Speaker
Description
Superconducting radiofrequency (SRF) niobium-on-copper (Nb/Cu) cavities are a key enabling technology for meeting the performance requirements of the Future Circular Collider (FCC). Within the ongoing CERN R&D programme, substantial progress has been made on 1.3 GHz test cavities through optimisation of Nb film deposition via high power impulse magnetron sputtering (HiPIMS), including the use of DC substrate bias and synchronised pulsed biasing. These approaches provide control over ion energy and ionic-species flux during film growth, enabling 1.3 GHz Nb/Cu cavities not only to meet, but to exceed, the FCC-ee performance targets when scaled to this frequency. The next challenge, and the focus of this talk, is the transfer of this process route to the FCC-ee operational 400 MHz cavity geometry. The larger cavity scale introduces additional constraints, making copper substrate preparation a critical element of the technology scale-up. In particular, electropolishing of large copper surfaces requires tight control of electrolyte chemistry, temperature, flow conditions, material-removal uniformity, rinsing, and passivation before coating. These steps determine the quality of the Nb-Cu interface and are therefore central to film adhesion, suppression of local defects, mitigation of field-limiting features and reproducible RF performance.