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The authors discuss means to verify the analysis and the limitations imposed by the low critical current densities in presently available high-temperature superconductors. Based on experimental data, it is concluded that : (1) high-temperature superconductors can be used as permanent magnets with trapped fields of at least 3 T at 77 K; (2) their energy product should be as high as 1.8 MJ/m/sup 3/ or 255 MG-Oe; (3) the magnetizing field has to be four times as large as the trapped field, or 12 T; (4) permanent-magnet use of high-temperature superconductors bypasses the problems of wire or tape manufacture; and (5) widespread use of this concept will depend on improved critical current densities. >

We have developed and demonstrated a novel method of commutating direct current out of a standard vacuum interrupter. The technique uses a high-current, gate-turn-off thyristor in series with the interrupter to extinguish the arc and allow the interrupter to clear. The commutation method has been used to design and fabricate a continuous duty switch that will switch off 1000 A in 100µs and develop 25 kV at a repetition rate of 5 Hz. We have completed preliminary tests and present the important design relationships and test data in this paper.

Exhibiting essentially zero resistivity at high current densities and large magnetic fields, modern superconducting materials offer intriguing possibilities in power engineering. However, in order to utilize this property in any large scale application, the superconductor must be stable during fault and overload transients in the total system. In order to know whether the superconductor itself is stable, one must understand and control the detailed mechanisms of magnetic flux motion which occur in the material. Such flux motion will give rise to a non-zero resistivity and hence will always involve heat generation, which in turn will perturb the low temperature environment necessary to sustain superconductivity. This paper discusses the essential differences between superconductors and ordinary conductors; the control of heat generation resulting from magnetic flux motion necessary to insure that the superconductor is stable; and some experiments on models of dc superconducting transmission cables which test the theoretical assumptions.

After a brief review of the reasons for and forms of secondary energy storage and of the elements and history of inductive or magnetic storage, we discuss the four distinct areas in which superconducting magnetic energy storage can be applied. Differences in energy transfer times place different requirements on the storage coil, on the switch or transfer element, and on the energy losses in the superconductor. We report on designs and experiments in one of these areas with 2 to 300 kJ units, and on the analysis and plans for an installation that is to provide 250 MJ of plasma compression energy for the theta-pinch controlled thermonuclear fusion test reactor. We point out those elements of inductive storage that need further development before a theta-pinch fusion reactor can become economically competitive. Finally, we compare the size and costs of the energy storage components of these systems with similar and with larger inductive storage systems that are to interact reversibly with electric utility networks.

The stability of the high-temperature superconducting oxides has been analyzed, using the methodology developed for conventional type II superconductors. The results are presented in graphical form for the temperature range from 4 to 100 K. For a 90-K superconductor the first flux jump field peaks above 7 T at 60 K, and for a 120-K superconductor it peaks above 12 T at 75 K. The maximum adiabatically stable thickness increases greatly. The linear dimension of the minimum propagating zone increases by a factor of 3 to 5, and the quench propagation velocity drops by four orders of magnitude. It is concluded that the high-temperature superconducting materials will have much higher stability than conventional type II superconductors and that their high flux jump fields will make ultrafine multifilamentary conductors unnecessary and improve the outlook for tape conductors. The energy required to create a propagating zone is increased. However, methods of coil protection will have to be modified. >

The MAPPS (MegAmpere Pulsed Power Supply) demonstration tests are described, and the results are reviewed. The observed limits to the switch performance and methods to improve performance are discussed. Testing of the 100 kA demonstrator was completed in July 1986. The measured transformer coupling during operation was 0.99. The switch successfully interrupted 460 A and developed 7.5 kV at five commutations per second. Leakage current through the gate-turn-off thyristor prevented the switch from reaching the 1000 A design goal. The switch performed well up to 460 A, commutating current 290 times during testing. Results indicate that energy storage transformers provide a method to store energy at low currents so that high-purity aluminium or superconducting conductors can be used. >

The temperature dependence of the critical fields of V3Ga has been measured up to 70 kOe using ac and dc magnetization techniques. For a material with a transition width of 26 mdeg at a transition temperature, Tc, of 14.19°K, one calculates from the slope (dHc2/dT=−43.2 kOe/deg) and curvature of the Hc2 vs T curve, the Ginzburg—Landau parameter, κ=40±2, the Pauli paramagnetism parameter α=2.28±0.04, and the spin—orbit scattering constant λso=0.6±0.1. No evidence for Hc3 was detected and the experimentally difficult upper limit to Hc1 appears to be about three times as large as calculated from the theories.

A number of NbTi superconducting wires for use in a quadrupole magnet have been evaluated in a mock-up configuration. These tests have included variations in insulation, helium cooling channels, winding techniques, copper to superconductor ratio, number of superconducting filaments, short sample critical current, wire size, and wire twisting. The most significant parameters for obtaining the design current at near 30 kG central field was found to be the short sample critical current and twisting of the conductor. One conductor performed reasonably well without having been twisted.

A superconducting quadrupole doublet for beam focusing has been operated with a 30‐kG field at the center of the 30‐cm‐long straight section of the magnets and a 3‐kG/cm field gradient. The magnets used twisted multifilament Nb–Ti superconductor imbedded in a Cu matrix of 0.050‐in. diameter and operate at currents up to 500 A. Each magnet is equipped with a persistent mode superconducting switch. The room‐temperature beam aperture of the magnet‐Dewar system is 15 cm. The focal length of the magnet pair is about 1 m for 500‐MeV pions. The magnets are mounted in a Dewar with thermally compensated supports to minimize motion upon cooling and a soft iron shield reduces external fields to less than 60 G.

Self field critical current densities of thick Nb 3 Ge films have been measured in the liquid hydrogen temperature range 13.8 - 21 K. Bulk critical current densities J c above 1.6 × 106A/cm2were observed at 14 K falling linearly at higher temperatures. The behavior extrapolated to zero critical current gave a critical temperature T c ∼ 19.2 K which is ∼1.5 K lower than the onset T c determined from the a.c. magnetic susceptibility. The samples were long sections of copper tubes, which had been coated on the inside with Nb 3 Ge by a chemical vapor deposition process. J c for various high T c materials are compared including calculations of adiabatic stability. Nb 3 Ge holds promise for a dc superconducting power transmission line operating near 14 K.

The design and operation of a simple self-switching and self-rectifying flux pump is discussed. The concept has been tested using a low temperature transformer and a 10 kOe, 30 A, Nb-Zr coil paralleled by a 45 A switching coil made from Pb-Sn solder.