Abstract: A radio frequency wave ion generating apparatus which provides a thin disc shaped plasma adjacent an ion attracting means is described. The radio frequency waves are preferably microwaves. Several improvements to microwave plasma generation are described including the steps of generation of a resonantly sustained microwave discharge inside a microwave/plasma coupler or cavity (11) to ignite the plasma; probe (14) and length tuning of the microwave/plasma coupler by means of a sliding short (12) during operation of the plasma and minimization of the plasma volume inside the coupler in a chamber (15) to produce the disc plasma. The apparatus is particularly useful for internal ion or free radical irradiation of various materials provided in the plasma or for ion acceleration outside the cavity for use as an ion engine or as an ion source for irradiation of the materials.