Impact of nitrogen doping of niobium superconducting cavities on the sensitivity of surface resistance to trapped magnetic flux

Future particle accelerators such as the SLAC Linac Coherent Light Source-II (LCLS-II) and the proposed Cornell Energy Recovery Linac require hundreds of superconducting radio-frequency (SRF) niobium cavities operating in continuous wave mode. In order to achieve economic feasibility of projects such as these, the cavities must achieve a very high intrinsic quality factor (Q(0)) to keep cryogenic losses within feasible limits. To reach these high Q(0)'s in the case of LCLS-II, nitrogen-doping of niobium cavities has been selected as the cavity preparation technique. When dealing with Q(0)'s greater than 1 x 10(10), the effects of ambient magnetic field on Q(0) become significant. Here, we show that the sensitivity to RF losses from trapped magnetic field in a cavity's walls is strongly dependent on the cavity preparation. Specifically, standard electropolished and 120 degrees C baked cavities show a sensitivity of residual resistance from trapped magnetic flux of similar to 0.6 and similar to 0.8 n Omega/mG trapped, respectively, while nitrogen-doped cavities show a higher sensitivity of residual resistance from trapped magnetic flux of similar to 1 to 5 n Omega/mG trapped. We show that this difference in sensitivities is directly related to the mean free path of the RF surface layer of the niobium: shorter mean free paths lead to less sensitivity of residual resistance to trapped magnetic flux in the dirty limit (l << xi(0)), while longer mean free paths lead to lower sensitivity of residual resistance to trapped magnetic flux in the clean limit (l >> xi(0)). These experimental results are also shown to have good agreement with recent theoretical predictions for pinned vortex lines oscillating in RF fields. (C) 2016 Author(s).