A New Method for Measuring the Low-Temperature Dielectric Property of a Low-Loss Material with a Superconducting Resonant Cavity

A superconducting 6.5 GHz cylindrical TE011 mode cavity was used to measure the small dielectric loss of polytetrafluoroethylene at liquid helium temperature. First, the Q value and the resonance frequency of the unperturbed niobium cavity was measured by the decrement method. Then a thin cylindrical rod sample, suspended from the magnetic loop, was inserted into it through the coupling port. By the use of a gate valve and a vacuum by-pass the sample can be drawn out and reassembled without introducing air into the cavity. From the decrease of the Q value and the shift of the resonance frequency, the complex dielectric constant was obtained ( ε '/ ε 0 =1.9 and ε ''/ ε 0 =4.4 × 10 -6 at 1.3 K). Loss tangent tan δ was estimated as 2.4 × 10 -6 .