Accelerator and particle therapy system

An accelerator, having a pair of cores formed in disk shape, a pair of magnetic poles fixed to circular surfaces of the cores, a pair of coils placed around the magnetic poles, and having a space between the pair of the magnetic poles for circulating and accelerating ion beams,

• wherein:
• the pair of the magnetic poles has a plurality of depression and projection structures arranged along orbits of the ion beams circulating, and the magnetic poles are plane-symmetric with respect to an orbit plane created by the orbits of the circulating ion beams as well as plane-symmetric with respect to a vertical plane perpendicular to the orbit plane,
• the pair of the magnetic poles has a depression structure of the plurality of depression and projection structures, in a position intersecting with the vertical plane, and a boundary surface between the depression structure placed in the position intersecting with the vertical plane and a projection structure adjacent to the depression structure has unanimously either a projection shape or a depression shape with respect to the vertical plane,
• wherein the core has an ion injection port in a location which is different from a center of an orbit-plane-side surface of the core and which includes the vertical plane,
• the plurality of depression and projection structures are each formed in a fan shape having a top at an injection point where the ion injection port is installed, and having an arc along an outer edge of the core,
• a depression structure of the depressions intersecting with the vertical plane has a short distance from the injection point to the arc, and a tangent to the boundary surface between the depression structure and a projection structure adjacent to the depression structure forms an acute angle with the vertical plane, and the acute angel monotonically increases as a contact point between the tangent and the boundary surface is moved from the injection point toward the outer edge of the core, and
• a depression structure of the depressions intersecting with the vertical plane has a long distance from the injection point to the arc, and a tangent to the boundary surface between the depression structure and a projection structure adjacent to the depression structure forms an acute angle with the vertical plane, and the acute angel monotonically decreases as a contact point between the tangent and the boundary surface is moved from the injection point toward the outer edge of the core.