Niobium can easily pick up hydrogen during cavity processing steps. It turns out that absorbed hydrogen segregates near the surface and upon cool-down to cavity operational temperatures of <4.4K may form normal conducting hydride nanoprecipitates. Due to proximity effects, such precipitates are also superconducting, but with altered properties and may have a lower breakdown field than the host niobium.

The model connects together experimental hints and provides both nanostructural origins and a physical mechanism of anomalous dissipation. Intensive experimental work is underway to test the proposal. If confirmed, the finding may pave the way for significant improvements in the quality factors of niobium cavities and enable cheaper and more efficient particle accelerators.

Full details can be found in the journal Superconductor Science and Technology.