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

Abstract

A model for the onset of the reduction in superconducting radio-frequency (SRF) cavity quality factor, theso-called Q-drop, at high accelerating electric fields is presented. Since magnetic fieldsat the cavity equator are tied to accelerating electric fields by a simplegeometric factor, the onset of magnetic flux penetration determines the onset ofQ-drop. We consider breakdown of the surface barrier at triangulargrooves to predict the magnetic field of first flux penetrationHpen. Such defects were argued to be the worst case by Buzdin and Daumens (1998Physica C 294 257), whose approach, moreover, incorporates both the geometryof the groove and local contamination via the Ginzburg–Landau parameterκ. Sinceprevious Q-drop models focused on either topography or contamination alone, the proposed model allows newcomparisons of one effect in relation to the other. The model predicts equivalent reduction ofHpen when either roughness or contamination were varied alone, so smooth but dirty surfaceslimit cavity performance about as much as rough but clean surfaces do. Still lowerHpen was predicted when both effects were combined, i.e. contamination should exacerbate thenegative effects of roughness and vice versa. To test the model with actual data, couponswere prepared by buffered chemical polishing and electropolishing, and stylusprofilometry was used to obtain distributions of angles. From these data, curves forsurface resistance generated by simple flux flow as a function of magnetic field weregenerated by integrating over the distribution of angles for reasonable values ofκ. This showed that combined effects of roughness and contamination indeed reduce theQ-drop onset field by ∼ 20%, and that contaminationcontributes 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 opticalimages have become commonplace. The model was extended to fit cavity test data, whichindicated that reduction of the superconducting gap by contaminants may also play a role inQ-drop.