Preliminary Surface Study of the Low-Temperature Baking Effect on Niobium. LAURA MACINTYRE (Norfolk State University Norfolk, VA 23504) ANDY T. WU (Thomas Jefferson National Accelerator Facility, Newport News, VA, 23606)

Study has shown that the performance of niobium (Nb) Superconductive Radiofrequency (SRF) cavities can be improved by baking the cavities in air or in ultra-high vacuum (UHV) at temperatures ranging from 100º to 150º C. This low-temperature baking improves the quality factor (Q_o) and the Q-slope which appears at high fields in Q_o vs. E_acc curves for Nb SRF cavities. So far, primarily x-ray photoelectron spectroscopy has been used to study this low-temperature baking effect in terms of the possible changes of the surface chemical information of niobium. However, the detailed mechanism responsible for the improved performance is not clear. It has been proposed that oxygen diffusion into the bulk of Nb or disassociation of Nb₂O₅ may be responsible for the improved performance. Direct confirmation from surface analyses has yet to be done. In this study, Nb samples treated by buffered chemical polishing were baked at 120º and 160º C in air and at 120º C in UHV and compared to an unbaked sample. Relevant analytical surface tools were used to observe the Nb samples. Metallographic optical microscope images and 3-D profilometer surface scans showed no significant changes in grain structure or surface morphology after baking. On the other hand, the number of field emissions was found to be reduced significantly after baking in UHV as observed using scanning field emission microscopy. Data obtained from secondary ion mass spectrometry showed that indeed the low-temperature baking induces oxygen diffusion into the bulk. This study may affect Nb SRF cavity treatment procedures in the future and eventually improve the performance of Nb cavities used in particle accelerators.