  The first step towards high critical currents in Bi-2212 wires was the
comprehension that the supercurrent is blocked over long lengths by
filament-diameter bubbles grown during the melt stage, which cause expansion of
the wire diameter and dedensification of the superconducting filaments. Whereas
the previous successful approach to reduce the problem of voids related to
bubbles was based on the application of a high overpressure during the heat
treatment, we fabricated Bi-2212 wires by applying a new concept of suitably
alternating groove-rolling and drawing techniques with the aim of densifying
the phase already during the working procedure prior to the heat treatment. We
here for the first time were able to reach in wires reacted with closed ends -
i.e. with gas trapped in the wire as it happens in long-length wires - the very
same values of critical current shown in short wires reacted with open ends.
This is the irrefutable evidence that, only by acting on the deformation
technique, we were able to raise the critical current by properly densifying
the superconducting powder inside the filaments already before the melt stage.
Whole-conductor current densities in our long length simulation wires already
reach 400 A/mm2 at 4.2 K and 5 T, which can be still easily increased through
architecture optimization. The actual breakthrough is that the densification is
optimized without further complex treatments through a technique which can be
straightforwardly applied to long-lengths wires.
