X-ray computed tomographic and focused ion beam/electron microscopic investigation of coating defects in niobium-coated copper superconducting radio-frequency cavities.

A combination of X-ray computed tomography (CT) and focused ion beam - scanning electron microscopy (FIB-SEM) is employed to investigate substrate and related surface defects in a niobium coated superconducting radio frequency (SRF) copper cavity. The cavity was manufactured by spinning, with subsequent application of a sputtering-deposited niobium coating (≈ 40 µm thick) on the internal surface. Before coating, the copper surface was pre-treated in several stages, ending with chemical polishing. CT and FIB-SEM identified furrows (≈ 20 µm deep) in the copper beneath the coating, with an alignment consistent with remnants of score lines from the spinning process. The furrows were filled with niobium and contained voids at the niobium/copper interface that extended a few microns into the niobium coating. The presence of the defects led to similar furrows at the niobium surface. The study reveals the importance of pre-treatment of the cavity internal surface to avoid defects that may have deleterious influence on the Q slope and durability of the niobium coating.