A plasma source apparatus for generating a beam of charged particles is disclosed. The apparatus comprises: a plasma chamber provided with an inlet for the ingress of gas and an aperture for the extraction of charged particles from the plasma chamber; a radio frequency (RF) plasma generation unit for generating a plasma inside the plasma chamber, the radio frequency plasma generation unit comprising first and second resonant circuits each tuned to resonate at substantially the same resonant frequency, the first resonant circuit comprising a first antenna and a first, RF power source adapted to drive the first resonant circuit at substantially its resonant frequency, and the second resonant circuit comprising a second antenna, whereby in use an RF signal is induced in the second antenna by the first resonant circuit due to resonant coupling, the second resonant circuit being configured to apply the induced RF signal to the plasma chamber to generate a plasma therein; and a particle accelerating unit for extracting charged particles from the plasma and accelerating the charged particles to form a beam, the particle accelerating unit comprising a second power source configured to apply potential between the plasma chamber and an accelerating electrode, the region between the plasma chamber and the accelerating electrode constituting an acceleration column. The second power source is adapted to output a high voltage relative to that output by the first, RF power source.