The present invention provides apparatus for acceleration of a charged particle beam which includes energy means for altering the energy of a circulating beam circulating in the apparatus and means for extracting output beams from the apparatus. Control means are arranged to be operable to alter the energy of the circulating beam using the energy means so as to be able to extract output beams of at least two energies from the apparatus using the means for extracting. Thus there is provided an accelerator, and an extraction method therefor, which is capable of extracting a small diameter beam having a varying energy level. This facilitates improved medical irradiation treatment using the beam.