient intensity on the inner walls of the cylindrical cavity for plasma formation, while allowing the beam, with a waist diameter of 100 μm at the focus to deliver most of its energy within the cavity. The beam propagated via multiple reflections from the plasma through the cavity. Diagnostics were performed on the beam transmitted through the target. Streak camera images were collected of the intensity of visible emission from the plasma along the axis of the target. Anomalous results were obtained with respect to the reproducible observation of maximum visible light emission from regions at the far end cavity from where the laser beam is injected. Another unforseen but interesting result was the small divergence of the beam transmitted through the cavity. Preliminary models were developed to attempt to explain the observations. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

In some high-field laser-plasma experiments, it is advantageous to accompany the main high-energy (~1 J) laser with a second high-energy pulse (~0.1 J) which has been frequency-shifted by ~10-20%. Such a pulse-pair would have a low walk-off velocity while remaining spectrally distinct for use in two-color pump-probe experiments. Moreover, by shifting the second pulse by ~plasma frequency, it is theoretically possible to exercise some amount of control over a variety of laser-plasma instabilities, including forward Raman scattering, electromagnetic cascading, and relativistic self-focusing. Alternatively, the two pulses may be counter-propagated so that the collide in the plasma and create a slowly-propagating beatwave which can be used to inject electrons into a laser wakefield accelerator. The design, characeterization, and performance of a hybrid chirped-pulse Raman amplifier (CPRA)/Ti-Sapphire amplifier are reported and discussed. This hybrid system allows for the generation of a high-energy (>200 mJ), broadband (15-20 nm bandwidth FWHM), short duration (>100 fs duration) laser sideband. When amplified and compressed, the Raman beam's power exceeds 1 TW. This sideband is combined with the primary laser system to create a bi-color terawatt laser system which is capable of performing two-color high-field experiments. This two-color capability can be added to any commercial terawatt laser system without compromising the energy, duration or beam quality of the primary system. Preliminary two-color laser-plasma experiments are also discussed. / text