Prototype HTS magnet system with sub-ppm field ripple wirelessly powered by HTS transformer-rectifier

Abstract High-temperature superconducting (HTS) magnets are promising for Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) applications since they can generate higher magnetic fields and work at higher temperatures. However, due to various sources of loss, HTS NMR/MRI magnets normally have to work in current driven mode, incurring significant cryogenic heat load and inferior magnetic field stability compared to persistent current operation of Low-temperature superconducting (LTS) magnets. This work demonstrates using a magnetic field controlled HTS transformer-rectifier and high-resolution NMR probe measurement feedback to wirelessly power closed-loop HTS magnets with high temporal stability. We achieve less than ±8 ppb/h long-term field drift and ±0.19 ppm/min short-term field ripple in an HTS test magnet with a time constant less than 30,000 seconds, setting a temporal stability record for quasi-persistent operation HTS magnets. By eliminating current leads and compensating for screening current drift, this work provides a potential engineering pathway of magnetic field stabilization for future HTS NMR/MRI applications.