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

Highlights: ► Construction details of a compact annulus magnet (YP1070) were reported. ► This is the first report of the YP1070 operation in a bath of liquid helium at 4.2 K. ► Spatial and temporal trapped field behaviors of the YP1070 were characterized. ► Temporal enhancement of the trapped fields was discovered at 4.2 K. -- Abstract: 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