Abstract: A cyclotron comprises a dee, an ion source cone provided with an ion outlet through which ions are emitted, and an ion puller electrode mounted on the dee to pull out ions from the ion source cone through the ion outlet by applying a voltage between the ion source and the ion puller electrode. The ion source cone can be moved from outside the cyclotron without breaking the vacuum of the cyclotron. The ion puller electrode has a sliding aperture member provided with an aperture and capable of being moved relative to the dee. An operating projection of the ion source cone is brought into engagement with the aperture member, and then the ion source cone is moved to move the aperture member to a desired position. Thus, the aperture of the ion puller electrode can be positioned at an appropriate position opposite the ion outlet by moving the ion source cone without requiring any special mechanism for moving the ion puller electrode.

Abstract: A pair of magnetic field adjusting center rods are inserted in central portions of a pair of pole pieces of a magnet included in a cyclotron. One of the magnetic field adjusting center rods is provided with an ion source receiving hole and the other is provided with a magnetic field adjusting recess coaxially with the ion source receiving hole. The magnetic field adjusting center rods are provided at their ends facing each other with magnetic field correcting projections, respectively. The magnetic field correcting projections project toward each other to form pole faces facing each other with a small air gap. Thus, irregularities in a magnetic field created between the pole pieces are corrected so that the dispersion of ion beams and the distortion of an orbit of ions can be prevented.