Effect of mild baking on superconducting niobium cavities investigated by sequential nanoremoval

Abstract

The near-surface nanostructure of niobium determines the performance of superconducting microwave cavities. Subtle variations in surface nanostructure lead to yet unexplained phenomena such as the dependence of the quality factor of these resonating structures on the magnitude of rf fields-an effect known as the ''Q slopes''. Understanding and controlling the Q slopes is of great practical importance for particle accelerators. Here we investigate the mild baking effect-120 C vacuum baking for 48 hours-which strongly affects the Q slopes. We used a hydrofluoric acid rinse alternating with oxidation in water as a tool for stepwise material removal of about 2 nanometers=step from the surface of superconducting niobium cavities. Applying removal cycles on mild baked cavities and measuring the quality factor dependence on the rf fields after one or several such cycles allowed us to explore the distribution of lossy layers within the first several tens of nanometers from the surface. We found that a single HF rinse results in the increase of the cavity quality factor. The low field Q slope was shown to be mostly controlled by the material structure within the first six nanometers from the surface. The medium field Q slope evolution was fitted using linear (/ B peak surface magnetic field) and quadratic (/ B 2 ) terms in the surface resistance and it was found that best fits do not require the quadratic term. We found that about 10 nanometers of material removal are required to bring back the high field Q slope and about 20-50 nanometers to restore the onset field to the prebaking value.

Unprecedented quality factors at accelerating gradients up to 45 MVm−1in niobium superconducting resonators via low temperature nitrogen infusion

“…The difference stays in the fact that with the low T treatment the nitrogen enriched layer is just few-tens nanometers deep, versus microns for the high T treatments. The fact that 120°C (which does not lead to antislope) vs 160°C samples differ in nitrogen concentration just in the first few nanometers, gives yet again a sense of how important the first nanometers underneath the oxide layer are for RF performance, in agreement with previous HF studies in [2]. From the SIMS measurements we notice also some other interesting differences: the 160°C sample shows (systematically for both samples analyzed) a thinner oxide Nb 2 O 5 (and also NbO 2 signal) of about half the depth compared to the undoped and 120°C case.…”

Ultralow Surface Resistance via Vacuum Heat Treatment of Superconducting Radio-Frequency Cavities

“…Residual resistance increases significantly, but it appears that this can be ameliorated by two HF rinses to return to ~2 nW, similar to the post-EP surface of TE1PAV005 or the N-doped surface of TE1RI006. This is consistent with the residual resistance increase observed in a 120 C baked cavity followed by subsequent HF rinsing [31].…”

Nanostructural features degrading the performance of superconducting radio frequency niobium cavities revealed by transmission electron microscopy and electron energy loss spectroscopy

“…The slightly increased niobium oxidation state in EP120C is consistent with the increased oxygen concentration right underneath the oxide, as found before. 16,32 This increased oxygen concentration may be a reason of the 1-2 nX higher residual resistance in 120 C baked cavities, which can be restored to the pre-120 C bake level by the hydrofluoric acid rinse 34 since the oxygen-rich layer gets converted to the newly grown oxide. Finally, a very important finding is lack of insulating Nb 2 O 5 along grain boundaries.…”