Characterization of etch pits found on a large-grain bulk niobium superconducting radio-frequency resonant cavity

Abstract

The performance of superconducting radio-frequency (SRF) resonant cavities made of bulk niobium is limited by nonlinear localized effects. Surface analysis of regions of higher power dissipation is thus of intense interest. Such areas (referred to as ''hotspots'') were identified in a large-grain single-cell cavity that had been buffered-chemical polished and dissected for examination by high resolution electron microscopy, electron backscattered diffraction microscopy (EBSD), and optical microscopy. Pits with clearly discernible crystal facets were observed in both ''hotspot'' and ''coldspot'' specimens. The pits were found in-grain, at bicrystal boundaries, and on tricrystal junctions. They are interpreted as etch pits induced by crystal defects (e.g. dislocations). All coldspots examined had a qualitatively lower density of etch pits or relatively smooth tricrystal boundary junctions. EBSD mapping revealed the crystal orientation surrounding the pits. Locations with high pit density are correlated with higher mean values of the local average misorientation angle distributions, indicating a higher geometrically necessary dislocation content. In addition, a survey of the samples by energy dispersive x-ray analysis did not show any significant contamination of the samples' surface. The local magnetic field enhancement produced by the sharp-edge features observed on the samples is not sufficient to explain the observed degradation of the cavity quality factor, which starts at peak surface magnetic field as low as 20 mT.

Detection of surface carbon and hydrocarbons in hot spot regions of niobium superconducting rf cavities by Raman spectroscopy

“…Several of these recrystallized foils were subsequently cold worked by bending back and forth ($ 90 angle) from 20 to 50 times. We also have Raman data from six different hot spot samples cut out of a single-cell cavity [11]. The hot spots displayed temperature rises $0:3-0:4 K above the He bath temperature at cavity magnetic field strengths of 90 mT.…”

“…One group of foils had been rolled from a piece of Nb plate used for SRF cavity fabrication and subsequently processed in a similar manner to that used on cavities including electropolishing and BCP etching [2,5,11]. In addition we examined foils as obtained directly from Alpha Aesar (as-received noted here as unprocessed).…”

“…The detailed processing history and cavity performance measurements can be found in Ref. [11]. Sample numbers were those assigned at Jefferson Lab and are referred to as, for example, Jlab #12.…”

“…The standard spectroscopy tools of electron microscopy, namely Auger and energy dispersive x-ray (EDX), showed little difference between the regions inside and outside the pits in preliminary tests [11]. One concern is that such spectroscopic tools cannot measure hydrogen, a gas which diffuses easily in Nb and is a well-known contaminant potentially affecting the quality factor Q of the cavity [5].…”

“…A particular case study of interest is a single-cell niobium SRF cavity, fabricated using a buffered chemical polishing (BCP), that exhibited medium field (20-100 mT) Q drop [11]. Localized regions of high dissipation, hot spots, developed as Q decreased, and optical and electron backscattered diffraction imaging showed clearly that such hot spots were correlated with a high areal density of etch pits, especially near grain boundaries.…”