A wide bore, high resolution NMR magnet with a proton resonance frequency of 900 MHz, field of 21.1 T, and stored energy of 40 MJ has been designed and fabricated at the NHMFL. The magnet has five Nb 3 Sn and five NbTi coils, each of which contains stainless steel strip heaters. An active protection system with analog quench detection circuitry is employed to determine the onset of a quench and to subsequently open a superconducting protection switch and energize a portion of the strip heater network. A passive protection system with diode sets and shunting resistors exist for redundancy. Details of the protection system are presented and its performance during the magnet's bucket test are discussed.

Resistive magnets offer very high field strengths, unmatched by superconducting technology. However, the spatial and temporal characteristics of raw magnetic fields generated by resistive high powered and water cooled magnets, are unadapted to most nuclear magnetic resonance ͑NMR͒ experiments. The National High Magnetic Field Laboratory has installed a 24 T (ϳ1 GHz 1 H), 32 mm bore, 13 MW resistive magnet to study the feasibility of utilizing such fields for NMR applications. Herein we present our efforts in identifying, characterizing, and improving the temporal properties of the magnets. The temporal instabilities arise mainly from two sources: power supply ripple and inlet cooling water temperature variations. To compensate for power supply ripple, flux stabilization was employed, whereas for long term variations, arising from variations in the water temperature, a field frequency lock unit was utilized. Moreover, a novel flow based water temperature control scheme was implemented. The stabilization and improved control reduced the initial 16 ppm peak-to-peak variation to ϳ2 ppm. Implementation of a field frequency lock unit further reduced the temporal variation to 0.8 ppm peak-to-peak. Sharp NMR linewidths -1.7 ppm at full width at half height of 2 H in liquid D 2 O -are observed in small volume samples, enabling moderate resolution NMR experiments to be performed at 24 T.

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