Study of Nb Surface Under Ultra High Vacuum After Heat Treatments For SRF Cavities
Specific heat treatments applied to superconducting radiofrequency (SRF) cavities, such as nitrogen infusion or Mid T baking, aim to improve the quality factor (Q0) at medium accelerating fields (10 to 20 MV/m). These treatments reduce the BCS surface resistance by tuning the mean free path of niobium over a few hundred nanometers, either by diffusing oxygen from the native oxide layer or by diffusing nitrogen after the dissolution of the oxide layer. However, these treatments preclude the usual chemical polishing, as it would reverse the beneficial effects of the heat treatments, making the cavities highly sensitive to surface contamination. In particular, the formation of niobium carbides, which can mask the expected benefits, strongly depends on the annealing conditions, surface preparation, and the materials history. Several hypotheses are considered regard-ing the origin of carbon: vacuum contamination, surface pollution, or internal migration from the niobium itself, potentially enriched with carbon during previous chemi-cal treatments (BCP, EP). This work aims to identify the primary source of carbon responsible for niobium carbide growth, using techniques such as X Ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and secondary ion mass spectrometry (SIMS). The study will also help pinpoint the key influencing parameters, thereby contributing to a better understanding of and potential mitigation strategies for their impact on SRF cavity performance.