Which inert gases can be used in an auxiliary discharge thermionic converter?

In an auxiliary discharge thermionic converter, inert gases such as neon, argon, or xenon are used between the electrodes.1

These devices utilize a third electrode to which voltage is applied to produce positive ions, which helps neutralize the space-charge effect that otherwise limits current density. This configuration allows the converter to operate at relatively lower temperatures, enabling the use of conventional fossil fuels as a heat source.1