| Scanning magnet is the core component of the proton therapy nozzle,which can accurately illuminate the target tumor area by changing the beam trajectory of the pencil beam scanning.Based on the research and analysis of equivalent air-gap scanning magnets,the electromagnetic heat and beam analysis of the tapered air-gap scanning magnets which can reduce the current of the slow scanning magnets are carried out in this paper.According to the performance requirements of the treatment nozzle,the online monitoring method of the magnetic field at the edge of the scanning magnet pole based on the Hall probe was studied.Finally,the scanning path optimization was studied according to the parameters of the scanning power supply.Based on the calculation and simulation of the equivalent air-gap scanning magnet,the shape of the air-gap of the slow scanning magnet is designed as a taper.The electromagnetic and thermal coupling analysis of the scanning magnets is carried out by using COMSOL,and the comparison with OPERA is made to verify the reliability of using COMSOL.Then,according to the static heating power of the magnet coil,the minimum water flow required by the coil is calculated.Considering the safety margin,the water flow rate of the two magnets is3.5 L/min and 8 L/min respectively.Then,the influence of different pole cutting parameters on the temperature rise of the magnet was compared.The results show that under the extreme condition of the maximum current and the scanning current changing rate of 80 A/ms for a long time,the high temperature rise of the magnetic pole and the melting of the adhesive can be prevented by proper pole cutting of the fast scanning magnet.Finally,according to the size parameters of the proton beam at the inlet of the treatment nozzle,the effects of tapered air-gap magnet and equivalent air-gap magnet on the beam spot size at different positions were compared.The two kinds of air-gap scanning magnets make the beam spot size of the 70 Me V proton beam at different positions differ by less than 1 %,and the size in the isocenter plane is less than 2.5 mm,which proves the effectiveness of the cone-shaped air-gap magnet.In order to meet the requirements of the beam distribution system for the accuracy and safety redundancy of the scanning position,the online monitoring scheme of the magnetic field at the edge of the scanning magnet pole is designed in this paper.According to the position accuracy of the proton beam in the isocenter plane and the switching time of the scanning point,the Hall magnetic field detector was selected,and the corresponding EPICS client was designed to obtain the magnetic field and temperature data of the probe.The problems of zero drift,temperature compensation and elimination of geomagnetic field of the Hall magnetic probe were analyzed.Through the simulation of the scanning magnet,it is found that there is a strong linear relationship between the fringe magnetic field and the center magnetic field at different positions,and the fringe magnetic field close to the magnetic pole which has little difference with the center magnetic field value is selected for online monitoring.In order to reduce the effect of scanning magnet current and magnetic field stabilization time on treatment time when switching between scanning points,intelligent algorithm was adopted in this paper to optimize the scanning path.Firstly,the ant colony algorithm,fast simulated annealing algorithm and genetic algorithm are introduced and implemented.Then the influence of the shape of the typical tumor and the magnetic stable death time on the path optimization was analyzed.The results show that when the scanning points are scattered and the time required to scan the same distance in two directions differs very little,the intelligent algorithm path has a better optimization effect than the zigzag scanning path. |
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