Quench detection and protection of the wide bore 900 MHz NMR magnet at the National High Magnetic Field Laboratory

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/sub 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 is discussed.