  The dynamics of an argon plasma in the gap of a thermionic diode is
investigated using particle-in-cell (PIC) simulations. The time-averaged diode
current, as a function of the relative electrical potential between the
electrodes, is studied while the plasma density depletes due to recombination
on the electrode surfaces. Simulations were performed in both 1D and 2D and
significant differences were observed in the plasma decay between the two
cases. Specifically, in 2D it was found that the electrostatic potential
gradually changes as the plasma decays, while in 1D fluctuations in the plasma
led to large potential fluctuations which changed the plasma decay
characteristics relative to the 2D case. This creates significant differences
in the time-averaged diode current. Furthermore, it was found that the maximum
time-averaged current is collected when the diode voltage is set to the
flat-band condition, 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.
