High order beam optics optimization for a superconducting gantry applied to proton therapy based on NSGA-III

Gantry is an indispensable part in the proton therapy facility to achieve multi-angle radiations. The introduction of superconducting techniques can significantly reduce the weight and volume of the gantry. A superconducting gantry with large momentum acceptance is under study at Huazhong University of Science and Technology. The first and second-order beam optics design has been accomplished. To narrow down the difference between optics design and beam tracking result in real magnetic fields, the high order optics optimization shall be performed. It is a many-objective optimization problem, and many knobs need to be fitted. The traditional fitting method in beam optics codes (e.g. MAD-X, TRANSPORT) is difficult to perform an efficient search in a large parameter space. This paper proposes an optimization approach based on the genetic algorithm NSGA-III for the high order optics of the superconducting gantry. Instead of the matrix coefficients fitting in the first and second-order optics design, the beam size and its symmetry in two planes are identified as the objectives. After the optimization with 1000 generations, a stable round beam can be achieved at the treatment terminal and the momentum acceptance reaches −6%/＋7% in the fifth-order optics.