The control grid structure for a convergent flow electron gun includes a corrective electrostatic lens associated with each of the beam passageways through the grid. The corrective electrostatic lenses increase in focusing strength taken from the outside of the beam toward the center of the beam to compensate for beamlet defocusing forces occasioned by the relatively large central aperture in the anode of the gun. In a preferred embodiment, the corrective electrostatic focusing lens structure comprises a second control grid portion interposed between a first control grid portion and the anode. The second control grid portion is thermally shielded from the cathode via the first grid portion for inhibiting unwanted thermionic emission from the composite control grid structure.