Multiscale mechanisms of SRF breakdown

Multiscale mechanisms of SRF breakdown and the dependence of the quality factor Q(H0) on the rf field amplitude H0 are discussed. We first consider a basic nonlinear dependence of the surface resistance Rs on the rf screening current density J  , which becomes crucial at rf fields of the order of the thermodynamic critical field HcNb∼200mT. The current-induced rf pairbreaking in the clean limit results in a quadratic field correction to Rs(H0) at low H0 and an exponential increase of Rs(H0) at H0 > THc/Tc. The effect of the nonlinear Rs(H0) on the rf thermal breakdown is considered. The field dependence of Rs(H0) reduces the breakdown field Hb below Hc and increases the medium and high field Q slopes. Then a model of nonuniform thermal breakdown caused by macroscopic hotspots on the cavity surface is proposed. It is shown that hotspots expand as H0 increases, resulting in additional mechanism of field dependence of Q(H0) which increases the Q slope and reduces Hb.