Model for initiation of quality factor degradation at high accelerating fields in superconducting radio-frequency cavities

Abstract

A model for the onset of the reduction in SRF cavity quality factor, the so-called Q-drop, at high accelerating electric fields is presented. Since magnetic fields at the cavity equator are tied to accelerating electric fields by a simple geometric factor, the onset of magnetic flux penetration determines the onset of Q-drop. We consider breakdown of the surface barrier at triangular grooves to predict the magnetic field of first flux penetration H pen . Such defects were argued to be the worst case by Buzdin and Daumens, [1998 Physica C 294 257], whose approach, moreover, incorporates both the geometry of the groove and local contamination via the Ginzburg-Landau parameter . Since previous Q-drop models focused on either topography or contamination alone, the proposed model allows new comparisons of one effect in relation to the other. The model predicts equivalent reduction of H pen when either roughness or contamination were varied alone, so smooth but dirty surfaces limit cavity performance about as much as rough but clean surfaces do. Still lower H pen was predicted when both effects were combined, i.e. contamination should exacerbate the negative effects of roughness and vice-versa. To test the model with actual data, coupons were prepared by buffered chemical polishing and electropolishing, and stylus profilometry was used to obtain distributions of angles. From these data, curves for surface resistance generated by simple flux flow as a function of magnetic field were generated by integrating over the distribution of angles for reasonable values of . This showed that combined effects of roughness and contamination indeed reduce the Q-drop onset field by ~20%, and that that contamination contributes to Q-drop as much as roughness. The latter point may be overlooked by SRF cavity research, since access to the cavity interior by spectroscopy tools is very difficult, whereas optical images have become commonplace. The model was extended to fit cavity test data, which indicated that reduction of the superconducting gap by contaminants may also play a role in Q-drop.

Field limit and nano-scale surface topography of superconducting radio-frequency cavity made of extreme type II superconductor

“…There exist several types of defect models that treat the suppression of B s [29,30,31,32,33,34]. In particular, Buzdin and Daumens [32] and Aladyshkin et al [33] studied the groove with triangular section and derived simple formulae for locally suppressed B s , which can incorporate a geometry of defect via an angle parameter and are useful for modeling surface topographies [35]. Their formulae are, however, derived under an assumption that the groove has an infinite depth, which can be applied to a defect with a depth much larger than penetration depth (> µm) [35], but can not be applied to that smaller than penetration depth (< O(10 2 ) nm).…”

Laser polishing for topography management of accelerator cavity surfaces

“…Sufficient local enhancement can exceed the critical field, causing a local phase change to the normal conducting state. Though the resulting normal‐conducting volume may be too small to trigger a general quench, the cumulative effect of many such small defects turning on with increasing gradient would be manifest as decreasing Q 0 at higher gradient (“Q slope”) . Efforts to explicitly model the effect of topography on Q 0 are beginning to meet success .…”

Enhanced characterization of niobium surface topography

“…Even when this enhancement is not sufficient to induce a full quench of the resonance, it may change the local loss. Though the resulting normalconducting volume may be too small to trigger a general quench, the cumulative effect of many such small defects turning on with increasing gradient would be manifest as reducing Q o [3] at a higher gradient. Recent studies [4] emphasize the importance of intrusions (pits, grain boundary crevices) as well as protrusions.…”