Conductors from Superconductors: Conventional Low-Temperature and New High-Temperature Superconducting Conductors
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subdivision of a given cross section of the superconductor into many filaments having a maximum diameter of no more than about 50 yam, since bigger filaments store more electromagnetic energy than can safely be deposited in the filament without locally heating it above its critical temperature (Tc). One advantage of hightemperature superconducting (HTS) materials is that they can operate at temperatures above —10 K. Since the specific heat is a strongly increasing function at low temperatures, this permits the safe filament size to greatly increase too. The need to minimize hysteresis losses, however, often provides a separate drive to minimize the filament diameter, as in the conductor of Figure 1, where there are some 7,000 filaments which are only 6 yam in diameter. The overall Cu:Nb-Ti ratio is about 1.5 :1. This represents a compromise between the need to minimize the dilution of the supercurrent density by Cu and the need to provide sufficient high-conductivity normal metal to pass the current when the magnet makes the transition from the superconducting to the normal state (a quench).
A third vital property is that the conductor can be fabricated in long lengths at reasonable cost. SSC strands are regularly made in continuous lengths exceeding 10 km, with little scrap loss, and for prices in the range of twice the raw material costs. A final necessity is that the mechanical strength and toughness of the conductor be adequate to endure wire fabrication, magnet winding, and magnet energization. Typical stresses that must be supported range up to ~250 MPa. The need to simultaneously satisfy all of these requirements is the reason why virtually all present-day superconducting magnets have been made from just two materials, Nb-47-50wt%Ti and Nb3Sn. Table I summarizes the principal properties of these two materials and compares them to properties exhibited by some potential competitors. An important distinction between Nb-Ti and all other materials is that Nb-Ti is strong and very tough, being itself a ductile bcc solid-solution alloy, while all other high-field superconductors are compounds having low dislocation densities and few slip systems and, consequently, a low level of toughness. On the other hand, the compounds can have transition temperatures and upper critical fields (Hc2) which are much higher. Superconducting behavior is bounded in H-T space by the phase boundaries indicated in Figure 2. Figure 2a shows that a low-temperature superconductor (LTS) enters the mixed, partially field-penetrated state at a very small field of order 10 mT. In the mixed state, the field penetrates as supercurrent vortices having a normal core with a well-defined diameter of two coherence lengths (f). The separation of the vortices continually diminishes toward Hc2, at which point the normal cores of the vortices overlap and superconductivity is destroyed. At 4 K, Hc2 for Nb-47wt%Ti is -11 T, about 25 T for Nb3Sn, about 50 T for PbMo6S8 and reaches phenomenal values which are as yet unmeasurable in the low-temperature l
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