John M. Houston

Theoretical Efficiency of the Thermionic Energy Converter

An analysis of the efficiency of the thermionic energy converter is given in terms of Vc and Va, which represent the potential differences between the top of the potential barrier between the electrodes and the Fermi levels of cathode and anode, respectively. The analysis applies to all vacuum converters, and to gas‐filled converters when the gas pressure is low enough so that the electron mean free path is greater than the cathode‐anode spacing. In special cases, the analysis may also apply at higher gas pressures. It is shown (with certain assumptions) that optimum values of Va and Vc exist. For Va, the optimum is determined by the onset of thermionic emission from the anode to the cathode. For Vc, the optimum is determined principally by radiation losses. It is concluded that no fundamental reason exists why efficiencies of 30% or more cannot be realized in converting heat to electricity by this method.

Thermionic Energy Conversion

Publisher Summary This chapter focuses on the thermionic energy conversion with description of idealized model of a thermionic converter and work function of various surfaces. A thermionic converter is a device which converts heat energy into electrical energy by utilizing the thermionic emission of electrons. The close-spaced high-vacuum converter appears to have neared the limit of its development at a power output of 1 watt/cm 2 and an efficiency of approximately 5%. Three electrode gas-filled devices should allow relatively low temperature operation, for example, in the range 1300–1700 K. Although the rare gases have a lower electron-neutral cross-section (that is, lower resistance), a Cs filling has the advantage that a low anode ϕ is maintained and that lower auxiliary-discharge voltages are required. Cesium-filled diodes are probably the most promising type of thermionic converter. Two promising approaches are now being explored for improving cesiated-cathode converters. The first of these is the use of electrode surfaces with the optimum crystallographic orientation. The second is the addition of gases such as halogens, oxygen, or hydrogen which increases the cathode emission and decrease the anode work function.

X-Ray Image Intensifiers

Publisher Summary This chapter describes that an X-ray image intensifier (XRII) is a device that converts an incident X-ray pattern to yield a visible-light image of brightness, substantially higher than a simple phosphor screen. In most XRIIs, the final image is considerably smaller than the incident X-ray pattern that facilitates the coupling of optical lenses to transfer the image to the final image receptor such as video pickup, eye, cine film, and many more. The brightness gain is achieved in two ways: first, by increasing the number of light quanta generated from a given region of the X-ray pattern and second, by reducing the size of the final image so that the quanta is emitted from a smaller area. The chapter also discusses that in almost all XRIIs, the X-ray pattern is converted to a light pattern using a phosphor and the brightness of the light pattern is then intensified. Many systems employ electron-optical processes to provide the quantum gain, even though this requires additional stages of quantum conversion; the light from the input phosphor is converted to an electron image by a photoemitter that forms the cathode of an electron-optical system. After gaining energy by passing through an electrostatic field, the electrons are converted back into a light image in another phosphor.

The Plasma Physics of Thermionic Converters

The plasma physics of thermionic converters is discussed in an attempt to develop a coherent physical picture of this aspect of the converter in all regions of operation. Emphasis is placed on the presentation of simple physical models consistent with experimental data: The usefulness of certain characteristic features of the I–V curve, in determining the physical processes governing converter operation, is pointed out. Nine regions of converter operation are identified by these features of the I–V curve and are discussed. The importance of two parameters of converter operation, the pressure—spacing product pd, and the relative amount of emitter‐surface ion generation β, in controlling converter behavior is emphasized. The particular values of pd and β control which of the nine regions of converter operation appear on a particular I–V curve.