Abstract: A circular accelerator comprises a target current value memory which stores a target current value of a beam current of charged particle which is extracted from an extracting device; and a frequency determination part in which a frequency change ratio is obtained by performing a feedback control based on an error signal between a detection signal of a beam current detector and a target current value which is stored in a target current value memory, and determines a subsequent frequency from the obtained frequency change ratio and a current frequency, wherein the subsequent frequency which is determined by the frequency determination part is stored in a frequency memory and a radio-frequency generator generates the subsequent radio-frequency of frequency which is determined.

Abstract: The circular accelerator comprises: a bending electromagnet that generates a bending magnetic field; a radio-frequency power source that generates a radio-frequency electric field in accordance with an orbital frequency of charged particles; a radio-frequency electromagnetic field coupling part connected to the radio-frequency power source; an acceleration electrode connected to the radio-frequency electromagnetic field coupling part; and an acceleration-electrode-opposing ground plate provided to form an acceleration gap between the plate itself and the acceleration electrode, for generating the radio-frequency electromagnetic field in an orbiting direction of the charged particles; wherein the bending electromagnet generates the bending magnetic field varying in such a way that the orbital frequency of the charged particles varies in a variation range of 0.7% to 24.7% with respect to an orbital frequency at the charged-particles' extraction portion, during a time of injection to extraction of the particles.