Experimental Setup The experimental setup essentially consists of five subsystems. These were the vacuum system, emitter heating system, temperature measurement system, output load instrumentation system and the data acquisition/processing system. Figure 2 gives a schematic of the overall experimental setup with all the components belonging to the various systems mentioned earlier. The Vacuum System. A turbomolecular pump was provided to rough the system to approximately 10~4 torr. An ion pump was then activated to take the pressure down to at least 10-8 torr. A vacuum of 10-8 torr or better is required to minimize electron deviation and also to improve efficiency of heating by electron bombardment. The vacuum level was monitored with an ion gauge whose output was directly transmitted to the computer in the data processing system through the data acquisition system. Thus the vacuum level was directly displayed on the monitor and stored along with other test data. The Emitter Heating System. The emitter was indirectly heated by electron bombardment (EB) heating. This was accomplished using a tungsten filament which was resistance heated by an AC power supply. To heat the filament, the power was gradually increased up to approximately 100 W, at which level the temperature would be greater than 2600 K and the filament would be hot enough to emit electrons with high kinetic energies. A floating DC bombardment power supply provided the necessary energy to direct the electrons from the filament to the electron bombardment target. The energy imparted by the impinging electrons heated up the EB target, which in turn provided the necessary heat input to the emitter by conduction. The Temperature Measurement System. The cesium reservoir and collector temperatures were controlled using compressed air for cooling and resistance heaters for heating. Microprocessor type controllers were used to control the power output to the resistance heaters. These controllers were supplied with the appropriate temperature signals by thermocouples placed in the diode to measure collector, guard and cesium reservoir temperatures. The emitter temperature was indirectly measured using an
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