Superconducting Radiofrequency Technology for Accelerators

6 High-Field Q Slope (HFQS) – Understanding and Cures

6.1 HFQS Summary

As mentioned in Introductory Chapter 2, Q versus E curves for niobium typically show three distinct regions of Q changes: low, medium, and high-field Q slopes [5]. As discussed in Chapter 4, the low field Q-slope (LFQS) arises from two-level-system (TLS) losses in the amorphous pentoxide. The losses increase for thicker oxide layers and substantially reduce when the layer is dissolved into the bulk with 300–400 °C baking. The medium field Q-slope (MFQS) shows falling Q with increasing field for most surface treatments, except for the interesting treatment of doping with N (or Ti), which shows rising Q, as discussed in Chapter 3. In this chapter, we address mainly the causes ...