HTS LEADS

HTS current leads represent the first large-scale application of high temperature superconductivity. This has occurred because even modest current density HTS material can be used to provide a significant reduction in the parasitic heat conducted into a cryogenic environment via the electrical leads used to provide current to the device. Fig. 4.16 illustrates a typical application.

Fig. 4.16. A conduction-cooled HTS magnet system used for magnetic separation, illustrating the use of HTS current leads to reduce heat load (LANL).

There are three classes of applications where HTS leads are seeing rapid introduction:

1. high current LTS magnets cooled by liquid helium
2. conduction-cooled LTS magnets
3. conduction-cooled HTS magnets

High Current LTS Magnets

The application where HTS leads bring the largest financial benefit is large, high current LTS magnets cooled by liquid helium. Examples include accelerator and detector magnets for high energy physics, SMES, and rotating machinery such as the Super-GM generator. These systems all require the transmission of large amounts of current between room temperature and approximately 4 K. In addition, the cryogenic systems on these devices have often been optimized to the point where the leads are the dominant heat load, hence the dominant source of operational cost, of the entire device. If HTS current leads are introduced into the system, the heat leak from room temperature can be intercepted at a much higher temperature (usually 60-77 K) rather than having to be removed at 4 K. Various ways of accomplishing this are illustrated in Fig. 4.17, which is taken from a paper by the CERN Large Hadron Collider (LHC) design team (Ballarino et al. 1996). Method "a" removes the heat with a 50-75 K helium gas stream used to cool the magnet shields. Method "b" uses two gas streams to lower the temperature of the warm end of the HTS lead to 50-60 K. This allows the use of a Bi-2212 lead at the cost of some system complexity. Method "c" uses a stream of helium gas to directly cool the copper portion of the lead. This results in better utilization of the enthalpy of the helium gas, again at the cost of some design complexity. The major point is that, independent of the cooling method, the total refrigeration load with the HTS leads is only a fraction of that in a purely conventional system. As will be reviewed below, these compelling economic advantages have led to the incorporation of HTS leads in, and often to HTS lead development in, almost all new programs involving large LTS devices.

Fig. 4.17. Suggested methods for cooling the 12.5 kA lead assemblies on the CERN Large Hadron Collider (Ballarino et al. 1996).

Conduction-Cooled LTS Magnets

As will be discussed later in this chapter, the simultaneous development of HTS leads and of Gifford McMahon (GM) refrigerators with capacities over 1 watt at 4.5 K has made possible the introduction of an entirely new class of superconducting magnet, the cryogen-free conduction-cooled magnet. Without the reduction in heat load offered by HTS leads, the realization of these systems would be much more difficult and, in fact, impossible in many cases.

Conduction-Cooled HTS Magnets

The "next turn of the crank" in superconducting magnet technology is the conduction-cooled HTS magnet, discussed in more detail later. In this case, the magnet would be possible even without HTS leads, but the economics of operation is greatly enhanced via the use of the leads, and every HTS magnet shipped to date has used them in some part of the system.

HTS Lead Technologies

There are two basic technologies for HTS leads: bulk rods of ceramic superconductor, and metal matrix superconducting composites. Both have developed to the point that they are offered for commercial sale. There are advantages and disadvantages to each.

Bulk ceramic leads (Fig. 4.18) are made by a variety of methods and of a number of different HTS materials, but the primary objectives are the same: to achieve a rugged ceramic structure with high critical current and low-resistance connections. The advantage of this approach is that the ceramics have intrinsically low thermal conductivity, so that leads may be made quite short for easier integration in the system. The disadvantages are that the ceramic rods are susceptible to breakage during installation, during operation, and (like the bulk structures used in fault-current limiters) during temperature excursions caused by driving the lead normal. In addition, it has proven difficult to provide very low resistance connections between the ceramic superconductor and the metallic connections at the ends of the leads. These disadvantages have been mitigated by careful system design in a number of magnets, and successful systems have been built using bulk leads.

Metal matrix composite leads (Fig. 4.19) essentially use the powder-in-tube technology used for BSCCO wire to manufacture a wire or tape incorporating a low thermal conductivity metal or alloy in place of the customary silver matrix. This approach has employed only Bi-2223 superconductor. The advantages of metallic leads are intrinsic ruggedness, high tolerance to thermal excursions, and very low contact resistances. These advantages are to be balanced against the disadvantage of the somewhat higher thermal conductivity of the composite material, which requires that a longer lead assembly be used to achieve heat leaks comparable to bulk leads.

Fig. 4.18. Bulk HTS leads manufactured by Furukawa Electric.

Fig. 4.19. Metal matrix HTS leads manufactured by American Superconductor Corp.

Japanese and German Lead Programs

HTS lead efforts in Japan are usually part of larger efforts directed toward specific applications; hence, it is difficult to determine budgets. It appears, however, that almost all of the parties participating in HTS materials work have developed current leads. In Germany, Hoechst has introduced a commercial line of bulk ceramic leads. Table 4.1 lists participants, materials used, types of lead technology, and applications.

Table 4.1
Manufacturers, Types, and Applications of HTS Leads in Japan and Germany

* Hasegawa 1996

Summary

HTS leads are clearly the first real large-scale HTS products. They have been produced by a number of companies worldwide and are available "off the shelf" in both bulk (Hoechst) and metallic (American Superconductor) form for currents up to 1,000 A. Future developments will include the reduction of heat leak as current density in the superconductor increases and the evolution of specialized leads for unique applications such as SMES and the high energy physics magnets.