Direct observation of hydrides formation in cavity-grade niobium

Abstract

Niobium is an important technological superconductor used to make radio frequency cavities for particle accelerators. Using laser confocal microscopy we have directly investigated hydride precipitates formation in cavity-grade niobium at 77 and 140 K. We have found that large hydrides were usually formed after chemical or mechanical treatments, which are known to lead to a strong degradation of the quality factor known as $Q$ disease. From our experiments we can conclude that hydrides causing $Q$ disease are islands with a characteristic thickness of $\ensuremath{\gtrsim}100\text{ }\text{ }\mathrm{nm}$ and in-plane dimensions $1--10\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. Our results show that mechanical polishing uploads a lot of hydrogen into bulk niobium while electropolishing leads to a mild contamination. Vacuum treatments at $600--800\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ are demonstrated to preclude large hydride formation in line with the absence of $Q$ disease in similarly treated cavities.