Evidence for long range movement of Bi-2212 within the filament bundle on melting and its significant effect on Jc

It is well known that the critical current density Jc of multifilamentary Bi-2212 wires tends to decline as the wire length increases, but the reasons for and the magnitude of this decline remain obscure and quantitatively unpredictable. Here we report on the Jc and mass density variation with length on ∼ 1 m long samples taken from two recent and representative wires, in which we find a strong decrease of Jc with distance from the end and a strong correlation between Jc and the local mass density. The mass density variations occur on length scales of centimeters, many times the nominal 15 μm filament diameter. The cause of the mass density variation appears to be internal gas pressure that generates bubbles which almost fill the filament diameter when the Bi-2212 melts. Control of this internal pressure seems to be vital to moderating or avoiding the length dependence of Jc.