High-Efficiency Superconducting Homopolar dc Generators

Abstract

The immediate requirement for flux pumps appears to be a design capable of producing 104 A at 100 W and high efficiency to energize large stabilized magnets. We have, however, successfully operated homopolar superconducting dc generators and also stabilized solenoids in helium vapor rather than liquid, and it appears reasonable to suggest that large magnets could be energized during the latter stages of cooldown, so that the power requirements for reasonable energization times may be less than this. Two designs for homopolar superconducting dc generators will be discussed and their performances reported on; both use permanent magnet and iron rotors. The first design has produced 1600 A and 20 W. The most significant feature of the design is the magnetic circuit used to generate in excess of 6 kG in the normal spot crossing the sheet. This enables reasonable power to be generated in a low-speed (300 rpm) design. There are, however, large losses attributed to sloshing of liquid helium. The second design is evolved from the first by enclosing the rotor in a vacuum container; special cooldown procedures are required. High efficiency is obtained (measured in excess of 100% at the time of writing, the excess being due to measurement error rather than a triumph over the second law of thermodynamics!). The design promises heavy currents and reasonable power outputs. The design allows topological manipulations of the sheet to allow adjustments of the current-carrying capacity of the sheet or output voltage (and hence power) or output impedance (and hence efficiency).