A Facility For Magnetic Field Penetration Measurements on Multilayer S-I-S Structures

Superconducting RF cavities made of bulk Nb has reached a breakdown field of about 200 mT which is close to the superheating field for Nb. As it was theoretically shown a multilayer coating can be used to enhance the breakdown field of SRF cavities. The simple example is a superconductor-insulator-superconductor (S-I-S), for example bulk niobium (S) coated with a thin film of insulator (I) followed by a thin layer of a superconductor (S) which could be a dirty niobium. To verify such an enhancement in a presence of a DC magnetic field at 4.2 K a simple experimental facility was designed, built and tested in ASTeC. The details of experimental setup and results of the measurements will be shown at the conference

Magnetic Field Penetration of Niobium Thin Films Produced by the ARIES Collaboration

Superconducting (SC) thin film coatings on Cu substrates are already widely used as an alternative to bulk Nb SRF structures. Using Cu allows improved thermal stability compared to Nb due to having a greater thermal conductivity. Niobium thin film coatings also reduce the amount of Nb required to produce a cavity. The performance of thin film Nb cavities is not as good as bulk Nb cavities. The H2020 ARIES WP15 collaboration studied the impact of substrate polishing and the effect produced on Nb thin film depositions. Multiple samples were produced from Cu and polished with various techniques. The polished Cu substrates were then coated with a Nb film at partner institutions. These samples were characterised with surface characterisation techniques for film morphology and structure. The SC properties were studied with 2 DC techniques, a vibrating sample magnetometer (VSM) and a magnetic field penetration (MFP) facility. The results conclude that both chemical polishing and electropolishing produce the best DC properties in the MFP facility. A comparison between the VSM and the MFP facility can be made for 10 $\mu$m thick samples, but not for 3 $\mu$m thick samples

RF Measurement and Characterization of European Spallation Source Cavities at UKRI-STFC Daresbury Laboratory and DESY

The Accelerator and Science and Technology Centre (ASTeC) is responsible for delivering 88 High Beta (HB) cavities as part of the European Spallation Source (ESS) facility in Sweden. The bulk Niobium Superconducting Radio Frequency (SRF) cavities operate at 704 MHz. They have been fabricated in industry and are currently being tested at Daresbury Laboratory and Deutsches Elektronen-Synchrotron (DESY). They will then be delivered to Commissariat à l’énergie atomique et aux énergies alternatives (CEA) Saclay, France for integration into cryomodules. To date, 50 cavities have been conditioned and evaluated and 36 cavities have been delivered to CEA. This paper discusses the experiences and testing of the cavities performed to date at both sites