Magnetic field simulation in the central region of the VINCY Cyclotron

The VINCY Cyclotron is under construction at the Vinca Institute of Nuclear Sciences, Laboratory of Physics, Belgrade, Serbia and Montenegro. One of the most difficult problems to solve is the magnetic field shaping in the central region of the machine in a wide range of field levels. The latest measurements showed a very large magnetic field depression in the central region. The following shimming elements are used for the field shaping in the central region: a central plug and a spacer ring between the sectors and the median pole plate. These elements should both release enough vertical space for the inflector and the RF electrodes and produce a field bump in the very center of the machine. Concerning this region, one has to mention the following facts: sectors are absent or very reduced, and the small gap between the sectors restricts remarkably the possibilities of producing a field bump. Therefore, the design of these elements is the most critical one. Optimization of the central plug gap affects both the vertical sparking probability and the beam dynamics at higher harmonic modes. The redesign of the magnetic structure of the central region relied solely on the detailed 3-dimensional calculations to speed up the experimental shimming procedure. The desired calculated magnetic field shapes, which include the influence of the plug, have been produced and confirmed