The role of near-surface dislocations in the high magnetic field performance of superconducting niobium cavities

Quantitative analysis of the near-surface dislocation content performed using electron backscattered diffraction for samples cut from niobium cavities limited by the 'high field Q -slope' effect show a high dislocation content, as well as a pronounced change in the near-surface dislocation content caused by the 100–120 °C annealing when such annealing has a healing effect on the high field Q -slope. We propose a model for the sharp degradation of the cavity Q at high field for niobium cavities based on dislocation-assisted magnetic flux entry at surface magnetic fields lower than those expected for ideal niobium. It is proposed that the elimination of the high field Q -slope by 100–120 °C annealing is caused by vacancy-assisted dislocation climb in the near-surface region.