Performance tests of Bi-2212 pancake coils fabricated by a lamination method

Bi-2212 pancake coils made by a lamination method were tested at various temperatures and magnetic fields. A small coil with 45 mm diameter was set at the center of a superconducting magnet system and tested at 4.2 K in various magnetic fields. E-J curves of the coils were well fitted with E = kJ(n) at all magnetic fields. n value of the E-J curve decreased with magnetic field, but was almost constant in fields above 10 T. Typical n-values were 17 at 0 T and 11 at 18 T. Estimation from the n-values indicates that a persistent current mode operation required for a NMR magnet is possible at high fields if the operating current density is kept less than about 30% of the critical current density J(c). A coil with 128 mm diameter was tested at temperatures up to 70 K with GM cryocooler. in order to investigate the stability of the coil, voltage of the coil was measured as a function of time at a fixed operating current. Below similar to 30 K, a maximum operating current density for which the coil was stably operated was almost equal to J,defined with 10(-13) Omega m. At 20 K J(c) was 4.6 x 10(4) A/cm(2) which was about 70% of that at 4.2 K, suggesting that the Bi-2212/Ag tape was promising as a conductor of cryogen free magnet. (C) 1998 Elsevier Science Ltd. All rights reserved.