The bubbling density, i.e. the number of bubbles formed per unit length, in over 1 m long Ag-sheathed Bi2Sr2CaCu2O8+x (Bi-2212) superconducting tape was quantitatively investigated. The bubbling seemed to be associated with the sausaging phenomenon which was observed during deformation of the tape, which mostly occurred in two different temperature ranges of the heat treatment. The diffusion mechanism for gas formation inside of the tape in each temperature range is discussed. A newly developed heat treatment process which is performed in an evacuated atmosphere while controlling the heating rate completely prevented bubbling. It was also confirmed that a critical current density of 1.2×105 A/cm2 at 4.2 K in 0 T can be realized in a double pancake coil fabricated by the present improved process.
