Low-heat-leak current leads for intermittent use

The paper reports on the development of current leads for an orbiting superconducting magnetic spectrometer that were required to achieve a zero-current heat leak of 0.03 W with a total coolant flow rate as low as 0.011 g/sec cooling only the coldest third of the length of a lead. Analysis of heat flow phenomena in an insulated current lead in thermal contact with cryogenic gas flow is given for prediction of lead performance. A basic lead design was then tested and modified by such techniques as tapering, adding superconducting sections, and operating above the optimum current, until a lead satisfying the low-leak requirements was obtained. Operating the insulated leads at well above their optimum current for short periods at high mass flow is possible, and instability behavior is predictable.