  A trapped field of over 3 T has been measured at 17.5 K in a magnetised stack
of two disc-shaped bulk MgB2 superconductors of diameter 25 mm and thickness
5.4 mm. The bulk MgB2 samples were fabricated by uniaxial hot pressing, which
is a readily scalable, industrial technique, to 91% of their maximum
theoretical density. The macroscopic critical current density derived from the
trapped field data using the Biot-Savart law is consistent with the measured
local critical current density. From this we conclude that critical current
density, and therefore trapped field performance, is limited by the flux
pinning available in MgB2, rather than by lack of connectivity. This suggests
strongly that both increasing sample size and enhancing pinning through doping
will allow further increases in trapped field performance of bulk MgB2.
