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This thesis deals with several theoretical aspects of the interaction of an intense femtosecond laser pulse with a plasma. A mechanism for the enhancement of the collisional absorption of light at high intensities is described, involving the direct excitation of collective modes of the plasma, and the importance of this mechanism for a solid-density laser-produced plasma is studied under a range of conditions. An intensity-dependent collision rate is used in a numerical calculation of the reflectivity of a steep-gradient plasma, such as might be produced by an intense femtosecond laser pulse, and the conditions required to maximize absorption at high intensities are determined. The relative contributions of field-induced ionization and collisional ionization in laser-produced plasmas are studied, and it is shown that the behaviour of a gaseous plasma is almost solely governed by the field-induced process. A model is developed to simulate the propagation of an intense femtosecond laser pulse through an initially neutral gas, and this model is used to make predictions about spectral modifications to the laser pulse. Under certain conditions the spectrum is significantly broadened and suffers an overall blue shift. Quantitative fitting of theoretical spectra to experimental results in the literature is attempted, but is complicated by associated defocusing effects in the plasma. Field-induced ionization can produce a gaseous plasma which is significantly colder, for the same degree of ionization, than a plasma produced by collisional ionization. One possible application for a cold highly-ionized plasma is in a recombination x-ray laser, and the propagation model allows the calculation of the plasma temperature, which is a crucial parameter in assessing the feasibility of such schemes.

Orientador: Antonio Jorge Abdalla / Banca: Marcelo dos Santos Pereira / Banca: José Vitor Candido dos Santos / Banca: Leonardo Fanton / Banca: Rafael Humberto Mota de Siqueira / Resumo: O objetivo deste trabalho é avaliar e comparar a resistência mecânica da união de aços dissimilares, o aço Maraging 300 com o aço 300M-ESR pelos processos de soldagens autógenas a Plasma (Plasma Arc Welding - PAW) e a Laser (Laser Beam Welding - LBW). As juntas foram submetidas à avaliação da resistência mecânica através de ensaios de tração e de dureza Vickers (HV) no cordão de solda e zona termicamente afetada. Foram também realizadas análises químicas e metalográficas das juntas soldadas, correlacionando a microestrutura com as propriedades observadas. Em ambos os processos foram aplicados tratamentos térmicos antes e após as soldagens, com o objetivo de endurecer os materiais e procurando aproximar a dureza de ambos os aços e da zona fundida (ZF) e zona termicamente afetada pelo calor (ZTA). Foram realizados vários testes com tempos e temperaturas para se definir quais eram os melhores tratamentos térmicos adotados para a equalização das propriedades mecânicas. Os tratamentos térmicos aplicados após a solda mostraram-se convenientes para o nivelamento dos valores das durezas, somente exibindo poucas perdas nas ZTAs dos aços Maraging. Com a aplicação destes tratamentos também foi possível notar uma equalização nas resistências à tração, em torno de 1300 MPa e aumentos consideráveis das mesmas, comparado com a mesma condição sem tratamento. As soldas mostraram-se eficientes para a união das chapas, porém, em algumas situações da soldagem a Laser, apresentou pequenas falhas,... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The objective of this work is to evaluate and compare the mechanical strength of welded joints made of the dissimilar steels such as Maraging 300 steel with 300M-ESR steel by the autogenous Plasma Arc Welding (PAW) and Laser welding process (LBW). The joints were submitted to the mechanical strength evaluation by tensile test and hardness Vickers (HV) at the weld bead and thermally affected zone. Chemical analysis and metallographic analysis of the welded joints were also performed, correlating the microstructure observed with the properties. In both processes, heat treatments were applied before and after welding, with the objective of at harden the materials and seeking to approximate the hardness of both steels and the fusion zone and heat affected zone. Several tests were carried using different times and temperatures to determine which ones were the best heat treatments to be adopted aiming the equalization of mechanical properties. The heat treatments applied after welding proved to be convenient for the leveling of the hardness values, only showing few losses in the HAZs of the Maraging steels. Applying of these treatments, it was also possible to note the equalization of tensile strengths, around 1300 MPa and considerable increases of the same, compared to the same condition without treatment. The welds showed to be efficient for the joining of the plates, however, in some situations the Laser welding presented small flaws, such as presence of pores, in the weld beads... (Complete abstract click electronic access below) / Doutor

