Abstract: A thermionic energy conversion method and apparatus is disclosed that eliminates or substantially reduces deficiencies caused by space charge or thermal radiation losses from the emitter to the collector. The method and apparatus uses an acceleration electrode having an aperture and disposed in the space between the emitter and collector. A high positive voltage is applied to the acceleration electrode to generate a positive electrostatic field. The shape of the facing surfaces of the emitter and collector and the shape of the acceleration electrode shape the electrostatic field to cause boiled-off electrons from the emitter to be accelerated and converged so that they all pass through the aperture without impinging on the acceleration electrode.