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

Cold diodes play a crucial role in various operational aspects of the superconducting magnet of magnetic resonance imaging (MRI) scanners. The forward and reverse characteristics and failure modes are studied for two types of high-power rectifier diode to find their suitability for 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 room temperature. The diodes show negative differential resistance (NDR) beyond 1 A of forward current at 4.2 K. The forward voltage stabilizes in the range of 1.22-1.24 V after attaining the turn-on voltage in the range of 5-8 V. The significance of NDR is analysed for the MRI magnet. The long-term stability of the forward voltage is studied at 77 K with 550 A of forward current. Two modes of diode failure are also studied to foresee any failure during the operation of the magnet.