Special conditions in a thermionic energy converter can be found, featuring unidirectional conduction in the backwards direction and very low work function and resisitivity. Such results have been reported on earlier (J. Appl. Phys. 70 (1992) 1489, Surface Sci. (1992)). The large back current and very low work function of the collector are attributed to the existence of Rydberg matter of cesium as a surface layer on the collector. Rydberg matter is an electronically excited form of matter, which has been predicted theoretically and recently reported on in several different experimental situations. We now proceed to study the plasma potential with a Langmuir probe and the collector temperature distribution in the open thermionic converter, with and without Rydberg matter present. The temperature results are that there is a very low temperature difference between the edge and the center of the collector in the Rydberg mode. This agrees with the low work function and good electron conduction in the Rydberg matter. The plasma probing experiments show that the potential gradient in the case of the Rydberg plasma is smaller than in the case of the normal plasma, which agrees with the low resistivity in the Rydberg matter.