Proximity breakdown of hydrides in superconducting niobium cavities

Abstract

Many modern and proposed future particle accelerators rely on superconducting radio frequency cavities made of bulk niobium as primary particle accelerating structures. Such cavities suffer from the anomalous field dependence of their quality factors Q 0 . High field degradation -so-called 'high field Q-slope' -is yet unexplained even though an empirical cure is known. Here we propose a mechanism based on the presence of proximity-coupled niobium hydrides, which can explain this effect. Furthermore, the same mechanism can be present in any surface-sensitive experiments or superconducting devices involving niobium.PACS numbers: 74.25.nn, 74.45.+c, 81.65.-b, 85.25.-j arXiv:1212.3384v3 [cond-mat.supr-con]

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

“…7,13,20 At this stage, it is not yet possible to definitively say, if it is the volume density or the size of the nanohydrides, which is affected. However, the removal of the HFQS suggests that it is likely the size.…”

“…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. 8,9 Nanohydrides may remain superconducting due to the proximity effect up to the breakdown field, which is determined by their size.…”

“…14,15 Both oxygen and vacancies may then serve as trapping centers for hydrogen, preventing the formation of lossy nanohydrides. 7 In this particular work, we present structural and analytic comparisons of Focused Ion Beam (FIB)-prepared cross-sectional samples from mild baked and unbaked cavity cutouts using various TEM techniques at room and cryogenic (94 K) temperatures. Temperature dependent nano-area electron diffraction (NED) and scanning electron nano-area diffraction (SEND) reveal the formation of stoichiometric non-superconducting small niobium hydride inclusions.…”

Precipitation of hydrides in high purity niobium after different treatments

“…The discussion of what defects can serve as nucleation centers is still open but we think that vacancies and hydrogen-vacancies complexes are among likely candidates. 5,12 We start with the assumption that hydrogen is uniformly distributed in the sample volume, and hydride nuclei are spaced by the distance L. The schematic used for this model is shown in Fig. 11.…”

“…[1][2][3][4] The effect was attributed to the formation of non-superconducting niobium hydride phases, and it was quickly found that Q disease can be "cured" and prevented by vacuum annealing at T > 600 C for several hours, presumably due to the decrease of the bulk hydrogen concentration. More recently, smaller hydrides were proposed 5,6 as a cause for another cavity limitation-a "high field Q slope," understanding of which is an area of active ongoing research.…”