Exercise field created by a cone portion

Field created by a portion of a cone.

Consider a portion of a cone of half-angle at the apex a and R1 limit rays and R2 (R1 <R2).
This system is loaded on the surface with non-uniform density:

a is a constant homogeneous to a length and r the radius of the cone at a point on its axis of symmetry.
Determine the electrostatic field at the top of the cone O.