Investigation for the vertical focusing enhancement of compact cyclotron by asymmetrical shimming bar

CYCIAE-100, a 100 MeV H− cyclotron under construction at China Institute of Atomic Energy (CIAE), is an AVF compact cyclotron. With energy above 70 MeV, the straight-edge sector magnet, instead of the spiral one normally used for AVF cyclotrons in this case, is still used to simplify the engineering procedures. The vertical focusing is likely not strong enough at the outer region and the Walkinshaw resonance may occur, if either the permeability decreasing tolerance in large scale pure iron castings and forgings, or the fabrication tolerance during the magnet construction, are seriously excessive. Theoretical investigation and numerical simulation results presented in this paper show that this kind of risk could be avoided by using a set of specially designed asymmetrical shimming bars between the pole edge and dummy Dee of the RF cavity at the outer region. In this way, the vertical focusing at outer radius region will increase substantially. This investigation provides a protective measure for the main magnet construction of CYCIAE-100.