Institute of Space and Astronautical Science has developed a cathodeless microwave discharge neutralizer for the microwave ion engine system. In order to clarify the electron emission mechanism of the microwave neutralizer, electron current characteristics as well as plasma parameters inside the neuralizer were measured. The electron current was greatly influenced by the material of the neutralizer orifice. Among several materials with different work functions, an orifice made of tungsten impregnated with barium-oxygen showed the best performance. It is concluded that by adopting a low-work-function material, the secondary-electron emission by the singly-charged-ion impact was emphasized, and thereby the electron current was increased.
