This paper describes the design study of a gantry for a carbon beam. Thedesigned gantry is compact such that its size is comparable to the size of theproton gantry. This is possible by introducing superconducting double helicalcoils for dipole magnets. The gantry optics is designed in such a way that itprovides rotation-invariant optics and variable beam size as well aspoint-to-parallel scanning of a beam. For large-aperture magnet,three-dimensional magnetic field distribution is obtained by invoking acomputer code, and a number of particles are tracked by integrating equationsof motion numerically together with three-dimensional interpolation. Thebeam-shape distortion due to the fringe field is reduced to an acceptable levelby optimizing the coil windings with the help of genetic algorithm.Higher-order transfer coefficients are calculated and shown to be reducedgreatly with appropriate optimization of the coil windings.
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