Design Study of a 3.5 T Superconducting Sector Magnet in a 450 MeV/nucl Booster Cyclotron for Carbon Ion Therapy

A radiation therapy system using carbon ions is proposed. It is based on an accelerator system of two coupled cyclotrons. The first cyclotron accelerates carbon ions (or other particles) up to 250 MeV/nucl. The second cyclotron is used to boost these carbon ions to 450 MeV/nucl for treatments at deep tumor sites. This separate sector cyclotron consists of six sector 3.5 T magnets with superconducting coils and four RF cavities. We report on our first results of a feasibility study of the superconducting sector magnets of this booster cyclotron. To prevent concave magnet pole edges and to achieve symmetry in the design of the magnet, the introduction of a radial gradient in the sector is proposed. A magnet geometry based on a configuration with tilted coils is presented. The field strength varies from 2.8 T to 3.5 T over 0.8 m. The viability of the magnetic design is checked with tracking simulations. The distribution of the large electromagnetic forces acting on the coil winding, evaluated using a 3D finite element analysis, is reported. The details of the superconducting coil, of the type of wires, and of the cable layout are presented. The conceptual design of the cryostat and the options for conductor protection against quench are proposed.