Le travail effectué dans le cadre de cette thèse porte sur l'étude d'un atténuateur de rayonnement X à gaz et du plasma produit à l'intérieur. Un atténuateur à gaz est composé d'une chambre remplie du gaz, généralement argon ou krypton à quelques centaines de millibars, qui absorbe la partie de basse énergie d'un spectre de rayonnement X de synchrotron, en réduisant la puissance reçu par les éléments optiques en aval sans affecter les propriétés de la partie de haute énergie du spectre. L'absorption des photons crée une région de gaz chaud et ionisé autour du parcours du faisceau X, en réduisant la densité du gaz localement. A détaillé bilan énergétique entre tous les processus impliqués c'est nécessaire pour être capable de prédire l'absorption et opérer et dessiner atténuateurs a gaz efficacement. Un modèle hybride que combine techniques de modélisation Monte Carlo et fluides à été développé pour déterminer le bilan énergétique et simuler l'absorption de rayonnement X. Le modèle a été valide expérimentalement pas études incluant absorption de puissance, spectroscopie optique d'émission et spectroscopie d'absorption par laser à diodes. Les résultats des simulation et expériences montre un plasma confiné autour du parcours du faisceau X, recombinant dans le volume de gaz et avec une température maximale de plusieurs centaines de Kelvin. Le modèle a été capable de prédire l'absorption de rayons X avec un erreur de entre 10 et 20%, qui permettre son utilisation comme première approximation pour le dessin et opération de atténuateurs a gaz et aussi comme point de partie pour modèles plus affinées. / The work done in the context of this thesis focuses in the study of an X-ray gas attenuator and the plasma produced within. An X-ray gas attenuator consists on a vessel filled with gas, usually argon or krypton at a few hundreds millibars, that absorbs the low energy fraction of a synchrotron X-ray spectrum, reducing the power received by downstream optical elements without affecting the properties of the high energy part of the spectrum. The absorption of the photons creates a region of hot, ionized gas around the X-ray beam path, decreasing locally the gas density. A detailed energy balance between all the involved processes is required to be able to predict the absorption and operate and design gas attenuators efficiently. A hybrid model combining Monte Carlo and fluid modelling techniques has been developed to determine the energy balance and simulate the X-ray absorption. The model has been validated by experimental studies including power absorption, optical emission spectroscopy and tunable laser absorption spectroscopy. The results of both simulation and experiments show a plasma confined around the X-ray beam path, recombining in the bulk of the gas and with a maximum temperature of several hundreds of Kelvin. The model was able to predict the X-ray absorption within a 10-20% of error, which allows its use as a first approximation for the design and operation of gas attenuators, and also provides a starting point for more refined models.

In this thesis, we report the various aspects of fabricating a Cold Atmospheric Plasma system, which can be used for Plasma Enhanced Chemical Vapour Deposition. The greatest advantage of this system is its vacuum free operation, which provides a cost e effctive alternative over conventional high vacuum systems. We have designed a reactor geometry for such a plasma system, in which, the contamination due to ambient air is kept at a minimum value using a low flow of Ar (500 sccm). Towards this end, we have modeled and simulated the flow pattern of Ar gas entering the reactor geometry and have studied its e effectiveness in removing air from the plasma zone. We have fabricated such a geometry and studied the contamination at different flow rates of Ar by observing the plasma optical emission. Further, the aspect of lamentation in atmospheric pressure plasma has been studied and we have identified a few process parameters which can convert a filamentary discharge to a diffused glow. Subsequently, a complete system was developed, including an in-house built high voltage power supply, to generate a plasma with low contamination and less number of laments. We have also carried out plasma diagnostics, specifically to estimate the Electron Energy Distribution Function (EEDF) of the plasma, by analysing the radiation emitted from an Ar plasma, acquired using an Optical Emission Spectroscope. The peaks in the spectrum were curve flatted with Voigt pro les and their widths and intensities were mapped to the electron number density and the EEDF of the plasma, using the mathematical models for Stark broadening and Corona population respectively. An optimization routine based on Nelder-Mead simplex algorithm was run to estimate the optimal values of these plasma parameters that produced a good match between the simulated spectrum and the experimentally acquired one. This analysis estimated that the value of electron number density in our plasma was in the range 0:82 1017 cm 3 to 3:56 1017 cm 3 and the electron temperature was in the range 0.36 eV -0.39 eV . It also predicted that the EEDF closely approximated a Maxwellian distribution. As a proof of concept, the fabricated reactor was used to deposit thin films of Polyacetylene over microscopic cover glass slides by polymerizing Acetylene gas in the cold plasma. Deposition rates as high as 1 m=min, were obtained during thin lm deposition of the polymer. The polymeric structure of the lm was studied using NMR and FT-IR. XPS measurement revealed 5% O2 inclusion in the samples. XRD showed no distinguishable peak, indicating the amorphous state of the films. The surface morphology investigated using SEM revealed highly porous broid kind of structures, which appeared to be agglomeration of particles with sizes in the order of few micrometers. P-type Polyacetylene lms were fabricated by doping them with 5.3% by atomic concentration of I2 vapours. The UV-Visible spectroscopy study revealed a bandgap of 2.05 eV for undoped and 1.49 eV for the doped Polyacetylene samples. The lms exhibited an increase in conductivity by two orders of magnitude; from 3:6 10 13 1cm 1 to 3:5 10 11 1cm 1 for un-doped and doped Polyacetylene samples respectively.