Nanostructural features degrading the performance of superconducting radio frequency niobium cavities revealed by TEM and EELS

TL;DR

This study reveals nanoscale hydrides and surface oxide modifications in niobium SRF cavities, linking defect structures to performance limitations, and demonstrates how specific treatments influence hydride formation and surface properties.

Contribution

The paper combines advanced TEM, EELS, and thermometry to identify nanoscale hydrides and surface oxide changes, providing new insights into cavity performance degradation mechanisms.

Findings

Nanoscale hydrides are present in electropolished cavities with high field Q slope.

120°C baking reduces hydride formation in cavities.

Hydrogen degassing and chemical polishing restore hydride levels.

Abstract

Nanoscale defect structure within the magnetic penetration depth of ~100nm is key to the performance limitations of niobium superconducting radio frequency (SRF) cavities. Using a unique combination of advanced thermometry during cavity RF measurements, and TEM structural and compositional characterization of the samples extracted from cavity walls, we discover the existence of nanoscale hydrides in electropolished cavities limited by the high field Q slope, and show the decreased hydride formation in the electropolished cavity after 120C baking. Furthermore, we demonstrate that adding 800C hydrogen degassing followed by light buffered chemical polishing restores the hydride formation to the pre-120C bake level. We also show absence of niobium oxides along the grain boundaries and the modifications of the surface oxide upon 120C bake.