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arXiv · 2410.02397

Role of microstructure on flux expulsion of superconducting radio frequency cavities

B. D. Khanal·S. Balachandran·S. Chetri·M. Barron·R. Mullinix·A. Williams·P. Xu·A. Ingrole·P. J. Lee·G. Ciovati·P. Dhakal
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Abstract

The trapped residual magnetic flux during the cool-down due to the incomplete Meissner state is a significant source of radio frequency losses in superconducting radio frequency (SRF) cavities. Here, we show a clear correlation between the niobium microstructure in elliptical cavity geometry and flux expulsion behavior. In particular, a traditionally fabricated Nb cavity half cell from an annealed poly-crystalline Nb sheet after an 800 $^\circ$C heat treatment leads to a bi-modal microstructure that ties in with flux trapping and inefficient flux expulsion. This non-uniform microstructure is related to varying strain profiles along the cavity shape. A novel approach to prevent this non uniform microstructure is presented by fabricating a 1.3 GHz single cell Nb cavity with a cold-worked sheet and subsequent heat treatment leading to better flux expulsion after 800 $^\circ$C/3 h. Microstructural evolution by electron backscattered diffraction-orientation imaging microscopy on cavity cutouts, and flux pinning behavior by dc-magnetization on coupon samples confirms a reduction in flux pinning centers with increased heat treatment temperature. The heat treatment temperature dependent mechanical properties and thermal conductivity are reported. The significant impact of cold-work in this study demonstrates clear evidence for the importance of microstructure required for high-performance superconducting cavities with reduced losses caused by magnetic flux trapping.

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B. D. Khanal, S. Balachandran, S. Chetri, M. Barron, R. Mullinix, A. Williams, P. Xu, A. Ingrole, P. J. Lee, G. Ciovati, P. Dhakal. 2024-10-03. Role of microstructure on flux expulsion of superconducting radio frequency cavities. https://arxiv.org/abs/2410.02397

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