Thesis (Ph. D.)--University of Wisconsin--Madison, 1983. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Second harmonic(2ω₀) and three halves harmonic(3/2ω₀) are studied in the interaction of CO₂ laser of frequency ω₀, with an underdense plasma. It is shown theoretically that filamentation can lead to sizable second harmonic generation. The 2ω₀-radiation is emitted into a forward directed cone, its angle determined by the wavelength of the fundamental radiation in the plasma filament. Experimental observations confirm the theoretical predications. Aside from the forward directed emission cone, non negligable 2ω₀ power is detected over a broad angular range in the backward direction showing a broadened red shifted spectrum. The dependence of the backward emitted 2ω₀-radiation on the plasma density, and the red shifted spectrum strongly support the argument that the backward emitted 2ω₀-radiation is related to the SBS(Stimulated Brillouin scattering) instability. Based on the wave vector matching conditions, the angular distribution of 3/2ω₀-radiation is predicted. The experimental results confirm the prediction. It is shown experimentally that the fusion of three plasma waves produced in the TPD(Two Plasma Decay) instability dominates the generation of 3/2ω₀-radiation when the incident beam energy is high. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Honors project (B.A.) -- Carson-Newman College, 2010. / Project advisor: Dr. Michael Seale ; Research advisor: Chris Niemann. Includes bibliographical references (pp. 24-25).

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-2 power law. The average phase velocities are around 4.5 km/sec and are slightly larger than the electron diamagnetic drift velocity. The waves are tentatively identified with the electron drift mode. Coherent signals due to long wavelength MHD modes are detected. Results from a Langmuir probe corroborate most of these data.

Neste trabalho foi produzido um plasma através da focalização da radiação de um laser CO2-TEA pulsado sobre um alvo sólido de ferro, colocado no interior deuma câmara de vácuo. Apresentamos resultados experimentais referentes às medidas de temperatura de íons, temperatura de elétrons, densidade de elétrons e velocidade de expansão do plasma. Utilizou-se a técnica de sonda de Langmuir para medida da densidade e temperatura de elétrons. Um analisador eletrostático multi-grade foi especialmente construído para a medida da função de distribuição de energia de íons. A velocidade de expansão do plasma foi inferida a partir do tempo de vôo do plasma. É mostrado que os resultados experimentais são consistentes com um modelo de expansão livre de um plasma em vácuo.

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.