BEAMSTopic

High-field Q-slope and anti-Q-slope in niobium

Superconducting niobium cavities lose almost no energy, but at strong fields the losses suddenly grow (the Q-slope), while cavities treated with nitrogen lose less as the field grows (the anti-Q-slope). Why either happens is still debated.

Why it matters

The quality factor at operating field sets the cryogenic cost of every superconducting linac, including XFELs and future colliders.

Review

G. Ciovati, G. Myneni, F. Stevie, P. Maheshwari, D. Griffis, High field Q slope and the baking effect: Review of recent experimental results and new data on Nb heat treatments, Physical Review Special Topics - Accelerators and Beams, 2010. https://doi.org/10.1103/PhysRevSTAB.13.022002 reference checked

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2 problems

Frequency and temperature dependence of the anti-Q-slope

Open

The loss reduction with field is strong in some cavities and absent in others, and depends on the cavity frequency and temperature. A theory must explain where it appears and where it does not.

Why low-temperature baking at about 120 C removes the Q-slope

Contested

Heating a niobium cavity at about 120 degrees Celsius for two days removes the high-field loss rise. What this mild bake changes in the metal is not fully known.