Sulfur Hexafluoride as a Fuel for Ionic Propulsion

Experiments on the generation of an intense beam of SF6(-) negative ions by an electron bombardment method are described. Theoretical considerations are given on the basic process s for the generation of negative ions, the characteristics of an electron bombardment ion source and the relevant properties of the sulfur hexafluoride molecule. The feasibility of introducing the gas in the ion source in the form of a well defined stream is demonstrated to ensure a high ion conversion efficiency for this ion source. Time-of-flight-mass spectrometer measurements indicated that only SF5(-) and SF6(-) ions were produced in that source and that the SF5(-) contribution to the total ion current was negligible. The negative ion current as function of the electron bombarding energy exhibits a complex resonance spectrum. The limitation to the maximum negative ion current produced by this method is set by space charge effects in the bombarding electron beam.