Physical Design Of A Large-momentum-acceptance Superconducting Gantry Applied To Proton Therapy

Due to the unique Bragg peak dose distribution of proton beams,the proton therapy achieves a precise radiotherapy on tumors,which has been an advanced radiotherapy method currently.As a core component of the proton therapy facility,a rotatable gantry provides multiple-angle irradiation of proton beams.The mainstream implementation of the gantry uses normal conducting magnets,resulting its heavy weight and a large size.Usually,total weight of a gantry is more than 150 tons.The superconducting magnet is an important direction of the new gantry design.This dissertation proposes a physical design of superconducting gantry beamline on the basis of alternating-gradient canted-cosine-theta(AG-CCT)magnets and the concept of localachromaticity.By using the superconducting magnets that contain no iron cores,the weight of a superconducting gantry can be reduced by about an order of magnitude.Meanwhile,the large momentum acceptance is realized through iterative designs and optimizations.Therefore,during the treatment the magnetic fields of the gantry require no adjustment synchronously with the beam energy,avoiding the risk of quench of the superconducting wires in the quick current change.Compared with the traditional gantry,the superconducting gantry is more complicated: a)Big difference between the low and high order optics.b)Obvious discrepancy between the optics model and realistic model.c)A large number of variables and objects in optimization.Therefore,step-by-step iteration mode is adopted in the design from the linear optics and the high order optics to the particle tracking in realistic fields,in order to verify the feasibility of beamline gradually.The main research contents of this dissertation include:(1)Finish the layout design of the gantry to lay the foundation for the subsequent optimization.This part is divided into three points: a)Through the demand analysis,the overall layout of the superconducting gantry is determined.The whole beamline consists of two bending sections;b)Through the first order optics design,the basic structure of the combinedfunction magnet is determined;c)Through the second order optics design,introducing the hexapole magnetic fields to increase the momentum acceptance of the beam line under the second order optics.(2)Finish the high order optics optimization.In this stage,the mechanism of high order effect is analyzed,and the specific order used in optimization is determined.There is some significant adjustment of optimization: adopt genetic algorithm for intelligent search and use objects based on beam spots to reduce the complexity.After optimization,the superconducting gantry shows momentum acceptance of-6%?7%.(3)Finish the design and optimization of the magnets in the second bending section on the basis of particle tracking in realistic fields.AG-CCTs are adopted to implement the combinedfunction magnets,meeting the requirement of field distribution in optics design.In the optimization of realistic models,the GPU acceleration is introduced for effective calculation of particle tracking in realistic magnetic fields.The speed-up ratio is about 100 in tests.Hence,the optimization can be completed in several days instead of half a year.The realistic beam tracking shows that the momentum acceptance of the AG-CCT-based second bending section can achieve-7%?5%.