Surface resistance effects of medium temperature baking of buffered chemical polished 1.3 GHz nine-cell large-grain cavities

Abstract

Three 1.3 GHz 9-cell large grain superconducting niobium cavities were experimented with the proposal of medium temperature baking, using buffered chemical polishing to remove the previous heat treatment impurity profiles. 2 K vertical tests of the cavities gave the average intrinsic quality factor of 2.7×1010 at 16 MV/m, as well as the maximum accelerating gradients of 20-22 MV/m. These promising values confirmed the effectiveness of an improved medium temperature baking recipe for niobium cavities which have been benefitting the superconducting radiofrequency community. Furthermore, the resistance analysis demonstrated that medium temperature baking reduced both the BCS resistance and the residual resistance of the cavities. Impurity analysis on niobium samples provided some proof that the former reduction was due to the shortened electron mean free path, while the later reduction was probably correlated with the mitigation of the increased interstitial impurity atoms.

Mid-T heat treatments on BCP’ed coaxial cavities at TRIUMF

“…In A similar shift in the Nb 2 O 5 layer has been observed on samples after a mid-T bake (Posen et al, 2020). Increased carbon impurities had also been observed (Yang et al, 2022) in mid-T samples and associated with weaker rf performance.…”

“…A decreasing R BCS at 4 K would be an enormous benefit for SRF based heavy ion accelerators operating in continuous wave (cw) mode at moderate peak surface fields, such as the ISAC-II or FRIB accelerators. The first reported results on BCP'ed 1.3 GHz nine-cell cavities (Yang et al, 2022) do not show the characteristic increase in Q 0 , but still a larger Q 0 than a baseline treatment. This is similar to the difference between EP'ed and BCP'ed N 2 -doped cavities (Dhakal et al, 2019).…”

“…This simplified process should also increase consistency in larger cavity productions. Since mid-T bakes show a decreasing R BCS with increasing rf field amplitude (Ito et al, 2021;Sha et al, 2022;Steder et al, 2022;Yang et al, 2022), this treatment is of high interest for low frequency SRF cavities that typically operate at a temperature of 4.2 K. At this operating temperature, the surface resistance R s is dominated by its temperature dependent component R BCS and the temperature independent part R Res contributes only a small fraction. A decreasing R BCS at 4 K would be an enormous benefit for SRF based heavy ion accelerators operating in continuous wave (cw) mode at moderate peak surface fields, such as the ISAC-II or FRIB accelerators.…”

In-situ synchrotron x-ray photoelectron spectroscopy study of medium-temperature baking of niobium for SRF application

“…The surface of the Nb samples treated together with cavities was studied primarily by SIMS that allowed determination of the concentration profile in the surface layer characterized mainly by the increase in oxygen-, carbon-and nitrogen-containing species. No change of hydrogen as compared to the baseline treatment was observed [23,24]. In [26] SIMS-depth profiling of the specially prepared samples using the recipes of the furnace baking [20] demonstrated a large increase in oxygen concentration and small quantities of C and N (though larger ones as compared to the electropolished niobium surface).…”

“…The mid-T baking was also tested on cavities at higher temperatures and different thermal pre-treatments on large-grain niobium cavities [23], with nitrogen-gas addition [24] and also on 650 MHz cavities [25]. The surface of the Nb samples treated together with cavities was studied primarily by SIMS that allowed determination of the concentration profile in the surface layer characterized mainly by the increase in oxygen-, carbon-and nitrogen-containing species.…”