Abstract
Nanoscale defect structure within the magnetic penetration depth of ∼100 nm is key to the performance limitations of niobium superconducting radio frequency 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 120 °C baking. Furthermore, we demonstrate that adding 800 °C hydrogen degassing followed by light buffered chemical polishing restores the hydride formation to the pre-120 °C bake level. We also show absence of niobium oxides along the grain boundaries and the modifications of the surface oxide upon 120 °C bake.
Highlights
Superconducting radio frequency (SRF) cavities are the state-of-the-art technology for particle acceleration implemented in most modern and future planned accelerators.1,2 SRF cavities are predominantly made of bulk niobium and are typically operated at temperatures of 2 K or below, deep in superconducting state of niobium, which has superconducting critical temperature Tc 1⁄4 9:25 K
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 120 C baking
Since these field dependencies are determined by the surface treatments and the magnetic field only penetrates 100 nm inside niobium in superconducting state at 2 K, the nanostructure within this thickness and its changes with treatments is key to understanding changes in surface resistance and Q
Summary
Superconducting radio frequency (SRF) cavities are the state-of-the-art technology for particle acceleration implemented in most modern and future planned accelerators. SRF cavities are predominantly made of bulk niobium and are typically operated at temperatures of 2 K or below, deep in superconducting state of niobium, which has superconducting critical temperature Tc 1⁄4 9:25 K. The magnitude of Q is determined by the average microwave surface resistance Rs, which consists of the strongly temperature dependent part RBCSðTÞ and a temperature independent (residual) component Rres. Recent investigations showed that for standard cavity preparation techniques as well as for a newly discovered nitrogen doping both RBCS and Rres depend on the surface rf magnetic field magnitude B / Eacc. Since these field dependencies are determined by the surface treatments and the magnetic field only penetrates 100 nm inside niobium in superconducting state at 2 K, the nanostructure within this thickness and its changes with treatments is key to understanding changes in surface resistance and Q
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
