| | We are embarking upon a theoretical investigation of the predicted behavior of ideal, clean superconducting surfaces exposed to large, slowly varying magnetic fields ¨C e.g. the niobium surface of a superconducting RF cavity. The peak operating fields of these cavities already exceed Hc1, and yet vortex penetration is not observed; theoretically we must study the metastable state between Hc1 and the instability threshold Hsh at which spontaneous nucleation of vortices at the surface becomes energetically favorable. The operating temperatures are not near Tc, and hence Ginzburg-Landau theory is not a controlled approximation; to get quantitative understanding of the superheating field, we must apply Gorkov theory or perhaps the Eilenberger equations. ('Line nucleation' formulas Hsh = Hc/kappa, which have discouraged the exploration of high kappa materials, have no theoretical basis.) We will report here on initial investigations using Ginzburg-Landau theory to test and validate numerical schemes for extracting the instability threshold for superconducting layers near Tc, and then sketch plans for applying these techniques to the full Eilenberger equations. We will conclude with speculations about the effects of microscopic and mesoscopic disorder. | |
