Research On The Tuning Of Cyclotron Frequency And Magnetic Field

Positron emission tomography(PET)is an advanced nuclear medical imaging technology,which is widely used in the early diagnosis of human tumors.However,the imaging work couldn't be completed without the equipment of a cyclotron.In China,almost all cyclotrons used for producing radioisotopes for PET diagnosis are imported from abroad.High-frequency system is an important part of cyclotron.The performance of the high-frequency system is directly detemiined by the performance of the high-frequency resonant cavity,which determines the performance of the accelerator to a large extent.During the actual operation of the cyclotron,the high-frequency resonant cavity is influenced by the beam load,gravity and thermal loss and other factors,which could cause the deformation of the cavity.And the defonmation of the cavity lead to the shift of the resonant frequency,which will cause the working frequency mismatch with the resonant frequency of the resonant cavity.Firstly,the basic principle of cyclotron and the basic parameters of the resonant cavity are introduced.The simulation researches are carried out for the resenant cavity of cyclotron.Especially,the resonant frequency and quality factor of the cavity are numerically analyzed and calculated.The results calculated by different softwares are compared with the actual measured parameters of the high frequency system.It is verified that the thermal loss causes the deformation of the cavity which leading to the shift of the resonant frequency.This thesis proves the validity of HFSS and CST used for the analysis of the resonant cavity,and provides a reference for the design and optimization of the resonant cavity.In order to solve the problem that the resenant frequency of the resonant cavity will change gradually during the operation of cyclotron,a scheme is proposed to match the frequency of the high frequency power source with the frequency of the accelerating electric field in the high frequency cavity in real time.Compared with adjusting the frequency of the cavity,this scheme does not need to install an automatic frequency stabilizing device for the resonant cavity,which simplifies the structure of the cavity and reduces the cost.The high-frequency system adopted does not have low-level control function.The low-level control directly avoids the danger of direct contact with high voltage and provides the premise of safety guarantee for the safe operation of the accelerator.However,the control functions of frequency scanning,amplitude stabilization,phase stabilization and fire protection need to be completed by PLC.The low-level control system controls the amplitude and phase accuracy of the acceleration field within ±1%and ±1°,which realizes the automatic control of its feed-forward power,reflection power,phase and cavity frequency.At the same time,in order to monitor the working state of the cyclotron remotely in real time,WinCC is used to complete the communication with the host user interface system,and realize the man-machine interaction.In addition,in order to ensure the stable operation of the high-frequency system,a complete interlocking protection mechanism is established,and different measures are taken to protect the possible types of faults.In order to meet the requirements of isochronous acceleration,the magnetic field strength should also be changed correspondingly when the high frequency operating frequency changes,that is,the magnitude of the magnet current needs to be changed accordingly,so that the particle cyclotron frequency matches the high frequency resonant frequency to overcome the sliding phase.Firstly,the static magnetic field model is established by finite element simulation software to simulate the average magnetic field of cyclotron under different magnet currents.Then the relationship between the magnetic field and the resonant frequency is theoretically analyzed.Finally,the relationship between the magnet current and the resonant frequency is obtained when the magnet current varies in a small interval.According to the optimal magnet current corresponding to different resonance frequencies,the automatic frequency tracking of magnet current is completed.In the case of ensuring the maximum carbon film beam,the experiment obtains the optimal magnet current corresponding to different resonance frequencies,which makes the theory validated.According to the relationship,the magnet current is automatically adjusted,which overcomes the slip phase and ensures the stable output of the Faraday beam.The method enables the magnet current to quickly and accurately find and track the cavity frequency,overcomes the slip phase caused by the frequency offset,and completes the stable output of the beam.