Simulations of Argon Plasma Decay in a Thermionic Converter

Thermionic generators convert heat to electricity with no moving parts. Thermionics comprise an electron emitting cathode and an electron collecting anode separated by a vacuum gap. Typically, a plasma is used to mitigate space charge. Here, we report on a comprehensive modeling effort of the dynamics of an argon plasma in the gap of a thermionic diode. We applied particle-in-cell simulations to characterize the time averaged diode current, as a function of the relative electrical potential between the electrodes, while the plasma density depletes due to recombination on the electrode surfaces. Simulations were performed in 1D and 2D and significant differences were observed in the plasma decay between the two cases. Specifically, in 2D, well defined plasma sheaths formed in front of both electrodes, while in 1D the sheath heights varied continuously. This creates significant differences in the time averaged diode current. In 2D simulations, the maximum time averaged current is collected when the diode voltage is set to the so-called flat-band condition, i.e. where the cathode and anode vacuum biases are equal. This suggests a novel technique of measuring the difference in work-functions between the cathode and anode in a thermionic converter.