The focus of this dissertation is to study the keV X-ray emission from different Z material to develop compact, quasi-continuous tabletop K sources for the application of in-line phase contrast imaging, X-ray diffraction experiments and X-ray microscopy. The emission occurs from plasma produced by focusing 120 fs, 800 nm and 300 J Ti:Sapphire laser pulses up to an intensity of few times 1016 W/cm2 on Cu, Fe and Ag metal targets. The main interest of these experiments is to characterize the fast electrons and X-ray emission from the laser plasma interaction. The investigation of X-ray emission energy and spectral characteristics is carried out by employing three detector systems which include filtered pin-diode, Charge Coupled Device and CdTe pulse height detectors. Scaling of photon flux as a function of incident laser energy is measured and related to the dynamics and absorption mechanisms involved in laser matter interaction. Specifically the construction and evaluation of K X-ray sources operated at 1 kHz repetition rate is reported with a photon flux of ~ 7109 photon/s, 3109 photon/s and 1.4107 photon/s around the K line for Cu, Fe and Ag respectively. These K fluences correspond to energy conversion efficiencies of around Cu = 310-5, Fe = 1.0610-5 and Ag = 210-7. The Cu and Fe K X-ray sources are applied to thin biological specimens and low atomic number (Z) materials to record in-line phase contrast images. It is demonstrated that the laser based K sources can serve as alternative sources for conventional X-ray radiography of biological samples in clinical applications. / Photonics and Plasmas

In this thesis electron generation is studied via laser plasma interaction known as laser wakefield acceleration in two regimes of weakly relativistic and highly relativistic laser intensity regimes. The plasma targets consisted of gas jets photonionized by rising edge of the laser pulse to densities as high as 10^20cm3. In the weakly relativistic regime, 210 mJ at 33 fs were focused to intensities of up to 310^18 Wcm2 on the gas targets of 2.4 mm length. In the highly relativistic regime, 3 J of energy compressed in 30 fs were delivered at intensity as high as 6.5 10^18 Wcm2 on targets of 2.4, 5, and 10 mm. Monoenergetic electrons in tens of MeV were observed in weakly relativistic regime, while electron energies as high as 300 MeV were observed in highly relativistic regime. Higher input laser intensity and prepulse levels were found to enhance electron production. Scaling of energy and stability of electron generation were also studied. / Photonics and Plasmas

Ce travail s'est attaché à démontrer théoriquement et expérimentalement l'intérêt d'une analyse spectrale de l'émission ionique radiale, croisée à l'observation quantitative des structures plasma excitées, pour cerner plus clairement les phénomènes en jeu au cours de l'interaction d'un laser avec un plasma. Le réglage fin des paramètres laser et plasma ont permis une étude paramétrique détaillée de l'interaction sur une large gamme de densité plasma. Lorsque l'on fait varier la densité plasma pic de très sous-dense à quasi-surdense, les distributions ioniques passent de piquées à maxwelliennes, et présentent de fortes modulations quand la durée de l'impulsion laser est proche de la demi-période plasma, i.e. proche de la résonance du sillage laser. Dans ce cas, le mécanisme d'accélération est différent de celui de l'explosion Coulombienne par l'intervention des effets du champ de sillage laser et du champ de la gaine plasma au bord du canal creusé par le laser. Par ailleurs, pour une densité plasma pic donnée et plus élevée que celle de la résonance, lorsque la longueur du gradient plasma est augmentée, l'accélération ionique est supprimée au profit de structures électromagnétiques fondamentales (soliton/vortex) clairement identifiées optiquement pour la première fois. Grâce à un protocole expérimental innovant, l'étude optique des champs magnétiques au sein de ces structures est à présent rendue possible. Enfin, proche de la densité critique, l'autofocalisation efficace du laser conduit à un dépôt très localisé de l'énergie laser, donnant naissance à une expansion électronique ultra-rapide (moins d'une picoseconde) et à la naissance d'un dipôle magnétique intense qui participe au chauffage des électrons. Pour ces études, des techniques et diagnostics ont été développés et testés simultanément avec succès. Il s'agit, d'une part, de jets de gaz submillimétriques à haute densité permettant une exploration localisée, stable et reproductible sur une large gamme de densités et de profils plasma, et d'autre part, d'interféromètre et polarimètre à hautes résolutions spatiales et temporelles afin d'observer la propagation laser et l'évolution des densités et champs au sein du plasma.

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No description available.

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.