Multiscale mechanisms of SRF breakdown

Abstract

Multiscale mechanisms of SRF breakdown and the dependence of the quality factor Q( H 0) on the rf field amplitude H 0 are discussed. We first consider a basic nonlinear dependence of the surface resistance R s on the rf screening current density J, which becomes crucial at rf fields of the order of the thermodynamic critical field H c Nb ∼ 200 mT . The current-induced rf pairbreaking in the clean limit results in a quadratic field correction to R s( H 0) at low H 0 and an exponential increase of R s( H 0) at H 0 > TH c/ T c. The effect of the nonlinear R s( H 0) on the rf thermal breakdown is considered. The field dependence of R s( H 0) reduces the breakdown field H b below H c 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 H 0 increases, resulting in additional mechanism of field dependence of Q( H 0) which increases the Q slope and reduces H b.