Investigation of the effect of structural defects from hydride precipitation on superconducting properties of high purity SRF cavity Nb using magneto-optical and electron imaging methods

Abstract

Nb is an elemental superconductor with a critical temperature of 9.3 K and is widely used to fabricate superconducting radiofrequency (SRF) cavities for particle accelerators. However, microstructural defects in Nb, such as grain boundaries and dislocations, can act as pinning centers for magnetic flux that can degrade SRF cavity performance. Hydrogen contamination is also detrimental to SRF cavity performance due to the formation of normal conducting hydrides during cool down. In this study, disc shaped Nb bi-crystals extracted from a high-purity large-grain Nb slice were investigated to study the effects of grain boundaries, hydrogen, and dislocations on superconducting properties. Grain orientation and grain boundary misorientation were measured using Laue X-ray diffraction and electron backscattered diffraction (EBSD) analyses. Cryogenic magneto-optical imaging was used to directly observe magnetic flux penetration below T_c =9.3 K. Damage caused by low temperature precipitation of hydrides and their dissolution upon reheating after cryogenic cycles was examined using electron channeling contrast imaging (ECCI), and EBSD. The relationships between hydride formation, dislocation content, grain boundaries, cryo-cooling, heat treatment, and flux penetration indicate that both grain boundary character and hydrogen content affect magnetic flux penetration. Such flux penetration could be facilitated by dislocation structures and low angle grain boundaries resulting from hydride precipitation and heat treatment.

Tuning microwave losses in superconducting resonators

“…At H c1 < H a < H s the Meissner state remains metastable due to the Bean-Livingstron barrier [22]. Many magneto-optical imaging investigations of type-II superconductors [229][230][231][232] have revealed premature local penetration of vortices at grain boundaries and other materials and topographic defects at the surface [54][55][56][57]. In turn, TDGL and nonlinear electrodynamic simulations have shown that surface defects can reduce the penetration field [233][234][235] and cause flux jets [236,237] being precursors of themomagnetic avalanches.…”

An Innovative Approach of Surface Polishing for SRF Cavity Applications

“…Test results have been published in [13] and were not really satisfactory because of Q-disease: the presence of a significant amount of hydrogen in niobium material leads to the formation of niobium hydride precipitates when exposed to low temperatures typically between 50 and 150 K [28]. These niobium hydrides are non-superconducting and lead to a strong degradation of the surface resistance [29][30][31]. Thus, the triggering of Q-disease on our samples made the evaluation of the impact of the polishing procedure on the surface resistance not relevant.…”