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

摘要

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 × 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℃ baked cavities show a sensitivity of residual resistance from trapped magnetic flux of ～0.6 and ～0.8 nΩ/mG trapped, respectively, while nitrogen-doped cavities show a higher sensitivity of residual resistance from trapped magnetic flux of ～1 to 5 nΩ/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 ＜＜ ξ_0), while longer mean free paths lead to lower sensitivity of residual resistance to trapped magnetic flux in the clean limit (ℓ ＞＞ ξ_0). These experimental results are also shown to have good agreement with recent theoretical predictions for pinned vortex lines oscillating in RF fields.