RF Characterization of Thin Film Sputtered Superconducting Resonators – Master Thesis Giovanni Caldarola

Author: Giovanni Caldarola ; Type of thesis: Master Thesis
Abstract: Long time has passed from the discovery of superconductivity, in 1911. Since then theoretical and experimental progress have been made continually. Nevertheless, there are still much to learn about superconductivity and the uses of superconducting materials. At INFN/LNL research and development is being done in order to understand the properties and uses of various superconducting materials to push forward the field of particle accelerator technology. These are used to fabricate radio-frequency (RF) accelerating cavities in order to minimize the power dissipated and increase their figures of merit, such as accelerating gradient (Eacc) and intrinsic quality factor (Q0 or Q) which is a convenient parameter for the number of oscillations it takes the stored energy to dissipate to zero.
Superconducting properties of high purity niobium makes it the preferred material for many accelerator projects using superconducting technology. In fact, niobium possesses very intriguing physical and mechanical properties, not only the highest superconducting transition temperature (Tc) of 9.26°K and the highest superheating field of 240 mT among all available superconducting pure metals but also excellent ductility, which enables machining to be done relatively easily. In addition, the high quality factor and cavity accelerating gradient are fundamental parameters as they affect the overall cost of the accelerator in a direct way.
The accelerating gradient of the superconducting niobium cavities has been remarkably raised in the past decades with an advance of the cavity fabrication technology.
Cavities used to-date are made of niobium either in bulk or niobium coated on copper and are operated at 1.8 and 4.2 °K where the BCS component of the surface resistance is reduced to minimum and the cavity works in the residual resistivity regime.
In Legnaro three laboratories are reserved for cavity treatments and analysis: the chemical, the sputtering and the cryogenic lab, which is dedicated to the whole process of RF testing.
The work will focus on the influence on cavity performance of external surface conditions (in bulk cavity) and of the interface between niobium and copper (in thin film sputtered cavities). Specifically the interest is aimed to investigate how to better allow heat transfer from the resonator to the helium bath

For this purpose, 6 GHz elliptical cavities have been used. The main advantages over a 1.3 or 1.5 GHz superconducting RF cavities (SRF) are saving time, cost and material. Furthermore, due to smaller dimensions, the processes involved have been characterized by a reduction of energy in thermal treatments and a fast cryogenic measurement. A spinning technology is used to create seamless bulk-Nb and bulk-Cu cavities [1].
As a parallel activity, RF characterization of a 101 MHz Nb-Cu QWR (quarter wave resonator) has been conducted and compared with previous tests, varying sputtering parameters.
During the RF test the cavities have to be cooled at cryogenic temperatures in order to reach the superconducting state. In the testing facility there are two apertures which can host a cryostat and allow that this task is accomplished. While 6 GHz elliptical cavities are tested at 4.2°K and then at 1.8°K, for the 101 MHz QWR a temperature of 4.2°K is sufficient.