||For these ten years, the high
gradient of niobium sc RF cavities seems to saturate around 40 MV/m.
The question is in the further technical issue or in the fundamental
critical field of niobium material. In this paper the author will try
to explain the saturation from the theoretical point of view.
Superheating field is reviewed as the fundamental critical field in RF
application. In next the author will analyse the temperature variation
of the superheating field especially on niobium material. The result is
compared with the experimental RF critical field of niobium bulk
cavity, which was measured in Cornell University and a good agreement
with the prospect ion of vortex nucleation is obtained. The same
analysis is extended Nb3Sn or lead cavities, and the consistent result
is obtained with the vortex nucleation as the theoretical critical
field in RF application.