Identification and minimization of sources of temporal instabilities in high field (> 23 T) resistive magnets

Resistive magnets offer very high field strengths, unmatched by superconduc ting technology. However, the spatial and temporal characteristics of raw m agnetic fields generated by resistive high powered and water cooled magnets , are unadapted to most nuclear magnetic resonance (NMR) experiments. The N ational High Magnetic Field Laboratory has installed a 24 T (similar to 1 G Hz H-1), 32 mm bore, 13 MW resistive magnet to study the feasibility of uti lizing such fields for NMR applications. Herein we present our efforts in i dentifying, characterizing, and improving the temporal properties of the ma gnets. The temporal instabilities arise mainly from two sources: power supp ly ripple and inlet cooling water temperature variations. To compensate for power supply ripple, flux stabilization was employed, whereas for long ter m variations, arising from variations in the water temperature, a field fre quency lock unit was utilized. Moreover, a novel flow based water temperatu re control scheme was implemented. The stabilization and improved control r educed the initial 16 ppm peak-to-peak variation to similar to 2 ppm. Imple mentation of a field frequency lock unit further reduced the temporal varia tion to 0.8 ppm peak-to-peak. Sharp NMR linewidths - 1.7 ppm at full width at half height of H-2 in liquid D2O - are observed in small volume samples, enabling moderate resolution NMR experiments to be performed at 24 T. (C) 2000 American Institute of Physics. [S0034-6748(00)00307-5].