Author: Lanza Giulia ; Type of thesis: PhD Thesis
Abstract: Superconducting radiofrequency resonators for particle acceleration have become a standard component for particle accelerators. This work described an in-depth 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 latter 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 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 1.5 GHz copper cavities has been explored. First, 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 was 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 was 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 were investigated by using a biased third electrode that promoted the positive ion bombardment of the growing film. A mixed Bias-Magnetron system was built using a positively charged metal grid which surrounded the cathode. Different film characteristics were studied and compared, focusing mainly on superconducting and resistive properties. Also morphological and microstructural properties were analyzed in a very valuable collaboration with “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 were prepared and measured in order to compare bulk and thin film results. Even though the work is still in progress all of the partial results to-date have been viii Abstract analyzed and commented, in order to extrapolate every possible information. The final result is a global overview of the sputtering coating techniques and a of the results obtained using each system. Suggestions for future efforts have been included as part of the conclusions. Continue reading

6 GHz Cavities: A Method to test A15 Intermetallic compounds RF Properties-PhD Thesis SILVIA MARIA DEAMBROSIS

Author: SILVIA MARIA DEAMBROSIS ; Type of thesis: PhD Thesis
Abstract: For superconducting alloys and compounds, at a given operating temperature, the best rf performances (low surface resistance and high relevant critical fi elds) are obtained for high Tc and low resistivity materials. Among the possible candidates, A15 compounds appear to be the most promising.
We needed a fast, easy and performing way to characterize A15 superconducting materials for their potential application to accelerating resonators. The idea is to build microcavities completely equal in shape to the real scale model. The rf characterization of samples is an useful diagnostic tool to accurately investigate local properties of superconducting materials. However, a common limitation of systems used for this, often consists in the di culty of scaling the measured results to the real resonator.
In this work we will proof that 6 GHz resonators can simply become our cavity shaped samples. Our attention was focused on two materials: V3Si that has a really high RRR value and Nb3Sn that is the only A15 material already used for a resonant accelerating structure.
The process parameters optimization necessary to improve the A15 phase superconducting properties, crystal structure and morphology is going on through the small sample production: this is fundamental but still not enough. Continue reading

Sputtering niobium films into a RFQ model & Sputtering of superconducting V3Si film-Foreign Institute Thesis ZHANG YAN

Author: ZHANG YAN ; Type of thesis: Foreign Institute Thesis
Abstract: Superconducting technique has been widely applied to linac particle accelerators for more than two decades. Cryogenic RF performance of SC cavities has been improved a lot due to improvement on purification of SC material, as well as SC cavity design, fabrication and surface treatment techniques. The Sputtering technique of SC cavities provided another chance to particle accelerators: the cost of cavity fabrication greatly decreased, while the performances of sputtering coated niobium cavities are competitive with those of bulk material SC cavities.
In this thesis some important features of RF cavities are briefly introduced; the difference in design of a SC cavity and that of a normal conducting cavity are indicated. The design parameters of a 144 MHz SC QWR and an 1.5GHz monocell spherical cavity are presented. The SC material for cavity fabrication, and measurement method of SC cavity are introduced, then the fabrication and surface treatment technique of SC cavities are discussed.
The application of sputtering technique in SC cavities is a recent development of SRF technique. After nearly two decades study, the sputtering coated niobium film SC cavities achieved a cryogenic RF performance close to that of bulk niobium cavities. The thesis introduced various sputtering techniques on this purpose from preliminary glow discharge, discusses the LNL, Peking University and Australia National University’s QWR sputtering configurations, and introduces LNL’s surface treatment technique for copper substrate cavity.
In the study of niobium sputtering for 1.5GHz monocell spherical cavity, different magnetron configurations were tried and measured a large amount of sputtered niobium samples. By improving the magnetron configuration and surface treatment technique of the substrate cavity, sputtered niobium cavities with better RF performance were obtained. It was found out that substrate surface treatment takes a very important role in the sputtering of a SC cavity, as sample measurement cannot give out helpful information of the RF performance, the study with substrate Continue reading