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Abstract: A model for the onset of the reduction in SRF cavity quality factor, theso-called Q-drop, at high accelerating electric fields is presented. Breakdownof the surface barrier against magnetic flux penetration at the cavity equatoris considered to be the critical event that determines the onset of Q-drop. Theworst case of triangular grooves with low field of first flux penetration Hp,as analyzed previously by Buzdin and Daumens, 1998 Physica C 294: 257, wasadapted. This approach incorporates both the geometry of the groove and localcontamination via the Ginzburg-Landau parameter kappa, so the proposed modelallows new comparisons of one effect in relation to the other. The modelpredicts equivalent reduction of Hp when either roughness or contamination werevaried alone, so smooth but dirty surfaces limit cavity performance about asmuch as rough but clean surfaces do. When in combination, contaminationexacerbates the negative effects of roughness and vice-versa. To test the modelwith actual data, coupons were prepared by buffered chemical polishing andelectropolishing, and stylus profilometry was used to obtain distributions ofangles. From these data, curves for surface resistance generated by simple fluxflow as a function of magnetic field were generated by integrating over thedistribution of angles for reasonable values of kappa. This showed thatcombined effects of roughness and contamination indeed reduce the Q-drop onsetfield by ~30%, and that that contamination contributes to Q-drop as much asroughness. The latter point may be overlooked by SRF cavity research, sinceaccess to the cavity interior by spectroscopy tools is very difficult, whereasoptical images have become commonplace. The model was extended to fit cavitytest data, which indicated that reduction of the superconducting gap bycontaminants may also play a role in Q-drop.



Autor: A Dzyuba, A Romanenko, L D Cooley

Fuente: https://arxiv.org/







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