Materials Investigation and Surface Design of Superconducting Radio-Frequency Accelerating Cavities at Cornell University

Superconducting radio-frequency (SRF) cavities, the key component in particle accelerators, require alternative materials and nanostructured surfaces beyond niobium. This would open new opportunities for high electric field, low surface resistance, and high operation temperature for a wide range of future accelerator applications. At Cornell University, we design and fabricate active niobium surfaces through alloying, doping, and oxide control inspired by theory.<br/><br/>First, we demonstrated Nb₃Sn films with low surface roughness and pure stoichiometry (two critical parameters for SRF cavities) through an electrochemical method both in sample studies and at cavity scale. We will report the chemical, structural, and superconducting RF properties of the electrochemically deposited Nb₃Sn films together with the growth mechanism analysis. Also, we will compare these results with the state-of-the-art vapor diffused films.<br/><br/>Second, we designed an inhomogeneous layer within the field penetration depth through doping of the niobium surface. Our theory collaborators at the Center for Bright Beams have demonstrated a significant improvement of superheating field via this doping method. We have achieved desirable doping profiles and promising DC superconducting properties on the sample scale using electrochemical and physical vapor methods. We will report the doping profiles and phase transformation under different doping conditions, and expect to obtain their corresponding RF results by the time of the conference.<br/><br/>Third, we investigated the surface oxide structures on niobium in order to generate an approach to control the surface nanostructure inspired by [T. Kubo and A. Gurevich, 2019]. We will discuss updates to our recent study on the oxide structural analysis under different processing conditions.<br/><br/>Lastly, it is challenging to achieve high quality films and surfaces on large-scale, intricately-structured cavity devices. To do this, we developed customized electrochemical, chemical vapor, and physical vapor deposition systems. The deposition system development will be covered throughout the presentation.