Effects of Curved Superconducting Magnets on Beam Stability in a Compact Ion Therapy Synchrotron

Abstract: Superconducting, curved magnets can reduce accelerator footprints by producing strong fields (>3T) and reducing the total number of magnets through their capability for combined-function multipolar fields, making them an attractive choice for applications such as heavy ion therapy. There exists the problem that the effect of strongly curved harmonics and fringe fields on compact accelerator beam dynamics is not well represented: existing approaches use integrated cylindrical multipoles to describe and model the fields for beam dynamics studies, which are invalid in curved coordinate systems and assume individual errors cancel out over the full machine. In the modelling of these machines, the effect of strongly curved harmonics and fringe fields on compact accelerator beam dynamics needs to properly included. An alternative approach must be introduced for capturing off-axis fields in a strongly curved magnet, which may affect long-term beam stability in a compact accelerator. In this article, we investigate the impacts of deploying a curved canted-cosine-theta (CCT) superconducting magnet in a compact medical synchrotron for the first time. We develop a method to analyse and characterise the 3D curved fields of an electromagnetic model of a CCT developed for the main bending magnets of a 27m circumference carbon ion therapy synchrotron, designed within the Heavy Ion Therapy Research Integration Plus European project, and the CERN Next Ion Medical Machine Study (NIMMS). The fields are modelled in the compact synchrotron in MAD-X/PTC to study their effects on beam dynamics and long-term beam stability. The insights gained through the methods presented allow for the optimisation of both magnet and synchrotron designs, with the potential to impact the operational performance of future ion therapy facilities.