Helium Vapor Cooled Current Leads

The introduction of current into a helium dewar with minimal heat input is a crucial factor in the economical operation of the system. The heat generated in the leads can be extracted by cooling them with the helium gas exhausted from the dewar. AMI leads for this purpose are used worldwide in both small and large installations. Current ratings range from 25 to 75,000 Amperes .

The heat capacity of helium vapor is used in these counterflow leads to minimize the liquid helium consumption for a specified operating current. Leads of this type have been shown to evaporate 2.8 x 10 ^-3 liters per hour per ampere for each pair when operated at the design current (40% less at zero current). However, a more conservative value of 3.2 x 10 ^-3\ liters per hour per ampere is recommended for design calculations. A voltage drop of approximately 0.22 volts per lead pair at the rated current is typical. A pressure difference of approximately 2 mm of mercury (0.03 psi) is developed through the leads at the rated current.

Break-away vapor cooled current leads are available for some amperage ratings and represent a popular configuration for persistent magnet operation. Break-away leads are supplied in two sections so that the upper section can be detached from the lower section during persistent magnet operation. Helium consumption is significantly reduced by break-away lead configurations since current lead thermal paths are eliminated. Automatic lead retractors can be incorporated for automation and control of the break-away lead configuration.