Numerical Calculation of Magnetic Field Enhancement and Impact of Surface Defects on Premature Entry of Magnetic Field in Thin Nb Films for SRF Cavities

In this work, we have investigated two series of Nb/Cu samples deposited by HiPIMS technique, and differing in Nb deposition conditions, Nb film thickness and Cu substrate polishing techniques. All the films were additionally irradiated by Nd:YAG laser to smooth their surfaces. The impact of the magnetic field enhancement at the surface defects on the premature start of magnetic field penetration into the superconducting film was studied, combining experiments and numerical calculations. Compared to previous study, improved numerical calculations by using the Finite Element Method (FEM) served to calculate the maximum field enhancement factor β
_m
, reflecting impact of the most crucial surface defects found in the samples. Magnetization measurements at 4.2 K in DC magnetic field, oriented parallel to the film, were employed to determine the start of the field penetration
H
_en
. The SEM and AFM analyses served to investigate the Nb surface morphology. In some samples, deviations from the
H
_en
(β
_m
) dependence were observed. It was found that the magnetic field penetration could start from the Nb/Cu interface rather than from the free Nb surface due to visibly better quality of the free Nb surface observed by SEM analysis, and that could lead to the deterioration of
H
_en
(β
_m
) dependence.