Superconducting dc transformer coupling

Abstract

Measurements of the ratio of secondary voltage to primary voltage as a function of the secondary current allow a determination of the superconducting dc transformer coupling force. The force was measured for tin-film transformers. The primary film thickness and the thickness of the SiO insulating layer were varied. The maximum coupling force appears to be given approximately by ${F}_{\mathrm{cm}}\ensuremath{\simeq}(\frac{{\ensuremath{\varphi}}_{0}^{2}}{8\ensuremath{\pi}{\ensuremath{\mu}}_{0}})\ifmmode\times\else\texttimes\fi{}{[{\ensuremath{\lambda}}_{p}coth(\frac{{d}_{p}}{{\ensuremath{\lambda}}_{p}})+{\ensuremath{\xi}}_{p}+{d}_{0}]}^{\ensuremath{-}1}$ ${[{\ensuremath{\lambda}}_{s}coth(\frac{{d}_{s}}{{\ensuremath{\lambda}}_{s}})+{\ensuremath{\xi}}_{s}+{d}_{0}]}^{\ensuremath{-}1}$. Here ${d}_{0}$, ${d}_{p}$, and ${d}_{s}$ are the oxide thickness, primary thickness, and secondary thickness, and other symbols are standard.