A TOFMS STUDY OF EXCITED CESIUM SPECIES FROM A THERMAL PLASMA-DEVICE

The flux of cesium species from a thermal cesium plasma is studied by time-of-flight mass spectrometry. Excited cesium species drift to an i onization region located in a separately pumped chamber. In this regio n some of the excited cesium species are field ionized by an electric field which has a field strength of 200 V cm-1 in the ionizing pulses. The ions formed are accelerated with less than 100 V into a flight pa th and are finally detected. Typical time-of-flight (TOF) mass spectra display two peaks. The first peak is due to Cs+ formed from singly ex cited cesium atoms, and the second peak is due to cesium clusters with 5-15 cesium atoms which have fragmented to atomic ions in the flight path. The emission of excited species increases with increasing emitte r saturation current, while it is generally quite low when the plasma shows a large back-current, i.e. in the mode with accumulation of Rydb erg species and Rydberg matter in the thermionic energy converter (TEC ).