الفهرس 
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Abstract
This thesis reports a numerical investigation of the phenomenon associated with CO2 laser-induced helium breakdown and plasma expansion. The analysis is based on the measurements carried out by Camacho et al. (Camacho et al. 2011) for the breakdown of helium by 9.621 μm CO2 laser at pulse duration (FWHM) of 64 ns. The gas pressure was varied from 12.0 kPa up to 101.0 kPa (90.0 torr to 757.0 torr). The study adopted an electron cascade model given in Gamal and Abdellatif (Gamal and Abdellatif 2014), previously developed by Evans and Gamal (Evans and Gamal 1980). The model numerically solves a time-dependent Boltzmann equation for the electron energy distribution function (EEDF) and a set of rate equations representing the change of excited states population. Furthermore, the applied model is adapted to account for the possible electrons generation and loss processes that might take place in the interaction region corresponding to the given experimental conditions. In this experiment, due to the far-infrared (FIR) laser (hυ” ” ” ” " ~ " ” ” ” ” 0.12 eV), multiphoton ionization (MPI) becomes improbable. Therefore, a previous breakdown technique is used as a preionization source to initiate the breakdown of helium. However, no experimental evidence was given to define the value of these initial electrons density (IED) regarding each gas pressure. Since their values can considerably affect the breakdown threshold dependence on the gas pressure, the calculations are carried out to determine the breakdown threshold intensity as a function of the gas pressure considering a wide range of the IED. It has been shown that the values of these densities, which obtained reliable threshold intensities in a reasonable agreement with the measured ones, vary between 1011 cm-3 to 1013 cm-3 on the tested pressure range, and they exemplified an unsystematic variation with the gas pressure. Both thresholds showed a fast DROP over the low-pressure region, while they reserved almost the same value over the remaining tested pressure range (30.0kPa- 87.0 kPa). Based on the determined IED, a comparative study of the electron energy distribution function (EEDF) and its parameters (viz, the electrons density, ionization rate, electron mean energy ….and so on) for selected gas pressure values covering the whole examined range is presented. These calculations target the individual influence of the physical processes responsible for helium breakdown by FIR laser radiation. The computations reveal that electrons’ growth occurs through collisional ionization of the ground and excited gas atoms. At the high pressure, however, the electron recombination losses are dominated. Electron energy loss by elastic collision and the loss of electrons through diffusion showed a negligible contribution under the experimental conditions under investigation. Finally, considering Gaussian spatial distribution of the laser intensity in the focal volume, the Boltzmann equation was solved numerically to investigate the following: (I) The distribution of the electrons density obtained at the end of the laser pulse along the axial (z) and radial (r) distances of the cylindrical focal volume. This enables an accurate estimation of the picture of the plasma size (breakdown region) and its location along the axial and radial distances of the focal volume as a function of the gas pressure. (II) Furthermore, clarification of the breakdown and non-breakdown areas following the electrons density concerning the attained degree of ionization. (III) The dependence of plasma expansion on both gas pressure and the value of the IED.