A microwave ion source is disclosed and includes a plain rectangular waveguide having a first section to which a microwave generator is coupled, a second section defining a discharge chamber and an intervening transformer section dimensioned to provide for transmission of microwaves between the first section and the second section substantially without impedance losses. The first and second sections have uniform rectangular internal cross-sectional shapes defined by a first dimension which, for both sections equals one half of the wavelength of the microwaves, and a second, smaller dimension which is less than the second section of the waveguide than in the first section.