Magnetic Field Design And Operation Stability Analysis In A Superconducting Cyclotron

Cancer is one of the most fatal diseases in the world,and its treatment has become a major problem in the medical field.According to statistics,about 70%of cancers require radiation therapy.The physical dose distribution and biological effects of radiation directly affect the accuracy and effectiveness of treatment.The"Bragg characteristics"possessed by protons can achieve precise control of its dose.With the help of good biological effects,proton therapy has become one of the most effective methods in the field of cancer treatment.The proton therapy system is a high-tech product integrating physics,mechanics,electricity,medicine,and other disciplines.As a core component,the performance of the beam accelerator is very important,and the superconducting cyclotron has become the mainstream in the field of proton therapy due to its compact structure,stable operation,and continuous beam current.The development of superconducting cyclotrons needs to solve the problems of continuous particle injection,stable acceleration,and high-efficiency extraction.Based on a250MeV isochronous superconducting proton cyclotron,this paper focuses on the design of isochronous magnetic fields,compensation strategies for magnetic field instability and beam phase detection,etc.,specifically include:（1）Complete the main magnet design according to the isochronous magnetic field design theory.By virtual materials and the electromagnetic simulation software OPERA,the isochronous magnetic field verification of the 2D model was realized.The final design of the magnet was completed by establishing the 3D model and combining the isochronous iterative method.The results showed that the isochronous magnetic field error was less than 10 Gs,meeting the desired error accuracy,and the beam integral phase shift was controlled under 10^o.By optimizing the magnetic pole structure,the beam can quickly pass through or avoid dangerous resonance lines,and the beam acceleration efficiency was guaranteed.According to the precession extraction theory,the harmonic magnetic field was obtained through magnetic pole cutting,and the amplitude of the first harmonic magnetic field was adjusted to 15 Gs,the phase was 135^o,and enough turn separation of 7mm was obtained.（2）Analyze the factors of magnetic field instability and propose the strategy of magnetic field compensation.The magnet deformation analysis was carried out with the help of Solid works,and OPERA was used to simulate the influence of the magnetic characteristic change and the eddy current effect on the isochronous magnetic field.The results showed that the maximum magnetic field change caused by the magnet deformation was1.2×10^-3T,and the eddy current effect was related to the current change rate.With the help of beam dynamics calculation software CYCLONE,the magnetic field compensation strategy was analyzed and verified,the results showed that the method of adjusting the coil current was better than adjusting the parameters of the resonant cavity.（3）The detection of the beam phase is the basis of the magnetic field feedback control.Combining the beam dynamics theory to analyze the characteristics of the beam movement,a capacitive probe was selected,and the signal extraction process was simulated with the help of CST.According to the signal characteristic analysis,the signal carrying the phase information of the extracted beam was obtained.Finally,the Simulink module was used to complete the design and simulation of the signal processing system.With the help of FIR filter,I/Q demodulation and other methods,the phase detection error was controlled below 0.5^o.