1000-A cryotrons for superconducting-magnet flux pumps

Cryotrons are being developed for a low temperature flux pump power supply to energize superconducting coils without the excessive helium boiloff characteristic of high current input leads. The cryotrons have been tested to 1200 A at frequencies up to 40 Hz. For 1000 A, the optimum operating frequency is presently 5 Hz (determined by the current transfer rate and the normal cryotron gate resistance). The cryotron gates are Pb-4% Sb and have a normal state resistance of 6 μΩ (resistivity 3.8×10−7 Ω-cm).