Influence of crystalline structure on RF dissipation in Niobium: flux trapping, hydride precipitate, doping behavior...

TL;DR

This paper synthesizes existing experimental results to explore how crystalline defects in niobium influence RF dissipation, flux trapping, and doping effects, aiming to improve SRF cavity performance predictions.

Contribution

It highlights the role of crystalline defects beyond grain boundaries in affecting superconducting properties and RF dissipation in niobium, providing new insights for material processing.

Findings

Defects other than grain boundaries impact flux trapping.

Crystalline defects influence early vortex penetration.

Doping effects are linked to defect structures.

Abstract

Bulk niobium is the material mostly used in RF superconducting cavities for accelerator. Predicting and reducing the surface dissipation in RF is mandatory, since it has a tremendous cost impact on most of the large accelerator projects. The theoretical approach of superconducting radiofrequency (SRF) behavior has been far less explored than DC behavior and is still based on the description of relatively simple systems, quite remote from the realistic material in use. Nevertheless, because the actual crystalline substructure is not taken into account, it is still difficult to predict surface dissipation accurately. Moreover, Niobium with its large lambda (~40 nm), exhibit an original behavior compared to the usual superconductors used in applied superconductivity, and generalities (e.g. pinning at grain boundaries) needs to be reconsidered. In this paper we hope to demonstrate that sources of dissipation usually attributed to external causes (mainly flux trapping during cooldown and hydrides precipitates) are related to the same type of crystalline defects which are not grain boundaries; that they affect the local superconducting properties and can also be at the source of early vortex penetration at the surface. We want also to stretch out how those defects can explain some of the discrepancies observed from lab to lab on recent results obtained i.e. in doping experiments. Understanding the origin and the role of these defects could provide indications for updating specification as well as fabrication follow up. Rather than new results the author wishes to present to the superconducting accelerators community the synthesis of multiple experimental results scattered in the literature that provide a new lighting on recently published work, in particular in the domain of SRF cavity doping as well as issues such as sensitivity to trapped flux during cooldown.