Q-factor optimization for high-beta 650 MHz cavities for PIP-II

High Q-factors are of utmost importance to minimize losses superconducting radio-frequency cavities deployed in continuous wave particle accelerators. This study elucidates the surface treatment that can maximize high-beta 650 MHz elliptical niobium cavities. State-of-the-art treatments applied many single-cell cavities, and resistance studies performed understand microwave dissipation at this unexplored frequency. The nitrogen doping is confirmed be necessary medium RF fields. We five-cell demonstrated extremely high were obtained fields with treatment. also adding a cold electropolishing step after N-doping crucial push quench field multicell higher gradients.