High-power cold diodes for the protection of a 1.5 T superconducting MRI magnet system

Abstract

The cold diodes play a crucial role in the various operational aspects of the superconducting magnet of an MRI scanner. The forward and reverse characteristics, failure modes are studied for two types of high-power rectifier diodes to find their suitability of application at 4.2 K in a whole-body 1.5 T superconducting MRI magnet system.The diodes are characterized at various temperatures ( 300 K, 77 K and 4.2 K) in the current range up to 600 A. The turn-on voltage of the diodes at 4.2 K is found to be increased by 7-12 times that of the room temperature. The diodes show negative differential resistance beyond 1 A of forward current at 4.2 K . The forward voltage stabilizes in the ranges of 1.22 -1.24 V after attaining the turn-on voltage in the range of 5-8 V. The significance of negative differential resistance has been analyzed for the MRI magnet. The long-term stability of the forward voltage has been studied at 77 K with 550 A of the forward current. Two modes of the failure of the diodes are also studied to foresee any failure during the operation of the magnet.