Once the remarkable performance of the C12A7:e- material using a negative pulsed DC signal for the cathode, and the charge coupling technique was described, additional tests were performed at different frequencies and duty cycles of the cathode signal.
A. Results of the anode current at 20 kHz with Argon
In the first test, argon was introduced with a gas mass flow of 5 sccm, while measuring the collected anode current as a function of the voltage anode. In this case, the voltage applied to the C12A7:e- emitter was modified from 210 to 250 V. In this case, two different pulsed DC signals were used: 20 kHz-20% duty cycle and 20 kHz-80% duty cycle.
As it can be seen in Fig. 7, a notable performance was obtained in all cases except when applying 210V where it was not possible to maintain stable the plasma discharge. For example, with a signal cathode of 250V-20kHz-80%, a collected anode current of approximately 76 mA at 50 volts was measured. On the contrary, with a signal of 250V-20kHz-20% an anode current of 45mA at 50 volts was obtained. This means that despite reducing the duty cycle by 60%, the anode current was only reduced by approximately 40%. In this way, it is verified how the use of a pulsed DC signal improves the performance of the cathode due to the capacitive behavior of the electride.
Although, the cathode current values are not represented in the following figures, the measured values were similar to the anode current values. This is due to the connection of the keeper to ground through a resistance. By this way, the keeper current is reduced, and the performance of the cathode is improved.
B. Results of the anode current as a function of the frequency and duty cycle with argon
Once it was verified how the duty cycle parameter affects the anode current, a complete study was carried out where a frequency and duty cycle sweep were performed for the cathode signal. The values represented in Fig. 8 correspond to a cathode voltage of 270V while using a voltage of 30V at the anode and with a mass flow of 5 sccm of argon. As it can be seen, the values of the anode current increase as the frequency increases, since the anode current increases as the number of voltage pulses increases. In addition, it can also be seen that as the duty cycle increases, the current also increases for all frequency values.
These data confirm that different cathode voltages with different frequencies and duty cycles can be used to obtain the same value of anode current, thus allowing a precise control of the anode current of interest.
C. Endurance test of C12A7:e- sample operating with argon.
Finally, the C12A7:e- emitter was tested for 120 hours (without any stop) in a plasma environment with Argon obtaining a great stability and performance due to the application of the charge coupling technique previously described. In this case, the test was performed with a signal cathode of 90 kHz-50% duty cycle. In Fig. 9 it is represented the evolution of the voltage cathode (Vrms), the anode and cathode current, and the sample temperature keeping a positively bias of 10 volts at the anode with a mass flow of 5 sccm of Argon. As it can be observed, the temperature increased from room temperature to approximately 125ºC. However, this temperature is measured in the metallic keeper situated near the emitter surface, and therefore, it differs from the real sample temperature that is closer to 300ºC.
During the final hours of the test, a slight decrease in the cathode and anode current was observed. After the experiment, the sample was measured and analysed detecting a minor change in the expected composition of the C12A7:e- surface. In addition, we observed some sputtering evidence in the sample surface so that the application of lower cathode voltages could be the preferred solution. For this reason, further tests are programmed to check the reliability and endurance of the electride sample for 300 hours.