Analysis of Magnetic Vortex Dissipation in Sn-Segregated Boundaries in Nb$_3$Sn Superconducting RF Cavities

Abstract

We study mechanisms of vortex nucleation in Nb3Sn SRF cavities using a combination of experimental, theoretical, and computational methods. Scanning transmission electron microscopy (STEM) image and energy dispersive spectroscopy (EDS) of some Nb3Sn cavities show Sn segregation at grain boundaries in Nb3Sn with Sn concentration as high as ∼35 at.% and widths ∼3 nm in chemical composition. Using ab initio calculations, we estimate the effect excess tin has on the local superconducting properties of the material. We model Sn segregation as a lowering of the local critical temperature. We then use time-dependent Ginzburg-Landau theory to understand the role of segregation on magnetic vortex nucleation. Our simulations indicate that the grain boundaries act as both nucleation sites for vortex penetration and pinning sites for vortices after nucleation. Depending on the magnitude of the applied field, vortices may remain pinned in the grain boundary or penetrate the grain itself. We estimate the superconducting losses due to vortices filling grain boundaries and compare with observed performance degradation with higher magnetic fields. We estimate that the quality factor may decrease by an order of magnitude (10 10 to 10 9 ) at typical operating fields if 0.03% of the grain boundaries actively nucleate vortices. We additionally estimate the volume that would need to be filled with vortices to match experimental observations of cavity heating.

Ab initio theory of the impact of grain boundaries and substitutional defects on superconducting Nb3Sn

“…Grain boundaries with low T c are a candidate mechanism that lowers the first vortex entry field in Nb 3 Sn SRF cavities, and enhancing flux pinning in Nb 3 Sn superconducting wires is the only opportunity to improve critical current densities [89,4]. Ginzburg-Landau simulations can build off of these local T c estimates to further our understanding of flux penetration and pinning at grain boundaries in Nb 3 Sn and how these processes are affected by grain boundary composition [42,44,49].…”

“…In dc applications such as superconducting wires, high grain boundary densities provide needed pinning centers to prevent vortices from limiting the critical current density [4,40,41]. In ac applications such as SRF cavities, significant dissipation arises once flux penetrates the cavity surface, and grain boundaries can lower the barrier for flux penetration [33,[42][43][44].…”

$\textit{Ab Initio}$ Theory of the Impact from Grain Boundaries and Substitutional Defects on Superconducting Nb$_3$Sn

“…Grain boundaries with low T c are a candidate mechanism that lowers the first vortex entry field in Nb 3 Sn SRF cavities, and enhancing flux pinning in Nb 3 Sn superconducting wires is the only opportunity to improve critical current densities [4,57]. Ginzburg-Landau simulations can build off of these local T c estimates to further our understanding of flux penetration and pinning at grain boundaries in Nb 3 Sn [21,23,30].…”

“…In dc applications such as superconducting wires, high-density grain boundaries provide needed pinning centers to prevent vortices from limiting the critical-current density [4,19,20]. In ac applications such as SRF cavities, significant dissipation arises once flux penetrates the material and grain boundaries can provide unwanted nucleation sites [15,[21][22][23].…”