Lanza, Giulia (2008) New magnetron configurations for sputtering niobium thin films into copper tesla-type superconducting cavities. [Tesi di dottorato]
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Superconducting radiofrequency resonators for particle acceleration have become a standard component for particle accelerators. This work proposes a deep study and development of an alternative to the more frequently used bulk niobium cavities: niobium thin film coated into a copper cavity. The first niobium-coated copper cavity was produced at
CERN in the early eighties. The sputter technology was chosen first in the pure diode configuration and subsequently in the magnetron configuration. The last one was adopted
for the successful series production of the LEP and LHC cavities.
In this work an intensive R&D effort has been undertaken to study the coating technique, to improve it with several equipments and understand the correlation between the
coating system applied and the film morphology, the superconducting properties and the RF film quality. Four different coating configurations for sputtering niobium films into 1.5GHz copper cavities has been explored. First of all, the standard technique applied for several years at CERN to coat the LEP cavities has been reproduced. Then, in order to
improve the Nb film quality, the application of three main ideas to the sputtering process has been investigated: i) making niobium atoms imping perpendicularly to the substrate
surface, ii) promoting the effect of plasma bombardment on the growing film, iii) increasing the sputtering rate. Therefore, three different and new sputtering configurations are described: the effect of Nb atoms arriving perpendicularly to the substrate is explored either by using a cathode that follows the cavity shape (Large Area Cavity Shaped Cathode) or by increasing the plasma confinement efficiency by means of a target parallel to the magnetic field lines (Ringed Shaped Cathode). The removal of adsorbed impurities from the
film surface and the increase of the film density are investigated by a biased third electrode that promotes the positive ion bombardment of the growing film. A mixed Bias-Magnetron has been built using a positively charged metal grid positioned all around the cathode.
Different film characteristics have been studied and compared, focusing mainly on superconducting and resistive properties. Also morphological and microstructural properties
have been analyzed with the very valuable collaboration of "Interdepartmental Laboratory of Electron Microscopy" (LIME), University of Rome "ROMA TRE", at the Mechanical
and Industrial Engineering Department. Four RF test on different accelerating cavities are reported and commented. In addition a 3-cell bulk niobium 1.3 GHz cavity has been prepared and measured in order to compare bulk and thin film results.
Even if the work is still in progress every partial results have been analyzed and commented, in order to extrapolate every possible information. The final result is a global overview of the sputtering coating techniques and OF their results, with at the end some suggestions for the future developments.
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