This paper presents the development status of a 20-cm diameter microwave discharge ion thruster which generates 25 similar to 30 mN of thrust with an electric power of I M By optimizing the discharge chamber length, magnetic field and propellant flow injection, ion currents of up to 530 mA at a net microwave power of 100 W had been obtained at a frequency of 4.25 GHz with a coaxial cable to circular waveguide transformer. Almost the same performance has been achieved with a new antenna directly inserted into the discharge chamber, which removes the need for a long circular waveguide. Higher frequencies up to 5.8 GHz and stronger magnets have been tested for performance improvement and turned out to be very promising. (c) 2005 Elsevier B.V. All rights reserved.