In Situ Study of Nb Oxide and Hydride for SRF Cavity Applications Using Aberration-Corrected STEM and Electron Energy Loss Spectroscopy

Nb is the preferred material for superconducting radio-frequency (SRF) cavities in next-generation particle accelerators, but also exhibits a high solubility for oxygen and hydrogen, especially within the RF-field penetration depth. Using electron diffraction, we observe superlattice reflections near the SRF cavities surface at 300 K and 95 K, which are related to the ordering of oxygen and hydrogen atoms in the niobium crystal. High-resolution scanning transmission electron microscopy images allow direct observation of light atoms, such as hydrogen, carbon and oxygen, and combined with EELS, we can reveal the element composition at grain boundaries and precipitates near the cavity surface. These changes in local composition will be related to the different cavity processing methods.