Bulk and Plate Annulus Stacks for Compact NMR Magnets: Trapped Field Characteristics and Active Shimming Performance.

We have constructed two "annulus" magnets, YP2800 and YB10; each consists of 2800 YBCO thin square "plate annuli" (YP2800) and 10 YBCO thick "bulk annuli" (YB10). Their trapped field characteristics, spatial and temporal, were investigated and compared, experimentally and analytically. Two sets of field-cooling tests were performed at 77 K: (1) maximum trapped field tests, where a 2-T background field was applied to investigate the maximum trapped field capability of the two magnets; and (2) reduced trapped field tests, where spatial homogeneity improvement of the two magnets was investigated after field cooling with a reduced background field. Also, a Z1 copper shim coil was designed, constructed, and operated, alone and with YP2800 and YB10. When it was operated with the annulus magnets at 77 K, a significant attenuation of the shim coil strength was observed due to the screening currents induced within the annulus magnets.

Field performance of a prototype compact YBCO "annulus" magnet for micro-NMR spectroscopy.

A prototype compact annulus YBCO magnet (YP1070) for micro-NMR spectroscopy was constructed and tested at 77 K and 4.2 K. This paper, for the first time, presents comparison of the 77-K and 4.2-K test results of our annulus magnet. With a 26-mm cold bore, YP1070 was comprised of a stack of 1070 thin YBCO plates, 80-µm thick and either 40-mm or 46-mm square. After 1070 YBCO plates were stacked ''optimally'' in 214 groups of 5-plate modules, YP1070 was ''field-cooled'' at 77 K after being immersed in a bath of liquid nitrogen (LN2) with background fields of 0.3 and 1 T and also at 4.2 K in a bath of liquid helium (LHe) with background fields of 2.8 and 5 T. In each test, three key NMR magnet field-performance parameters-trapped field strength, spatial field homogeneity, and temporal stability-were measured. At 4.2 K, a maximum peak trapped field of 4.0 T, equivalent to 170 MHz 1H NMR frequency, was achieved with a field homogeneity, within a |z| < 2.5 mm axial space, of ~3000 ppm. YP1070 achieved its best field homogeneity of 182 ppm, though at a reduced trapped field of 2.75 T (117 MHz). The peak trapped fields at 4.2 K were generally ~10 times larger than those at 77 K, in direct proportion to ~10-fold enhancement in superconducting current-carrying capacity of YBCO from 77 to 4.2 K. Temporal stabilities of ~110 and ~17,500 ppm/h measured at 77 K, with trapped fields respectively of 0.3 and 1 T, show that temporal stability deteriorates with trapped field strength. Also, temporal enhancement of trapped fields at 4.2 K was observed and reported here for the first time.