  The magnetic field $B$ dependence of the critical current $I_c$ for the
vortex phase of a disordered superconductor is studied numerically at zero
temperature. The $I_{c}(B)$ increases rapidly near the upper critical field
$B_{c2}$ similar to the peak effect (PE) phenomenon observed in many
superconductors. The real space configuration across the PE changes
continuously from a partially ordered domain (polycrystalline) state into an
amorphous state. The topological defect density $n_{d}(B)\sim e^{\alpha B^{k}}$
with $k>1$ for $B\geq 0.4B_{c2}$. There is no evidence of a phase transition in
the vicinity of the PE suggesting that an order-disorder transition is not
essential for the occurrence of the PE phenomenon. An alternative view is
presented wherein the vortex system with high dislocation density undergoes
jamming at the onset of the PE.
