One of the challenges of using superconducting magnets is the connection of the magnet to a room temperature power supply. This is accomplished via current leads. The trick is that current leads should ideally have a low heat leak, since they connect room temperature to cryogenic temperature, while at the same time they should have low electrical losses since, depending on the magnet design, they may carry kiloamps of electrical current.
Until the advent of high temperature superconductors, most current leads were based on a vapor cooled design. In this design, the bottom of the current lead sits in a bath of liquid helium and is connected to the superconductor in the magnet. Cold helium vapor boiling off from the bath flows up through the current lead out to room temperature, intercepting the heat leak coming down the lead. The current lead is generally made of copper that is finely divided by many cooling passages through which the helium gas moves.
In some cases, the current lead design uses bundles of fine copper conductor contained within a tube through which the helium passes. The current leads are designed to minimize the conduction heat leak down the resistive conductor and the heat created by the resistive losses due to the electrical current flowing in the conductor. However, even current leads that are properly optimized can represent a significant heat load to the liquid helium space in the superconducting magnet system. Resistive vapor cooled leads are a well established technology and have been designed to carry 75 kA of current or greater.
The development of high temperature superconductors (HiTc) has had a major impact on current lead design. In fact, current leads incorporating HiTc superconductors represent an early successful commercial application of these new superconductors. The advantage of the HiTc leads is that not only do they permit the use of superconductivity up to 50K–77K, thus reducing the overall heat generated by resistive losses, but also in many cases the HiTc conductors are poor thermal conductors and thus reduce the conduction heat load into the liquid helium.
One popular design for current leads using HiTc materials is a binary lead in which the current in the lower half of the lead, below 50K, is carried by the HiTc material and the section of the lead between room temperature and 50K is carried by a more standard vapor cooled resistive lead.
An example of the maturity of the HiTc technology may be found in the Large Hadron Collider at CERN. In the LHC, more than 1000 of the 1300 leads used incorporate HiTc superconductors into their design. These range from 600 A to 13 kA in capacity. Other projects using or planning to use such leads include the ITER and JT-60SA Tokamaks and the high field series connected hybrid magnets being developed at the National High Magnetic Field Lab in the US.
Commercial vendors of both resistive and HiTc leads may be found in the CSA Buyer’s Guide. A good fundamental description of the optimization of resistive current leads may be found in “Superconducting Magnets” by M.N. Wilson (Clarendon Press 1983). Recent papers on current leads and their applications include: “HTS Current Leads: Performance Overview in Different Operating Modes,” A. Ballarino, presented at the 2006 Applied Superconductivity Conference; “Current Lead Optimization for Cryogenic Operation at Intermediate Temperatures,” L. Bromberg et al.; “Commissioning of the Cryogenics of the LHC Long Straight Sections,” A. Perin et al.; and “Development of Cryocooled Binary Current Lead in Low Temperature Superconducting Magnet System,” Y. S. Choi et al., all published in Adv. Cryo. Engr. Vol. 55 (2010).
A different approach to current leads utilizing the Peltier effect is described in “Evaluation of Thermoelectric Properties of BiTe Alloys for the Optimization of Gas-Cooled Peltier Current Leads,” T. Fujii et al., Adv. Cryo. Engr. Vol. 55 (2010).
After publication, my colleague and fellow Cold Facts columnist Glen McIntosh pointed out that in my last column on current leads I had inadvertently left out one of the most important early papers on vapor cooled current leads. This paper was by K. Efferson (K. R. Efferson, Rev. Sci. Inst., 38:1776 (1967)). This paper was so influential that many people originally referred to vapor cooled current leads as Efferson leads. Thanks go to Glen for catching this oversight.
