High fieldQslope and the baking effect: Review of recent experimental results and new data on Nb heat treatments

Abstract

The performance of superconducting radio-frequency (SRF) cavities made of bulk Nb at high fields (peak surface magnetic field greater than about 90 mT) is characterized by exponentially increasing rf losses (high-field Q slope), in the absence of field emission, which are often mitigated by low-temperature (100-140 C, 12-48 h) baking. In this contribution, recent experimental results and phenomenological models to explain this effect will be briefly reviewed. New experimental results on the high-field Q slope will be presented for cavities that had been heat treated in a vacuum furnace at high temperature without subsequent chemical etching. These studies are aimed at understanding the role of hydrogen on the highfield Q slope and at the passivation of the Nb surface during heat treatment. Improvement of the cavity performances, particularly of the cavities' quality factor, have been obtained following the hightemperature heat treatments, while secondary ion mass spectroscopy surface analysis measurements on Nb samples treated with the cavities revealed significantly lower hydrogen concentration than for samples that followed standard cavity treatments.

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

“…Also, it will be valuable to examine what slip systems are activated, and to determine how the geometrical arrangement of dislocations affects pitting. Initial studies on large-grain singlecell cavities heat treated at 800 C=3 h, without subsequent BCP, showed improved Q 0 values over baseline rf tests, after BCP [17].…”

Cryogenic test of a proof-of-principle superconducting rf-dipole deflecting and crabbing cavity

“…The heating process was initiated after the furnace reached a pressure of 10 À6 torr. From experience a good level of degassing is achieved with a hydrogen partial pressure below 10 À6 torr at the end of the process [35]. The cavity was etched again in a light BCP process to remove $10 m to eliminate the contamination due to the high-temperature heat treatment.…”

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

“…HFQS can be removed by the empirically found "mild baking" at 100-120 C in ultra high vacuum (UHV) for 24-48 h. 1,5 Several models for the HFQS were proposed in the past, but most were shown to contradict at least one of the experimental observations. 6 The most recent promising model is based on the formation of lossy niobium nanohydrides in the penetration depth. 7 In the past, larger non-superconducting hydrides were identified as an origin of hydrogen Q disease.…”