Nanostructural features degrading the performance of superconducting radio frequency niobium cavities revealed by transmission electron microscopy and electron energy loss spectroscopy

Abstract

Nanoscale defect structure within the magnetic penetration depth of ∼100 nm is key to the performance limitations of niobium superconducting radio frequency (SRF) cavities. Using a unique combination of advanced thermometry during cavity RF measurements, and TEM structural and compositional characterization of the samples extracted from cavity walls, we discover the existence of nanoscale hydrides in electropolished cavities limited by the high field Q slope, and show the decreased hydride formation in the electropolished cavity after 120 • C baking. Furthermore, we demonstrate that adding 800 • C hydrogen degassing followed by light buffered chemical polishing restores the hydride formation to the pre-120 • C bake level. We also show absence of niobium oxides along the grain boundaries and the modifications of the surface oxide upon 120 • C bake.

Influence of crystalline structure on rf dissipation in superconducting niobium

“…Hydrides have also been evidenced by Raman spectroscopy: characteristic vibrational bands from NbH and NbH 2 have been observed on cold as well as hot spots [119], but their density seems to be higher on hot spots. The same trend has been observed by TEM [120].…”

“…We can thus infer a scenario explaining in the same time the baking effect and doping experiments: if the interstitial content near the surface (∼rf penetration depth) is increased, the interaction of H with those interstitials will prevent accumulation of hydrogen and reduce the apparition of hydrides. Recent TEM experiments show a decrease in hydride formation after 120°C baking [120]. Hydrides being poor superconductors, they can promote early penetration of a vortex, thus by reducing their presence, one can reach higher fields without early nucleation of vortex, which is basically what is observed after baking.…”

Niobium near-surface composition during nitrogen infusion relevant for superconducting radio-frequency cavities

“…Combined with its resistance to corrosion and ductility, Nb has become the material of choice for SRF-based technology. The concentration of impurities (hydrogen, oxygen, nitrogen, and carbon) within the rf penetration layer (≈40 nm for pure Nb) is believed to be the key factor regarding the performance of cavities, either by lowering the electron mean-free path [4] and energy gap [5] or by trapping potential hydrogen which can cluster and precipitate niobium hydrides upon cooldown [6,7]. Traditionally, Nb-based SRF cavities were subjected to several production steps, including different polishing cycles like buffered chemical polishing (BCP) and electrochemical polishing (EP), as well as heat treatments at 800°C and 120°C [1,8] in a high vacuum in order to lower the hydrogen content inherent to the material [9].…”

“…The former treatment, known as nitrogen doping, includes annealing polycrystalline Nb cavities for a few minutes at 800°C in 3 × 10 −2 -8 × 10 −2 mbar of N 2 . Such treatment conditions lead to the formation of β-Nb 2 N inclusions at the surface as well as promoting the growth of NbO [6,10]. By etching approximately 5 μm by EP, therefore removing the aforementioned inclusions, cavities displayed record-breaking quality factors at intermediate accelerating field gradients up to 20 MV=m.…”

Vacancy-Hydrogen Interaction in Niobium during Low-Temperature Baking

“…The threshold value for the high-field Q-slope is 90-100 mT, which relates to a maximum niobium hydride size of roughly 10 nm 12 . This mechanism is proposed as the origin of the so-called'high-field Q-slope' , describing an increase of surface resistance above an onset field, which can be overcome by a 120 °C bake for 48 h in a pressure below 10 −6 mbar which is an empirical cure 5,[14][15][16][17] .…”