Effect of surface metallurgy on the penetration depth and RF breakdown field of superconducting niobium

The surface metallurgy of niobium is discussed including factors such as impurity distributions at the surface and surface topography. Measurements of the penetration depth are presented both in the clean limit, where there is about a 20A region of oxygen at the surface, after high temperature heat treatment, which affects the boundary condition on the GL order parameter; and in the dirty limit where heating in an oxygen atmosphere near 10 to the -6th power torr can greatly affect the surface roughness. Auger measurements of the surface conditions in a 1% Zr in Nb alloy are also presented. A discussion is given of preliminary results in a special apparatus where RF breakdown experiments can be performed on small samples and correlated with the metallurgical factors mentioned.