Superconductivity Physics, Electrical Cables, and Industry James L. Smith

t does not surprise us that we can material because the spectrum of conducting at about 4 kelvins, many see through glass. After all, we possible energy levels of the mobile superconducting materials have come Isay, glass is transparent to the electrons has a gap at an energy such to light for which those limitations light that travels to our eyes from that the collisions cannot occur. A are reduced. For example, in 1987 a objects on its far side. But what superconductor is “transparent” to material was discovered that becomes does it mean to say that glass is the flow of electricity. superconducting at a temperature “transparent” to photons of visible Since collisions between the mo- above the boiling point of liquid ni- light? It means that the photons in- bile electrons and the fixed electrons trogen (77 kelvins, or −320°F). The teract very little with the electrons in within a conductor are the source of discovery of “high-temperature” su- glass. And why do they interact so the conductor’s electrical resistance, perconductivity was a momentous little? Because the spectrum of pos- is certainly a prop- event because is very sible energy levels of the electrons erty to be desired of wires used to much cheaper than the helium re- has a gap extending over the range carry electricity. But superconduc- quired to cool previously known su- of energies possessed by photons of tivity has its drawbacks: The phe- perconductors to their transition visible light. Similarly, the electrons nomenon occurs only at low tempera- temperatures. of a superconducting material that tures, in low magnetic fields, and By July of 1988 Congress was in- move through the material under the when currents are low. However, fused with high-temperature-super- influence of an applied voltage do during the eighty-two years since conductivity fever—a feeling that not undergo collisions within the mercury was found to become super- the technology of high-temperature

1993 Number 21 Los Alamos Science 215 Superconductivity: Physics, Electrical Cables, and Industry

superconductors, unlike that of semi- the same effect as electrical resis- atures below their transitions tem- conductors, must not be lost to for- tance and thus defeats the purpose peratures might prove advantageous eigners, that the United States had of using a superconductor. Progress for the Superconducting Super Col- another chance to do it right, that has been made in combating flux lider, which must be cooled to very here was an opportunity for govern- creep with line-like (one-dimension- low temperatures to maintain its high ment to help industry. The efforts of al) imperfections that “pin” the flux vacuum. At the Laboratory last win- New Mexico Senator Pete Domenici quanta against movement. Acting ter, we used very high magnetic and Laboratory Director Sig Hecker on ideas from IBM, workers at Oak fields (produced by compressing led to legislation that established Su- Ridge and Brookhaven national lab- carrying a large current with perconductivity Pilot Centers at Ar- oratories have made tiny holes in su- explosives) to measure the field re- gonne, Los Alamos, and Oak Ridge perconductors with energetic, highly quired to drive YBa2Cu3O7 out of its national laboratories. Special agree- ionized atoms and found that such superconducting state at the lower ments between the pilot centers and holes do pin flux effectively. The temperatures. We found that the industry were allowed that gave fact that each known high-tempera- field required is very high indeed— more rights to industry, more protec- ture superconductor exhibits either 140 teslas, or 3 million times the tion to ideas. This was to be a test weak links or flux creep but rarely earth’s magnetic field. Only the case of marriage between govern- both is tantalizing, as is the lack of Russians at Arzamas-16 (one of their ment and industry. any scientific reason to rule out the nuclear-weapons laboratories) have But making wire capable of carry- possibility of a high-temperature su- carried out a similar experiment; ing practical amounts of current perconductor that exhibits neither they reported a slightly higher mag- from the high-temperature supercon- problem. It is important to note that netic field. Apparently the competi- ductors proved daunting. Nature over the last five years incremental tion between the weapons laborato- had charged a price for supercon- success at licking those difficulties ries in the United States and Russia ductivity at higher temperatures. has occurred around the world. has not stopped with the end of the Among the possible wire materials There have been no touchdowns, but Cold War. are the copper-oxide compounds progress has been steady. The excitement created by the dis- YBa2Cu3O7, Bi2Sr2Ca2Cu3O10, and The disappearance in high-tem- covery of high-temperature super- Tl2Ba2Ca2Cu3O10 and many doped perature superconductors of both conductivity has now faded, and Edi- versions of those compounds. They weak links and flux creep at temper- son’s 99-percent perspiration is all exhibit “weak links” or “flux atures well below their transition somewhere near half spent. Ameri- creep.” Weak links—the imperfect temperatures demonstrates that those can Superconductor and Intermag- bits of material between the perfect phenomena are indeed the price that netics General, industrial partners of crystalline grains that make up all must be paid for high-temperature the three national laboratories, are wires—are so called because their superconductivity. At temperatures producing strips of flexible, silver- superconductivity is more subject to in the vicinity of the transition tem- clad tape more than a hundred me- degradation by magnetic fields and peratures of the older superconduc- ters long that can carry over 10 am- currents than that of the perfect tors (say below 20 kelvins), the peres even after being wound on a grains. The solution is to find a pro- high-temperature superconductors do drum and then unwound. It is clear cessing protocol that will at least retain their lack of electrical resis- that the cost of feeding electricity to partially align the imperfect grains tance in record high magnetic fields. a major new building (one requiring, and reduce the amount of intergran- That property may prove to be very say, 20 megawatts of power) in a ular material. Some progress has useful when the cost of cooling to crowded city (Boston or New York, been made. Flux creep refers to the the lower temperatures must be paid say) with an underground cable of motion of magnetic-flux quanta— for other reasons, and we are work- such superconducting wire would be tiny whirlpools of current that exist ing to optimize wire for such appli- similar to the cost of feeding with an throughout a current-carrying super- cations. For example, the ability of oil-cooled cable, the type of cable conducting material. The motion high-temperature superconductors to now used despite the risk of oil occurs when the material is carrying maintain their superconductivity in leaks. In a city where electricity a practical current. Flux creep has very high magnetic fields at temper- generation is physically distant from

