Role of internal gases and creep of Ag in controlling the critical current density of Ag-sheathed Bi2Sr2CaCu2Ox wires

High engineering critical current density JE of > 500 A/mm2 at 20 T and 4.2 K can be regularly achieved in Ag-sheathed multifilamentary Bi2Sr2CaCu2Ox (Bi-2212) round wire when the sample length is several centimeters. However, JE(20 T) in Bi-2212 wires of several meters length, as well as longer pieces wound in coils, rarely exceeds 200 A/mm2. Moreover, long-length wires often exhibit signs of Bi-2212 leakage after melt processing that are rarely found in short, open-end samples. We studied the length dependence of JE of state-of-the-art powder-in-tube (PIT) Bi-2212 wires and gases released by them during melt processing using mass spectroscopy, confirming that JE degradation with length is due to wire swelling produced by high internal gas pressures at elevated temperatures [A. Malagoli et al. Supercond. Sci. Technol. 24, 075016 (2011) and A. Malagoli et al. Supercond. Sci. Technol. 26, 055018 (2013)]. We further modeled the gas transport in Bi-2212 wires and examined the wire expansion at critical stage...