Noncontact Evaluation of HTS Cylinder for Compact Cryogen-Free NMR

Herein, a compact cryogen-free nuclear magnetic resonance (NMR) system is developed using a stacked high-temperature superconducting (HTS) bulk system. To compensate for magnetic field uniformity, an HTS cylinder is installed inside the measurement space. The cylinder is wound with REBCO tapes placed on the surface of a copper cylinder. The uniformity of its critical current density is important for NMR signal detection. Such an operation requires an examination of the quality of the critical-current-density distribution of the cylinder. A noncontact measurement method is required for such an examination. When a magnetic field is applied, the superconductor exhibits a demagnetization effect. Moreover, if any defect exists in any part of the superconducting tape, that particular part would contain no demagnetization, thus disturbing the relevant magnetic field distribution. In this study, this phenomenon is investigated using DC and AC magnetic fields. The magnets and sensor are placed inside the HTS cylinder, and magnetic fields are applied to the HTS cylinder surface. The results confirm that the proposed method is effective and applicable for evaluating the critical-current distribution.