Design And Implementation Of A Fast Kicker Magnet Applied To A Proton Therapy Facility

Proton therapy is now recognized as an accurate radiotherapy method for cancer treatment owing to the unique Bragg peak effect of protons.In a proton therapy facility using the pencil beam spots scanning technique,it is essential to achieve fast beam on/off function with 100 microseconds response time to reduce the undesirable radiation dose during the exchange of spots and energy layers.In a cyclotron-based proton therapy facility HUST-PTF which is proposed by Huazhong University of Science and Technology,this fast beam switching function is implemented by a kicker magnet and a downstream beam dump.This paper introduces the design and development of the fast kicker magnet and the pulsed power supply.A high-precision dynamic magnetic field measurement system is also developed.The measurement results can fulfil the design requirements of the kicker function.This kicker magnet system has been applied to HUST-PTF,which laid a solid foundation for the beam commissioning.The main components in this paper are listed as follows:(1)According to the overall parameters of HUST-PTF,the physical requirments of the fast kicker magnet are introduced.A systematically study on the eddy current in the magnet core and the excitation coil are proposed,and it also points out that the dynamic performence of the inductance should be taken into consideration during commissioning of the pulsed power supply.To reduce the effect of eddy currents and further temperature rise,the material of soft ferrite is selected for magnet yoke and the vacuum pipe is manufactured with ceramic material.The static magnetic field is measured by Hall probe and the results meet the physical requirements of the kicker magnet.(2)According to the physical requirements of the magnetic field and the dynamic performance of the soft ferrite core,the parameters of the pulsed power supply for the fast kicker magnet is proposed,and the scheme design,simulation analysis and debugging optimization of the pulsed power supply are carried out.To solve the high voltage slopes and high frequency voltage oscillations,a multi-stage output filter scenario is applied.By taking the dynamic parameters of the magnet inductance,a fast transition from the rising state to the steady state is realized.This pulsed power supply has been manufactured and debugged with the kicker magnet,and the results of the factory test can meet the design goal.(3)By comparing the properties of the single coil and an array of coils applied to the magnetic measurement,the latter scheme is adopted to reduce the precision and synchronization requirements of measurement system.Due to the poor characterizes of hand-wound coils on the accuracy and uniformity,a searching coil array scheme based on printed circuit board(PCB)technique is adopted to measure the dynamic performance of the kicker magnet.And the design considerations of the dynamic magnetic field measurement system are introduced in detail.The measurement results show that: 1)The designed kicker magnet has a good following features and little field distortion during the fast change of magnetic fied,which fulfils the physical requrements of the kicker function.2)The measurement results of the field distortion are the same as the simulation results in Maxwell 2D,which verifies that the field distortions are manily caused by the relaxation process of current density in the exciting coils.