  Trapped magnetic flux is known to be a significant contribution to the
residual resistance of superconducting radio frequency (SRF) cavities. The
additional losses depend strongly if the vortices are depinned by the RF. The
depinning is affected by the purity of the material, and the size of the
pinning centers, as well as the cavity operation frequency. One may define a
depinning frequency, above which significant depinning occurs. This publication
presents a derivation of the depinning frequency from experimental data. We
find a depinning frequency of 673 MHz for RRR 110 niobium. On this basis the
currently used model is extended to describe the trapped flux sensitivity as
function of residual resistance ratio (RRR) and operation frequency while also
accounting for the pinning center size and the treatment history of the cavity.
Moreover, the model offers an explanation for the significantly higher trapped
flux sensitivity reported for nitrogen doped and 120 C baked cavities.
