Temporal Enhancement of Trapped Field in a Compact NMR Magnet Comprising YBCO Annuli.

Temporal "enhancement" of trapped fields was observed in the central region of a compact NMR magnet comprising a stack of 2800 YBCO "square" annuli (YP2800), field-cooled at 4.2 K. This paper presents an analytical model to simulate the trapped field enhancement in YP2800. First, based on an inverse calculation technique, the current distributions in the 560 5-plate modules of YP2800 were computed from the measured trapped field distribution. Then, YP2800 was modeled as a set of "three magnetically-coupled subcoils": the "bottom" coil (C, 140 modules); the "middle" coil (C, 280 modules); and the "top" coil (C, 140 modules). With the index resistance of each coil included, the circuit model shows that the average current in C "slowly" increases, induced by "fast" current decays in C and C. As a result, the center field in YP2800, dominated by the C currents, increases in time. The simulation agrees reasonably well with the measurement, which validates the analytical model.