Development of superconducting RF cavities toward higher performance

Superconducting RF technology has been developed based on electropolishing and ultra-pure water high pressure rinsing. Recently, new technologies, so-called Nitrogen doping and Nitrogen infusion, have been proposed and an improvement in cavity performance was achieved. Nitrogen doping realizes a drastic reduction in surface losses and has become the new standard for CW accelerators. Nitrogen infusion is another attractive technology. It has the possibility of improving the acceleration gradient too. The procedures for current standard surface treatment, Nitrogen doping and Nitrogen infusion are described, and experimental results are shown. New knowledge regarding how to handle magnetic flux to improve surface resistance is also discussed. (author)