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Cold diodes and resistors in parallel with the coils are keys components in the passive quench protection system of low-current (high inductance) superconducting magnets. The diodes prevent current from flowing in the circuits that are in parallel with sections of the superconducting coil when the magnet is being charged or discharged. The use of diodes and resistors in parallel with the superconducting coil sections reduce the peak voltages to ground and, in some cases, the hot-spot temperature in the magnet during a quench. This paper presents measurements of the onset (low current) forward voltage of several power diodes as a function of temperature and magnetic field.Index Terms-Cold power diodes, quench protection system, superconducting magnet, varistor discharge system. 1051-8223

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There is a rule of thumb that has been used for many years in the design of conventional iron dominated magnets. If the magnet uses a copper conductor, the RMS copper current density should be about 5 A for a magnet system with a minimum capital cost. If the cost of electric power is considered, the optimum conductor current density will be lower. The rule of thumb current density has its origin in papers published over forty years ago. The question has been asked, "Has the optimum current density changed since the 1960's?" In order to set the design parameters for the conventional magnets in the Facility for Rare Isotope Beams (FRIB) at Michigan State University (MSU), a study was done on magnet cost optimization based on the conductor current density. The magnet capital cost optimization was done considering the cost of conductor, iron, power supplies, and the cooling system. In addition, capital cost plus operating cost optimization was done for magnets operating lifetimes of 50000 hrs ( 10 yrs of accelerator operation).