Effect of surface roughness on critical current of niobium films

In the development of ultra-high precision superconducting devices, the housing geometry and surface finish that defines the circuit often determines ultimate performance. This can be more critical for non-planar geometries, such as cylindrical surfaces, in which preserving cylindricity can be more important than surface smoothness (our cylindricity requirement is pm along 100 mm cylinder length). Worse than added cost of polishing, there is also the risk that the polishing process will degrade cylindricity. We have investigated the critical current of niobium thin film traces as a function of quartz surface roughness (as defined by ASME standards). Preliminary results show that the critical current decreases with increasing surface roughness, with a more pronounced decrease as the surface roughness becomes comparable to 400 nm film thickness. We will discuss our results based on a simple theoretical model.