Influence of crystalline structure on rf dissipation in superconducting niobium

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 Nb=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.

Vacancy-Hydrogen Interaction in Niobium during Low-Temperature Baking

“…The mean lifetime τ mean of sample 1 exhibits here an excess compared to sample 6 representing increased open volume of vacancy cluster and near-surface defects. Lattice defects such as grain boundaries, vacancies and interstitials are known to be pinning sites for flux lines when the material undergoes the phase transition and the magnetic flux is expelled 60 . Hence, it is likely that the origin of the cavity quench originates from vacancy clusters found in the sample 1.…”

Vortex Dynamics and Dissipation under High-Amplitude Microwave Drive

“…This implies that any defect in the material with a dimension of the order of ξ or larger could pin vortices. Due to their small transverse dimension (approximately 1 − 5 nm), grain boundaries and single dislocations cannot be considered efficient pinning centers, whereas dislocation tangles and material nonuniformity can extend for as much as hundreds of nanometers, allowing for more efficient pinning [55,56].…”

“…The anisotropy parameter a i is introduced in order to emulate elongated pinning centers such as dislocation tangles occurring along the direction of the vortex flux. As described above, dislocation tangles are expected to be the most energetically favorable pinning sites in niobium, since their transverse dimension is of the order of ξ [55,56,61].…”

“…U ave is defined as the pinning energy that translates to a maximum pinning force per unit length (f p,ave ) comparable to values found in the literature. Usually, for niobium, the pinning force per unit length varies depending on the material's impurity content and the crystallographic structure in the range 10 −6 − 10 −4 N/m [56,62,63].…”

Direct evidence of microstructure dependence of magnetic flux trapping in niobium

“…The increase in residual resistance due to the trapped magnetic field has been studied in several SRF cavities with respect to the starting Nb material 24 , surface preparation 18 , and nitrogen diffusion conditions 25 . Theoretically, a multi-scale collective pinning mechanism is suggested for rf dissipation in SRF cavities 26 , and a recent review indicates pinning possibilities could be related to dislocation structures 27 .…”