Neste trabalho são apresentados e discutidos resultados referentes à formação e ao equilíbrio da corrente de plasma no TBR, Tokamak projetado e construído no Instituto de Física da Universidade de São Paulo. As curvas de ruptura, determinadas para hidrogênio, argônio e hélio são comparadas com um modelo simples, apresentando um acordo razoável. É verificada a influência de campos magnéticos espúrios na formação do plasma e são encontradas condições que auxiliam no processo de ruptura. São mostrados perfis temporais característicos da corrente de plasma e voltagem de loop para disparos onde se obtém equilíbrio. É feita uma comparação dos resultados experimentais com um modelo para descarga em tokamaks aquecidos ohmicamente. Tendo a carga efetiva como parâmetro, os resultados apresentam um razoável acordo com o modelo. / In this work are presented and discussed results of the formation and equilibrium of the plasma current in TBR, a small tokamak, designed and contructed at the Instituto de Física da Universidade de São Paulo. The measured breakdown curves for H2, A and He are compared with the predictions of a simple model with reasonable agreement. The influence of stray magnetic fields in the plasma formation is investigated and conditions are chosen to facilitate the breakdown. The time profile of loop voltage and plasma current for shots with plasma equilibrium are shown. A comparison is made between experimental results and analytical-numerical model for tokamaks discharges with ohmic heating. Reasonable agreement is obtained when Z, effective atomic number, is assumed as a parameter.

This thesis presents investigations of interactions of a 380 fs, 248.6 nm KrF laser beam focused to intensities up to 10¹⁸ W/cm² with gas targets of Helium and Neon. At these intensities, the targets are rapidly ionised by the process of optical field ionisation. The purpose of these investigations is two-fold. Firstly, we determine the relative importance of plasma heating mechanisms and investigate the scaling of electron temperature with density to ascertain whether an optically-ionised XUV recombination laser, utilising the 3s-2p transition in a Lithium-like ion, is viable. Such a laser requires, for the case of Neon, the production of very cold (<50 eV) electron temperatures at electron densities of the order of 5 x 10²⁰ cm^-3 , and optical-ionisation to the Helium-like stage of Neon (Ne⁸⁺). The results are compared and contrasted to computer simulations of the electron temperature. Secondly, we investigate the process of harmonic generation. We show that, contrary to commonly held beliefs, it is possible to generate shorter harmonic wavelengths from ions using a short wavelength laser than from atoms using longer wavelengths. We also show high harmonic generation efficiencies from neutral Helium at lower intensities (~10¹⁵ W/cm²) away from the laser focus. The results are compared to, and found to be matched well by, simulations which predict the single atom/ion harmonic response.

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