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In recent years, it has been pointed out that the maximum acceleratinggradient of a superconducting RF cavity can be increased by coating the innersurface of the cavity with a multilayer thin-film structure consisting ofalternating insulating and superconducting layers. In this structure, theprincipal parameter that limits the performance of the cavity is the criticalmagnetic field or effective $H_{C1}$ at which vortices begin penetrating intothe superconductor layer. This is predicted to depend on the combination of thefilm thickness. We made samples that have a NbN/SiO$_2$ thin-film structure ona pure Nb substrate with several layers of NbN film deposited using DCmagnetron sputtering method. Here, we report the measurement results ofeffective $H_{C1}$ of NbN/SiO$_2$(30 nm)/Nb multilayer samples with thicknessesof NbN layers in the range from 50 nm to 800 nm by using the third-harmonicvoltage method. Experimental results show that an optimum thickness exists,which increases the effective $H_{C1}$ by 23.8 %.

Evaluation of the superconducting characteristics of multi-layer thin-film structures of NbN and SiO$_2$ on pure Nb substrate