Development Of A High Throughput Facility For The RF Characterisation Of Superconducting Thin Films

Seal, Daniel and Burt, Graeme (2025) Development Of A High Throughput Facility For The RF Characterisation Of Superconducting Thin Films. PhD thesis, Lancaster University.

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Abstract

Superconducting radio frequency cavities are used in particle accelerators because they can accelerate charged particles efficiently with high duty cycle or continuous wave operation. For more than 60 years, bulk niobium has been the material of choice for these cavities; however, these require large capital and operational costs and now perform close to their theoretical limits. With future accelerators requiring more sustainable technology, an alternative approach using thin film superconductors deposited on copper cavities is being explored. This would allow significant capital savings, and the use of alternative materials, such as Nb3Sn, NbTiN, V3Si, offer the potential for higher temperature operation and accelerating gradients that exceed Nb. The development of thin film coatings on full-sized cavities is expensive and time-consuming. Therefore, optimising deposition parameters in cavities alone is not viable. Instead, thin film optimisation should be performed on planar samples, which are quicker, easier and cheaper to develop. For this, quick measurements of the superconducting properties is vital. Most importantly, measurements of the RF properties of samples, such as surface resistance, are vital to predict the RF performance before cavity depositions. A handful of dedicated RF test facilities have been designed around the world; however, many offer a slow sample turnover rate. In order to speed up the rate of thin film development, a new RF facility was developed and commissioned using a unique 7.8 GHz Choke Cavity design. A simple sample mounting procedure and low-effort operation allow for RF measurements of up to three samples per week in this facility, thus providing an important tool in the multi-parameter optimisation process of thin films. It is capable of testing samples 90 − 130 mm in diameter to measure the average surface resistance as a function of either sample temperature or peak sample magnetic field. This work presents details of the Choke Cavity and simulations that demonstrate its operation. The design and operation of the cryogenic facility are then presented with details of a sample workflow from substrate preparation and deposition to RF testing. Measurements of bulk Nb and thin film Nb on Cu samples were performed to commission the system and demonstrate the capabilities of the system. This demonstrated the ability to measure surface resistance at temperatures in the range 4 − 20 K and sample peak magnetic fields up to 3 mT, with a minimum resolvable surface resistance of ∼ 0.1 μΩ and typical uncertainties of ≈14%. Additional studies, with further superconducting measurements and surface analysis, have been performed with Nb and Nb3Sn thin films on Cu to demonstrate how the system can be used to optimise deposition parameters. For samples produced at Daresbury Laboratory, the Nb/Cu study demonstrated optimal RF performance can be achieved for a deposition temperature of 530 °C, whilst the RF performance for the Nb3Sn/Cu samples is optimal for lower magnetron powers down to 50 W. In addition, samples deposited with a Nb3Sn-Nb-Cu bilayer showed a decrease in RF performance compared with the single layer Nb3Sn.