Sources of positive ions for space charge neutralization at low temperatures

Abstract

In order to operate a thermionic power converter from a heat source which utilizes fossil fuels burned in air, it is desired to have a converter which operates at a temperature of 1200°C or lower. At these low temperatures, it is difficult to obtain an adequate source of positive ions for space charge neutralization. Surface ionization of cesium, which is the most common source of positive ions in a thermionic converter, is not effective at these low temperatures. Two methods for providing positive ions have been investigated experimentally: 1. (1) The thermionic emission of positive ions from solid compounds or mixtures of compounds and 2. (2) The application of repeated short pulses of electric power to the converter to ionize the cesium vapor in the interelectrode space. Thermionic emission of positive ions has been studied by many investigators. One of the best ion emitters reported in the literature is beta eucryptite (Li 2O, Al 2O 3, 2SiO 2) which will emit significant current densities of lithium ions in the region from 1000°C to 1200°C. We have studied ion emission from the eucryptite analog of other alkali metals and from mixtures containing alkali metal compounds. Ion emission current densities of several milliamperes per cm 2 have been obtained, but the lifetime of the emitter has often been less than anticipated. Space charge neutralization electrons has been demonstrated in a preliminary experiment using a three element structure with the ion emitter located between the electron emitter and collector. Pulsed operation of a thermionic diode has been studied using a modified commercial diode to which cesium vapor was introduced at controlled pressures. During the application of a 10 to 20 V pulse, a discharge occurs in the cesium and ions are formed. After the pulse is terminated, the ions formed neutralize the space charge and allow a high electron current to flow. The experimental tube has a nickel matrix emitter and a cesium coated copper collector so that the relative work functions of the emitter and collector do not allow useful power output densities to be obtained. However, an external bias voltage was used to adjust the interelectrode potential difference to simulate a thermionic converter. The diode current after the pulse was terminated was typically about 300 mA compared to a steady state current before the pulse of about 0·3 mA. Preliminary results indicate a charge transfer between emitter and collector between pulses which is 30 times the charge transfer during the pulse. Further work to optimize this ratio and to provide the pulse feedback of a portion of the diode output power is in progress.