Thermionic ignited mode

Figure 4. Characteristics of an ignited mode thermionic converter. Ignited mode motive diagram shown at top. The actual J-V curve is displaced from the ideal curve (bottom) by an amount equal to the voltage losses in the converter.

The motive diagram for the potential energy of electrons in an ignited mode thermionic converter has a more complicated shape, as shown at the top of Figure 4. The presence of positive ions in the plasma creates a minimum in the electron motive inside the interelectrode gap. There are narrow collisionless sheaths (the order of a Debye length in thickness) at both the emitter and collector edges of the plasma. 

Figure 5. Fluid mechanical analog for ignited mode thermionic converter.

Calculated Power and Efficiency. The simplified analytical models of thermionic characteristics have been used to project the converter efficiency and power density with the barrier index as a parameter. These projections are shown in Figures 8 and 9 as functions of the emitter temperature. The dashed lines in these two figures are for a constant current density of 10 A/cm. If the current density is adjusted to maximize the efficiency at each temperature, the calculated performance is represented by the solid lines. Typical present generation themionlc converters operate with Vg near 2.0. Ignited mode converters in laboratory experiments have demonstrated practical operation with 1.85 < Vg < 1.90. Other laboratory devices with auxiliary sources of ions and/or special electrode surfaces have achieved Vj < 1.5, but usually not under practical operating conditions. 

Thermionic conversion is a technology that needs, and can immediately use, research on high temperature properties of alkali metals. Electron transport properties of alkali vapors and characteristics of atomic clusters are particularly Important. Improved understanding in these areas could lead to performance improvements that would more than double the output power density and efficiency of cesium ignited mode thermionic converters. 

As for the ignited mode operation, the increased electrons and ions in the space between both electrodes contibute to induce the breakdown because the probability of collisional ionization is proportional to the electron and cesium atom density. Therefore, the larger output current will be obtained when the ignited mode operation will take place by increasing the cesium gas pressure and emitter temperature and by illuminating the intense light on the thermionic energy converter. 

FIGURE 4 Motive diagrams and J-V characteristics for (a) vacuum, (b) unignited, and (c) ignited modes ofoperating a thermionic... 

Thus far, essentially all practical thermionic converters have operated in the ignited mode. This conventional converter has demonstrated power densities of 5 to 10 W/cm and efficiencies of 10 to 15% for emitter temperatnres between 1600 and 1800 K. 

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