Theoretical analysis of ion-enhanced thermionic emission for low-temperature, non-equilibrium gas discharges

When thermionic emission is used in a gas discharge, the ions exiting the discharge will influence the local electric field at the cathode. This effect is similar to the Schottky effect, as the potential field due to the ion can effectively reduce the potential barrier ( work function) at the cathode and increase emission. In this work, this enhancement phenomenon is treated theoretically to understand how Schottky emission is enhanced due to the presence of an ion-so-called ion-enhanced Schottky emission. The effect of a stationary ion is determined through the analytical and numerical solution of the potential energy fields involved in the problem, including the applied field, electron image charge, and the ion potential field. The time-averaged enhancement due to an ion drifting towards the cathode is calculated, and an effective emission coefficient is predicted. These analyses show that a single stationary ion can significantly enhance thermionic emission, more than doubling the emission current depending on its location, but the enhancement is less significant (similar to 5-10%) when the motion of the ion is considered. However, the enhancement effect is more pronounced at moderate electric fields (similar to 10(5) V m(-1)) due to the residence time of the ion near the cathode. These results reveal the impact ions in the discharge may have on emission and how this needs to be considered when designing devices that integrate thermionic emission and discharges.