  We investigate the self-consistent electrodynamic structure of a particle
accelerator in the Crab pulsar magnetosphere on the two-dimensional poloidal
plane, solving the Poisson equation for the electrostatic potential together
with the Boltzmann equations for electrons, positrons and gamma-rays. If the
trans-field thickness of the gap is thin, the created current density becomes
sub-Goldreich-Julian, giving the traditional outer-gap solution but with
negligible gamma-ray luminosity. As the thickness increases, the created
current increases to become super-Goldreich-Julian, giving a new gap solution
with substantially screened acceleration electric field in the inner part. In
this case, the gap extends towards the neutron star with a small-amplitude
positive acceleration field, extracting ions from the stellar surface as a
space-charge-limited flow. The acceleration field is highly unscreened in the
outer magnetosphere, resulting in a gamma-ray spectral shape which is
consistent with the observations.