This thesis documents the development of plasma waveguides for high-intensity laser pulses. Initial work concentrated on the development of the discharge-ablated capillary waveguide, based on the work of A. Zigler (Zigler, A., Y. Ehrlich, C. Cohen, J. Krall and P. Sprangle, J. Opt. Soc. Am. B 13, 68). The waveguide was shown to be capable of guiding picosecond laser pulses with an intensity of 10¹⁶ W cm^-2 over a length of 10 mm. The pulse energy transmission of the capillary was increased from 48% to 70% when the discharge was fired. An interferometry-based measurement technique was developed, allowing measurement of the electron density profile formed in the capillary waveguide. These measurements were used as input to a numerical simulation that predicted the propagation of intense laser pulses through partially-ionised plasma waveguides. Numerical simulations accurately reproduced the picosecond pulse guiding results, and gave important insights into the properties and severe drawbacks of partially-ionised waveguides. Previous work on partially-ionised plasma waveguides has not fully explored the implications of the propagation of intense pulses through the partially-ionised plasma. For polypropylene waveguides, it was shown that for pulses with an intensity of 10¹⁶ W cm^-2, the waveguide is not capable of high-quality guiding. However, for pulses with an intensity of greater than 10¹⁷ W cm^-2, high-quality guiding is predicted through the partially-ionised waveguide in a new regime called "quasi-matched guiding". A novel gas-filled capillary discharge waveguide was designed and built. The device was shown to form a guiding channel inside a capillary pre-filled with gas. Interferometry measurements of the electron density profile formed in a hydrogen-filled capillary discharge waveguide showed that an approximately parabolic plasma waveguide could be formed in an essentially fully-ionised hydrogen plasma. The device was used to guide femtosecond laser pulses, with an intensity of 10¹⁷ W cm^-2, over distances of 20 and 40 mm, with a pulse energy transmission of 92% and 82% respectively. For the 20 mm-long waveguide, the peak intensity in the output plane of the waveguide was 70% of that at the waveguide input. These results indicate the lowest coupling and insertion losses of any waveguide published to date. The gas-filled capillary discharge waveguide is shown to be capable and versatile, and is suited for use as a tool in other applications. The use of the waveguide in the fields of XUV lasers and laser wakefield acceleration is discussed.

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.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.

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Title: The spectroscopy study of focused short-wavelength laser-matter interactions Author: Tomas Burian Department: Department of Surface and Plasma Science Supervisor: Ing. Libor Juha, CSc., Department of Radiation and Chemical Physics, Institute of Physics of the Czech Academy of Sciences, Prague Abstract: This thesis is focused on processes occurring during and immediately after an interaction of intense short-wavelength laser pulses with matter. Extreme states of matter (warm dense matter - WDM and hot dense matter - HDM), induced by EUV/SXR/X-ray lasers of two kinds, i.e., free-electron lasers (FEL) and plasma-based lasers, were investigated by emission spectroscopy over various spectral ranges and mass spectroscopic techniques. Absorption/transmission experiments revealed an effect of saturable absorption of soft X-ray laser radiation in aluminium WDM. Then, an ionisation potential depression (IPD) in dense plasmas was investigated by means of X-ray emission spectroscopy. Results obtained with the X-ray FEL-produced plasma exhibit very good agreement with computer simulations considering the Ecker-Kröll model. Analysis of optical emission spectra (OES) supports the key role played by fast recombination processes in the ablation plume created by focused short-wavelength laser beams on the solid target...

