Quenches in a high-temperature superconducting tape and pancake coil

Stability and normal-zone propagation experiments have been performed on single-length and double-pancake coils made of Bi-2223/Ag high-temperature superconducting tape. Experiments were conducted with liquid nitrogen and gaseous helium cooling in temperatures from 5 to 77 K. No distinctive normal-zone propagation was observed in the single-length conductor tests. Non-uniform critical currents were, however, observed over the length of the conductor. To dramatize the effect on the stability of a magnet wound with this kind of non-uniform conductor, a pancake coil was wound with conductors composed of a two-tape bundle on the inner turns and a single-tape on the outer turns. Quenches of the coil were observed when currents higher than the critical current of the single-tape section were applied and held for more than a few minutes. The coil quenched without a distinctive 'normal' front propagation or a large temperature gradient.