As we have mentioned before [1], the chemically-polished Niobium cavities performances are improved by the cavity heating ( 105°C for 48 hours ): the BCS resistance decreases and the Qo drop is smoothed off.
Identical results are also achieved with electropolished cavities. So the elimination of steep slope for high accelerator fields is the result of the cavity heating and not of the polishing kind.
Magnetic penetration depth measurements show the increase of the electron mean free path after heating and lead to consider the global thermal effect as the origin of the high accelerator field slope.
This interpretation allows to fit the experimental data, and the dRs/dT improvement, via the Rbcs decrease, can explain the slope smoothing off.
[1] Proceedings of the 6th EPAC, Vol. III p. 1885, Stockholm SWEDEN (1998)
Keywords: High gradients - Surface resistance - Magnetic penetration depth
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