Method of reducing axial beam focusing

A system presenting a minimised diameter of the magnet poles of a cyclotron for acceleration of negative hydrogen ions for production radioactive tracers, comprising

• an electromagnet system consisting of a pair of coils and a yoke including a first (1) and a second (2) circular magnet pole presenting pole sectors (4) and valley regions (6), at least two opposing valley sectors containing RF acceleration electrodes (8, 9) and the first and second magnet poles (1, 2) and the included RF electrodes (8, 9) being positioned in a vacuum casing for forming a cyclotron accelerating system for negative ions released from a central ion source when applying a proper RF accelerating voltage to the RF electrodes (8, 9), whereby an ion beam is deflected and focused between the first and second magnet poles by an applied strong magnet field by means of the electromagnet system;
• whereby each magnet pole forms four sector portions and four valley portions, the distance between the four sector portions being of the order 15-30 mm for creating a high magnetic field with a low number of Ampere-turns and a distance in the four valleys for allowing a suitable space for the ion beam vacuum conductance for achieving a necessary vacuum when accelerating the ion beam;
• and the electromagnetic field being adapted to saturate the four sector portions but not the valleys to transform an azimuthal magnetic field shape from being “square-wave”-shaped to becoming approximately sinusodial.