Enhancement in Quality Factor of SRF Niobium Cavities by Material   Diffusion
  An increase in the quality factor of superconducting radiofrequency cavities is achieved by minimizing the surface resistance during processing steps. The surface resistance is the sum of temperature independent residual resistance and temperature/material dependent Bardeen-Cooper-Schrieffer (BCS) resistance. High temperature heat treatment usually reduces the impurities concentration from the bulk niobium, lowering the residual resistance. The BCS part can be reduced by selectively doping non-magnetic impurities. The increase in quality factor, termed as Q-rise, was observed in cavities when titanium or nitrogen thermally diffused in the inner cavity surface. 
Pashupati Dhakal, Gianluigi Ciovati, Peter Kneisel and Ganapati Rao   Myneni
physics.acc-ph