  Advances in SRF technology over the last 40 years allowed achieving
accelerating gradients ~ 50 MV/m corresponding to peak surface magnetic field
close to the theoretical limit of niobium. However, the quality factor
decreases significantly with increasing accelerating gradient. This decrease is
expected since increasing the rf field increases the number of quasiparticles
and therefore the rf losses. Recently, a new phenomenon of increase in quality
factor with the accelerating gradient has been observed when SRF cavities are
doped with certain non-magnetic impurities. In particular, the diffusion of
nitrogen into the niobium cavities inner surface has been has been successfully
implemented into the commercialization of SRF technology. The quest is still
ongoing towards process development to achieve high accelerating gradient SRF
structures with high quality factors for future high power accelerators. Here,
we present the review of the research and development via nitrogen diffusion,
materials analysis and current theoretical understanding in high Q0 SRF
cavities.
