An experiment has been performed in order to determine the dominant ionic species in a cesium plasma in the Torr pressure range. The device consists of a planar thermionic converter with a small (25 &mgr; diam) hole in the collector. Ions that pass through the hole are accelerated by an ion gun and analyzed in a magnetic mass spectrometer. It is found that atomic cesium ions are the dominant species in both the ignited and unignited modes of operation of the discharge. The molecular cesium ions Cs2+and Cs3+were also detected in the ignited mode. In addition, measurements were made of the total ion and electron currents that pass through the collector hole. Here the hole acts as a probe of the plasma adjacent to the collector.