Helical Resonators for Measuring Dielectric Properties of Materials

A theoretical and practical investigation is given of superconducting helical resonators with quality factors greater than 10⁸ which allow the determination of dielectric loss tangents in the frequency range of 0.1 to 5 GHz and below 15 K with high accuracy. The underlying measurement theory is an extension of perturbation theory. They are evaluated for the fields in a shielded multiple quarter-wave helical resonator with a cylindrical dielectric specimen inside the helix. Measurement results on optimized resonators made from superconducting Nb and Nb₃Sn are reported as well as further applications.