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Comparative surface studies on fine-grain and single crystal niobium using xps, aes, ebsd and profilometry

As the surface magnetic field in niobium cavities approaches the theoretical critical field, RF losses grow sensitive to increasingly subtle features of the material and the surface. A striking example is the familiar onset of the high field Q-slope, where RF losses increase exponentially with field. A surprising feature of the high field Q-slope is its positive response to a mild baking at 100-120degC. But the Q-slope returns after the first 20 nm of the niobium metal surface is converted to loss-less pentoxide via anodization. The latter result suggests that the cause of the fast growing losses resides in the first 20 nm of the RF surface. Although there are several propositions, the exact mechanism for the high field Q-slope is not yet fully understood and demands further research. We are conducting surface analytic studies with optical profilometry, EBSD, XPS, Auger and SIMS to shed light on the mechanism of the high field Q-slope. We are comparing the behavior of fine-grain niobium with single crystal niobium, buffered chemical polishing (BCP) treatments with electropolishing (EP) treatments and properties before and after 110degC bake. Our approach is based on identifying lossy regions, dissecting of these regions and range of analysis.