ACCELERATOR AND PARTICLE THERAPY SYSTEM

1 . An accelerator that accelerates an ion beam while circulating the ion beam by a main magnetic field and an accelerating radiofrequency electric field, the accelerator comprising:

a main magnetic field magnet that has a plurality of magnetic poles arranged to face each other and excites the main magnetic field in a space interposed between the magnetic poles;

a magnetic channel that extracts the ion beam from an inside of the main magnetic field magnet toward an outside of the main magnetic field magnet;

a displacement unit that displaces the ion beam circulating in a main magnetic field region where the main magnetic field is excited to an outer side of the main magnetic field region; and

a disturbance magnetic field region that is provided in an outer peripheral portion of the main magnetic field region and excites a magnetic field which disturbs the ion beam displaced to the outer side and guides the ion beam to the magnetic channel, wherein the magnetic channel includes a predetermined mechanism that suppresses a magnetic field gradient generated radially inward in the circulating ion beam region.

2 . The accelerator according to claim 1 , wherein

the magnetic channel includes a septum located on a radially inner side of the main magnetic field magnet and an anti-septum located on a radially outer side with a beam extraction passage interposed between the anti-septum and the septum, and

the predetermined mechanism is achieved by setting a radial width dimension of the septum to be smaller than a radial width dimension of the anti-septum, and setting a radial width dimension of a predetermined region where a median plane, orthogonal to an axial middle of the main magnetic field magnet, intersects with the septum to minimum.

3 . The accelerator according to claim 1 , wherein

the magnetic channel includes a septum located on a radially inner side of the main magnetic field magnet and an anti-septum located on a radially outer side with a beam extraction passage interposed between the anti-septum and the septum, and

the predetermined mechanism is achieved by forming a recessed part in a predetermined region where a median plane, orthogonal to an axial middle of the main magnetic field magnet, intersects with the septum.

4 . The accelerator according to claim 1 , wherein

the magnetic channel includes a septum located on a radially inner side of the main magnetic field magnet and an anti-septum located on a radially outer side with a beam extraction passage interposed between the anti-septum and the septum, and

the predetermined mechanism is achieved by setting a radial width dimension of the septum to be smaller than a radial width dimension of the anti-septum, and forming a recessed part in a predetermined region where a median plane, orthogonal to an axial middle of the main magnetic field magnet, intersects with the septum to set a radial width dimension of the septum to minimum.

5 . The accelerator according to claim 3 , wherein the recessed part is formed over a predetermined length range from a position corresponding to an inlet side of the beam extraction passage.

6 . The accelerator according to claim 3 , wherein a traverse section of the recessed part is formed to spread from a bottom center of the recessed part toward an opening side.

7 . A particle therapy system comprising the accelerator according to claim 1 .