Speaker
Description
Nb$_3$Sn coatings on copper offer a promising route beyond established niobium-on-copper (Nb/Cu) thin film superconducting radiofrequency (SRF) cavities, particularly for future accelerators where reducing cryogenic power demand is a key objective. In the Future Circular Collider (FCC) context, Nb$_3$Sn/Cu is being explored as a potential pathway towards operation above 2 K, by exploiting the higher critical temperature of Nb$_3$Sn and its lower expected BCS surface resistance compared with Nb. However, Nb$_3$Sn is not a direct replacement for elemental Nb. Its performance depends on the formation of the ordered A15 phase, near-stoichiometric composition, controlled microstructure, low surface roughness, and stable film–substrate interfaces. Work at CERN has shown that dense Nb$_3$Sn films can be deposited on Cu via high power impulse magnetron sputtering (HiPIMS). However, their superconducting properties remain highly sensitive to substrate effects and processing conditions. The thermal expansion mismatch between Nb$_3$Sn and Cu can generate residual stress, potentially affecting long-range atomic order and reducing the critical temperature relative to the 18.3 K bulk value. Cu interdiffusion is another critical parameter. While it may assist A15 phase formation, diffusion towards the RF surface can create normal-conducting regions and degrade performance. Diffusion barriers such as Ta are therefore being investigated, although their effectiveness depends on obtaining the appropriate crystalline phase. To better understand the conditions required for high-performance Nb$_3$Sn films, this work draws insight from the deposition of Nb and Ta coatings, by investigating deposition conditions with and without in situ substrate heating on different substrate materials, enabling the specific influence of the Cu substrate to be more clearly isolated and understood. These studies provide a basis for identifying the substrate, interface, and processing conditions that govern Nb$_3$Sn/Cu performance in SRF applications.