Author: Deambrosis Silvia ; Type of thesis: Master Thesis
Abstract: Preparation of cavity walls has been one of the major problems in superconducting radio-frequency (SRF) accelerator technology. Accelerator performance depends directly on the physical and chemical characteristics at the SRF cavity surface. The ambitious objective of this project is to study a cavity surface preparation process which is superior in terms of cost, performance, and safety, to the wet chemical process currently in use. Plasma based processes provide an excellent opportunity to achieve these goals. Plasmas are chemically active media. Depending on the way they are activated and their working power, they can generate low or very high “temperatures” and are referred correspondingly as cold or thermal plasmas. This wide temperature range enables various applications for plasma technologies: surface coatings, waste destruction, gas treatments, chemical synthesis, machining … many of these techniques have been industrialized. A large number of important industrial plasma applications are carried out close to atmospheric pressure, in many cases in atmospheric air. The fascinating possibility to perform cleaning and/or etching processes of RF cavities without the need of any vacuum pumping system has to be deeply explored realizing different atmospheric con gurations as corona plasma, rf resonance plasma, plasma jet and torch. Thermal plasmas (especially arc plasma) were extensively industrialized, principally by aeronautic sector. Cold plasma technologies have been developed in the microelectronics but their vacuum equipment limits their implantation. To avoid drawback associated with vacuum, several laboratories have tried to transpose to atmospheric pressure processes that work under vacuum for the moment. Their researches have led to various original sources. In the textile sector, a number of plasma applications are conceivable and some have been tested in laboratory scale. The chemical functionality and/or the morphology of a ber surface can be altered in order to improve very di erent properties to tailor them for certain demands. The wettability can be increased to achieve a better impregnation or a deeper dying or, in contrast; it also can be decreased to create a water repellent behavior. New chemical functionalities on the surface can promote the reactivity with dyes. The water free removal of sizings seems to be possible. These are only a few examples that demonstrate the potential of this technology. We decided to try to ignite a resonance atmospheric plasma into 1.5 GHz superconducting niobium cavities to perform a feasibility study. The second step has been the attempt to understand what really happens to the resonant structure internal surface. The most powerful tool consists in the atmospheric plasma treatment and fast rf characterization of 6 GHz small resonators.
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