Spontaneous quenches of a high temperature superconducting pancake coil

A double-pancake coil made of Bi-2223/Ag high temperature superconducting (HTS) tape was constructed with an embedded heater and graded conductors to study the stability and quench propagation in HTS coils. The experiments were performed with liquid nitrogen and gaseous helium cooling in temperatures ranging from 5 to 77 K. The coil was very stable, and no "normal" zone was sustained or propagated with local pulsed heating. However, spontaneous quenches of the coil were experienced. This was found to be the result of having the coil current higher than that of the lower I/sub c/ sections of the coil for a long time. This quench process took minutes to develop-much longer than would be expected in a low temperature superconducting coil. The quench behaved more like a spreading and continuous heating of an increasingly larger partially resistive section of the coil than like a sequential "normal" front propagation.