Accelerator Seminar: Shreyas Balachandran

Microstructure dependence of magnetic flux trapping in Niobium: Attention needed to upstream Nb processing for high Q SRF Nb cavities

Abstract: Elemental type-II superconducting niobium is the material of choice for superconducting radiofrequency cavities used in modern particle accelerators, light sources, detectors, sensors, and quantum computing architecture. Current challenges to obtaining the highest energy efficiency or high-quality factors (high Q) in rf applications reduce power dissipation due to residual magnetic field. Flux can be trapped due to incomplete magnetic field expulsion during cool down the superconducting state. New SRF cavity processing recipes that use surface doping techniques have significantly increased their cryogenic efficiency. However, the performance of SRF Nb accelerators still shows vulnerability to a trapped magnetic field. In this talk, we report the observation of a direct link between flux trapping and incomplete flux expulsion with spatial variations in microstructure within the niobium. Fine-grain with an average grain size of 10–50 µm leads to flux trapping in the absence of dislocation structures in grain interiors. Larger grain sizes beyond 100–400 µm do not lead to preferential flux trapping, as observed directly by magneto-optical imaging. The deformed Nb sample’s microstructure variations suggest the importance of initial microstructure, processing, and heat treatment on Nb’s ability to expel magnetic flux. Our study’s result indicates that complete control of the niobium microstructure will help produce higher performance superconducting resonators with reduced rf losses related to the magnetic flux trapping. Understanding the processing life cycle from ingot to sheet to cavity fabrication and final high-temperature heat treatment is crucial to obtain consistent Nb sheets for cavity production for high Q and high gradient cavities planned for ILC beyond.

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