La caractérisation de matériaux par spectroscopie optique d’émission d’un plasma induit par laser (LIPS) suscite un intérêt qui ne va que s’amplifiant, et dont les applications se multiplient. L’objectif de ce mémoire est de vérifier l’influence du choix des raies spectrales sur certaines mesures du plasma, soit la densité électronique et la température d’excitation des atomes neutres et ionisés une fois, ainsi que la température d’ionisation. Nos mesures sont intégrées spatialement et résolues temporellement, ce qui est typique des conditions opératoires du LIPS, et nous avons utilisé pour nos travaux des cibles binaires d’aluminium contenant des éléments à l’état de trace (Al-Fe et Al-Mg). Premièrement, nous avons mesuré la densité électronique à l’aide de l’élargissement Stark de raies de plusieurs espèces (Al II, Fe II, Mg II, Fe I, Mg I, Halpha). Nous avons observé que les densités absolues avaient un comportement temporel différent en fonction de l’espèce. Les raies ioniques donnent des densités électroniques systématiquement plus élevées (jusqu’à 50 % à 200 ns après l’allumage du plasma), et décroissent plus rapidement que les densités issues des raies neutres. Par ailleurs, les densités obtenues par les éléments traces Fe et Mg sont moindres que les densités obtenues par l’observation de la raie communément utilisée Al II à 281,618 nm. Nous avons parallèlement étudié la densité électronique déterminée à l’aide de la raie de l’hydrogène Halpha, et la densité électronique ainsi obtenue a un comportement temporel similaire à celle obtenue par la raie Al II à 281,618 nm. Les deux espèces partagent probablement la même distribution spatiale à l’intérieur du plasma. Finalement, nous avons mesuré la température d’excitation du fer (neutre et ionisé, à l’état de trace dans nos cibles), ainsi que la température d’ionisation, à l’aide de diagrammes de Boltzmann et de Saha-Boltzmann, respectivement. À l’instar de travaux antérieurs (Barthélémy et al., 2005), il nous est apparu que les différentes températures convergeaient vers une température unique (considérant nos incertitudes) après 2-3 microsecondes. Les différentes températures mesurées de 0 à 2 microsecondes ne se recoupent pas, ce qui pourrait s’expliquer soit par un écart à l’équilibre thermodynamique local, soit en considérant un plasma inhomogène où la distribution des éléments dans la plume n’est pas similaire d’un élément à l’autre, les espèces énergétiques se retrouvant au cœur du plasma, plus chaud, alors que les espèces de moindre énergie se retrouvant principalement en périphérie. / Interest in the characterization of materials by laser induced plasma spectroscopy (LIPS) is growing with new applications emerging at an ever increasing pace. The purpose of this thesis is to verify the influence of the selection of spectral lines according to measured parameters of the plasma: electron density and excitation (neutral and singly ionized atoms) and ionization temperatures. Our measurements are conducted under typical operating conditions of LIPS: spatially integrated and temporally resolved. We used two binary aluminum targets containing trace elements (Al-Fe and Al-Mg). First, we measured the electron density using Stark broadening of lines from several species (Al II, Fe II, Mg II, Fe I, Mg I, Hα). We observed that the absolute density had a different temporal behavior depending on the species. The ionic lines giving electron densities systematically higher (up to 50 % at 200 ns after plasma ignition), and decreasing faster than densities derived from neutral lines. Densities obtained from trace elements Mg and Fe are lower than densities obtained from the commonly used line Al II at 281.618 nm. In parallel, we studied the space-integrated electron density evolution found from hydrogen Hα line and observed that it has a temporal behavior similar to the density obtained by the Al II line at 281.618 nm. Thus the two species probably share the same spatial distribution within the plasma. Finally, we measured the excitation temperature of iron (neutral and ionized, in trace amount in our targets), and the ionization temperature, using Boltzmann and Saha-Boltzmann plots, respectively. As previously described by Barthélémy et al. (2005), it appears that the different temperatures converge to a single value (considering error bars) after 2-3 microseconds. The different temperatures measured from 0 to 2 microseconds do not overlap, which could be explained by a departure from local thermodynamic equilibrium (Barthélémy et al., 2005), or by considering an inhomogeneous plasma where spatial distribution differs from one species to another, so that high energy species are found from within the plasma’s centre, which is hotter, while the lower energy species are found mainly in the periphery.