emitter temperature plays an important part in the bulk or volume ionization of cesium atoms. Figure 12 represents the optimized output current and power density characteristic envelopes with respect to the emitter temperature. From this figure it can be seen that the peak power density occurs at about 0.45 V. In the earlier series of collector optimization tests, the peak power density occurred at 0.39 V. The broken line indicates that portion of the power density envelope that would have existed had ignition in all cases of emitter temperature occurred above 0 V. Figure 13 indicates the variation of short circuit current density and peak power density as a function of the non-dimensional temperature T*. In this case, the non- dimensional temperature is the ratio of the maximum emitter temperature (1700 K) to the emitter temperature for a given test. The slopes of the two plots indicate that there might have been a peak at a temperature greater than 1700 K but due to material bonding limitations it was not advisable to go to higher temperatures. Thus, for all
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