Mechanism of Pit Formation on Surface of Superconducting Niobium Cavities During Buffered Chemical Polishing

Superconducting radio-frequency niobium cavities processed using buffered chemical polishing (BCP) sometimes show typical W-shaped pits on their surface, which may greatly limit their performance. However, the causes of such pits and effective solutions are not fully understood. In this study, we reproduced the formation of W-shaped pits on the cavity surface through niobium sample BCP experiments, directly observed the sample surface's evolution during the polishing process and the polished surface's morphology, and analyzed the cause of W-shaped pits in detail: the formation and attachment of bubbles on the niobium surface during the BCP process. Then, we systematically investigated the effects of different process parameters on the bubbles and pits, including the acid ratio, temperature, and flow rate. We also investigated how the formation of bubbles and pits was affected by the Nb facing orientation and grain size. This study provides insights into the mechanisms by which bubbles and W-shaped pits are formed on niobium surfaces, and highlights possible directions for reducing pit defects in Nb cavities processed using BCP treatment.