Electromagnetic Evaluation of HTS RF Coils for Nuclear Magnetic Resonance

We propose a Helmholtz-type probe coil that uses a high-temperature superconductor (HTS) for use as 700-MHz nuclear magnetic resonance (NMR) RF coils with a high unloaded quality factor (Q_u). We analyzed the microwave resonant properties of a one-turn coil set face-to-face. The coils were made of a YBa₂Cu₃O_7-δ (YBCO) film deposited on an r-plane sapphire substrate. The film thickness was 300 nm. The resonant frequency was found to split into in-phase (lower frequency) and out-phase (higher frequency) modes. We focused on the self-resonant frequency of the one-turn coil. We simulated the horizontal magnetic field caused by the RF coil. Although the coil could generate a sufficiently large transverse magnetic field in the in-phase mode, such a large field could not be generated in the out-phase mode. Therefore, we decided to use the in-phase mode. We also measured and simulated Qu to confirm that it was large enough for use in the 700-MHz NMR system.