Sputtering hollow cathode discharges designed for laser applications : experiments and theory

This thesis presents experimental and modelling studies of hollow cathode discharges (HCDs). This type of plasma source has several applications, for instance in the field of atomic spectrometry, vacuum micro-electronics, UV generation, ion sources, plasma processing, surface treatments and laser technology. More specifically, this study is devoted to the application of HCDs in laser technology. In this application field the HCD acts both as a metal vapour source by means of cathode sputtering, and as an active medium to excite metallic laser transitions. To improve and optimise the laser performance, a proper understanding is needed of the various processes and mechanisms that take place in the hollow cathode plasma and how these depend on the settings of the external parameters. Experiments: Several laser tubes were designed, built and studied experimentally. The impact of the external parameters settings on the laser operation were investigated by monitoring the behaviour of the IR copper ion line (780.8 nm), the transition with the highest gain. In this way the features of the optimal conditions and the best geometry could be determined experimentally. Modelling: The Plasimo-MD2D platform was used to construct a model that facilitates in-depth studies of the plasma behaviour and that can be used to find the optimum location in parameter space for the operation of these discharge types. The versatility of the model makes it possible to simulate the plasma processes for different HCD geometries, gas compositions and operation parameters. Analytical model : An analytical model was constructed with the aim to interprete the numerical results. The model is focused on the main plasma properties such as the plasma density, potential and current. It is based on elementary theories of glow discharges and extends these to account for the axial non-uniformity of longitudinal HCDs. The modelling results were validated with experimental observations. This comparison is guided by the analytical model that was constructed for that purpose. The insights obtained by this validation have lead to an improvement of the numerical model that can now be successfully employed as an optimisation tool for HCD lasers in particular, and more in general, for the improvement of various other HCD applications.