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