Speakers
Description
Superconducting Radio Frequency (SRF) cavities are critical components in particle accelerators, responsible for accelerating charged particles to high energies. Traditionally, these cavities use bulk niobium (Nb) due to its exceptional superconducting properties at cryogenic temperatures. However, limitations like localized defects can hinder performance by causing power absorption and abrupt transitions (quenching) from the superconducting state. To address these limitations, thin-film Nb superconductors deposited on copper (Cu) substrates are emerging as promising alternatives. These thin films offer several advantages, including high thermal conductivity, high critical fields, and reduced Nb usage. Assessing the purity of Nb in SRF cavities is crucial for optimal performance. A key parameter used for this purpose is the Residual Resistance Ratio (RRR). This ratio compares the material’s resistance at cryogenic temperatures to its room-temperature resistance. Higher RRR values generally indicate higher purity and, consequently, better superconducting properties. While established methods like 4-probe DC and
AC measurements effectively measure RRR in bulk Nb, they are not suitable for thin films. This lack of a suitable, non-destructive technique poses a significant challenge in characterizing thin-film Nb superconductors for SRF applications. This study presents a non-destructive approach for measuring the electrical conductivity (and hence RRR) of thin-film Nb superconductors. The method utilizes planar eddy current sensors, which interact with the sample without any physical contact, making them ideal for delicate thin films. The study investigates the change in the impedance of the sensor coil when placed near a thin-film Nb-coated Cu sample. As the sample is brought closer to the coil, eddy currents are generated within the Nb film due to the interaction with the sensor’s electromagnetic field. These eddy currents, in turn, affect the impedance of the sensor coil. The focus of the study lies on the frequency dependence of the resistive component of the impedance. This component directly relates to the sample’s conductivity and offers valuable information for characterizing the Nb film. The reactive component, on the other hand, provides less information. Experiments show a distinct minimum in the difference between the resistances measured with the Nb-coated Cu target and the Cu substrate only called ∆R. This minimum is observed consistently across various Nb film thicknesses (ranging from 0.3 to 3 micrometers) and temperatures (both room temperature and cryogenic temperatures). This unique signature allows for the extraction of conductivity data using a well-established theoretical model. This work presents experimental data on the difference in resistance ∆R measured at various frequencies for thin-film Nb coatings of different thicknesses on Cu substrates. The measurements were conducted at both room temperature and cryogenic temperature. The electrical conductivity of the Nb films was then determined by analyzing the minimum point observed in the ∆R data using the theoretical model.
Acknowledgments
The Science and Engineering Research Board, Government of India funded this work (Reference Grant No. CRG/2021/000398).
| Submitters Country | India |
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