We have measured the current-voltage (I-V} characteristics of several high-temperaturesuperconducting materials with widely difkrent morphologies {bulk Ag/Pb-Bi-Sr-Ca-Cu-0 tapes, thin films of Y-Ba-Cu-O, and melt-textured, bulk Y-Ba-Cu-0 samples). The I-V curves were taken at several magnetic fields ranging from 0 to 8 T. The measurements were carried out at three temperatures (4.2, 27, and 77 K) where the samples were immersed in liquid cryogens to ensure good thermal equilibrium. We compared our experimental results to the predictions of dissipation in superconductors made by the following physical models: modified Ambegaokar-Halperin, flux creep, vortex glass, collective flux creep, and a power law. The fits were extremely good for the first model and were not nearly as good for the others. Using the modified Ambegaokar-Halperin model, the critical current I"the normal-state resistance R",and y, which is proportional to the pinning potential U(H, T), were obtained for each material. Since the Ambegaokar-Halperin model is the only one which uniquely defines I"we conclude that its use puts this parameter on a solid physical basis.
A 7.25 T laboratory magnet utilizing Bi 2223 conductor has been designed, built, and installed at the Naval Research Laboratory. Operating at 21K at full field, the coil provides field homogeneity of * 1% in a 2-inch warm-bore.The system i s conduction cooled with a pair of Leybold single stage cryocoolers that allow cooldown in less than 36 hours and allow extended fast ramp opert,.ion. Operation at a total refrigerator input power of 6 kW is facilitated by the use of ASC CryosaverTM H T S current leads. The fully integrated system consists of the magnet, cryogenic system, control and protection system and power supply. This paper presents information on the magnet design, construction and subsequent testing.
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