Inverse Approach for RRR Measurement in Thin-Film Nb-Coated SRF Cavities Using Planar Inductor Eddy Current Sensors

Superconducting Radio Frequency (SRF) cavities are crucial components in particle accelerators and are typically constructed from bulk niobium (Nb) due to their excellent superconducting properties. However, bulk Nb is prone to localized defects, which can degrade performance. Thin-film (TF) Nb coatings on copper (Cu) substrates have emerged as a promising alternative, offering reduced material costs and potentially improved performance. The Residual Resistivity Ratio (RRR), a key indicator of material purity, plays a vital role in determining the thermal and superconducting properties of Nb. Traditional methods for measuring RRR, such as the 4-probe DC method, are not suitable for thin films due to the risk of damaging the coating, necessitating the development of non-destructive techniques. This study explores the use of planar inductor eddy current sensors to measure the RRR of thin-film Nb coatings through a non-contact method. Finite Element Analysis (FEA) using Ansoft Maxwell was conducted to simulate the impedance variations of the coil in the presence of Nb-coated Cu targets at both room and cryogenic temperatures. The resistance change (ΔR) of the coil was analyzed across frequencies, and a distinct minimum (F_min). A relationship was identified between F_min, conductivity, and RRR, allowing the determination of the conductivity of TF Nb and, consequently, its RRR. The results showed that increasing the conductivity of the TF Nb shifts F_min to lower frequencies, while the thickness of the Nb coating has no effect on the minimum ΔR.