Abstract
Bulk niobium is the most common material used in the fabrication of rf superconducting cavities for accelerators. Predicting and reducing the rf surface dissipation in these cavity structures is mandatory, since it has a tremendous cost impact on the large accelerator projects. In this paper the author hopes 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 that affect the local superconducting properties and can be at the source of early vortex penetration at the surface. We also want to show how these types of defects can explain some of the discrepancies observed from other laboratories in niobium cavity doping experiments. Understanding the origin and the role of these defects could provide direction for improving material specifications as well as improving fabrication control from sheet material to completed cavity. In particular, we will demonstrate that dislocation entanglements, due to the fabrication damage layer, have the strongest impact for the pinning behavior of trapped flux, as well as hydrogen segregation in cavity niobium. The author wishes to present to the superconducting radio frequency (SRF) accelerator community the synthesis of experimental results scattered in the literature, completed with some personal results. The results of this effort provide a new perspective on recently published work in the domain of SRF cavity doping and sensitivity to trapped flux during cooldown. I will also try to draw whenever possible, some conclusions about other types of superconductors used for SRF applications including $\mathrm{Nb}/\mathrm{Cu}$ thin films and to discuss their possible change of behavior with field or frequency. I will concentrate on surface and material science aspects since the experimental results on rf cavities have already been treated elsewhere.
Highlights
Most of the present models for determining the surface resistance of superconductors in the rf regime (SRF) do not take into account the most common crystalline defects found in bulk niobium
Ginsburg-Landau (GL) developments that account for some nonuniformity and nonlinear behavior are abundant in the literature, but they are valid only close to the critical temperature TC or the second critical field HC2, unless one deals with linearized GL model that applies at any T, but only for very small coherence length ξ
As pinning and elastic forces are in equilibrium, the pinning force automatically increases. This effect is mainly observed on thin films where the surface/volume ratio is high [28], and it could play a paramount role for alternative superconductors ðNbN; Nb3Sn; MgB2; ...Þ in SRF applications: most of these materials are fabricated into relatively thin films with relatively small grain sizes at the origin of some roughness at the nm level
Summary
Most of the present models for determining the surface resistance of superconductors in the rf regime (SRF) do not take into account the most common crystalline defects found in bulk niobium. The majority of the numerical developments found in the literature have been conducted for high Ginzburg-Landau parameter κ (small ξ) superconductors [3,4,5]. Their numerical predictions are generally poor for niobium with its large ξ (∼40 nm) and κ close to 1, especially in the SRF operating condition (Meissner state, 2–4 K).
Sign-in/Register to view highlights & summary 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.
