| Mirco-Structures of RF Surfaces in the Electron-Beam-Weld Regions of Niobium (2007) | |||||||||||||
Abstract | |||||||||||||
| Micro-structures of RF surfaces in the electron-beam-weld (EBW) regions of niobium samples were studied. These surfaces were polished with different techniques, namely buffered chemical polish (1:1:2 BCP) and electrolytic polish (EP). It was found that there are three distinctively different regions near the EBW, characterized by different grain sizes, shapes, and orientations. Strikingly, we found grain boundaries within the EBW are nicely lined up so that they form an angle of about 60 degree with respect to the weld seam. Upon being polished with BCP, these grain boundaries evolve into micro-steps with sharp edges, due to preferential etching of different crystallographic planes. There is evidence that the step height increases up to possibly more than 30 microns with repeated chemical polish. In contrast to the result with BCP, dimensions of surface irregularities were reduced with EP and sharp edges were rounded off. These observations have important implications in explanations of the high-field-Q-drop of niobium superconducting cavities. | |||||||||||||
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