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Superconductivity: Physics, Electrical Cables, and Industry

new construction (such as Baltimore, tives all seem to want access to the where electricity generation is to the enthusiasm, ideas, and capabilities of south and growth is to the north), su- our scientists and seem willing to put perconducting cables with twice the up with managerial and legal obsta- current-carrying capacity could re- cles to gain that access. They do un- place existing underground cables derstand that high-risk development and thus eliminate the need for over- can be the charge of the Laboratory, head lines around the edge of the that the design engineering of a com- city or for miles of new excavation petitive product comes later, and that through the city. Furthermore, con- the Laboratory can therefore make struction of new high-voltage over- most of its findings available to all. head lines may be halted by concerns The experiment of government of the American public about their helping industry is continuing, and aesthetics and safety, and under- apparently government has the most ground superconducting cables now to learn, just as we expected. offer a solution at the right time. Steadily albeit slowly, the problems In the future, when weak links of high-temperature superconductiv- and flux creep are licked and materi- ity are being solved. als are available that maintain their high-temperature superconductivity in higher magnetic fields, electric motors can be lighter and more effi- cient and electric energy can be ac- cumulated at night as a magnetic field in large coils and used in the day when demand is greater. Here much work and the risk of failure re- main. Even the effects of mechani- cal strain and the details of propaga- tion of thermal transients through coils made of high-temperature su- perconductors are subjects that re- main largely untouched by re- searchers. The modeling capabili- ties of the national laboratories and their abilities to field large-scale tests can help industry, and we have James L. Smith received his B.S. in physics been talking to companies about from Wayne State University and his Ph.D. in physics from Brown University. He is currently those areas of concern. Chief Scientist of the Laboratory’s Superconduc- There remain “cultural” differ- tivity Technology Center. During his twenty ences between national laboratories years at the Laboratory, he has received numer- ous awards, including two Distinguished Perfor- and corporate America. We tend to mance Awards from the Laboratory, the Interna- want to understand down to the last tional Prize for New Materials from the Ameri- detail; they need understand only can Physical Society, and an E. O. Lawrence Award from the DOE. He is a Laboratory Fel- enough to get a product on the mar- low, a Fellow of the American Physical Society, ket. And researchers at the national and the North American Editor of the venerable laboratories are not accustomed to Philosophical Magazine. His continuing re- search interest is the study of superconducting paying attention to proprietary infor- materials from high to extremely low tempera- mation. Yet company representa- tures and in very high magnetic fields.

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