Effects of illumination on thermionic energy converter characteristics

The thermionic electron current density J(e) from hot tungsten in a cesium atmosphere has a maximum at emitter temperatures T-E lower than 1400 K and a cesium gas temperature TCs of around 500 K. Solar light can excite or ionize cesium atoms because of the atom's low excitation and ionization potentials. These phenomena can be applied to the development of a novel thermionic energy converter (TEC) using exclusively solar light or operated at a low T-E together with other heat sources. In the present work, the effect of illumination of a xenon lamp with a broad spectrum similar to solar light is examined from the viewpoint of the improvement of low T-E TEC. It is found that illumination can improve TEC characteristics under the conditions that 1) space charge neutrality alpha < 10(-2), 2) thermionic electron current density J(e) > 10(-3) A/cm(2) and 3) the emitter work function phi(E) > 2.2 eV. These conditions ensure negative space potential formation in TEC. Electrons produced in the potential well by illumination can be accelerated at the electron sheath near the collector and ionize cesium atoms. As a result, illumination can accelerate the transition to the ignited mode and/or increase the TEC output current.