| Radiotherapy is one of three most common types of cancer treatment,which is used to treat nearly 50-70%of cancer patients.Currently,photon therapy(x and gamma rays)is the most widely used in radiotherapy.However,photon therapy has fundamental limitations:1)Dose decreases with penetration depth exponentially,2)healthy tissue in front of the tumor absorbs greater dose that increases the risk of inducing new cancer.Different from the photon,proton has an excellent dose distribution with Bragg peak.Thus,if the proton beam is precisely controlled to place the Bragg peak within a tumor,the tumor tissue can be destroyed at utmost,while the surrounding healthy tissue can be spared.Therefore,proton therapy can treat tumor like surgery,which is currently the most advanced non-invasive precision therapy for cancer.Accelerator,as the core component of proton therapy equipment,its performance is directly related to the stable operation of the system and the requirement for medical treatment.Cyclotron operates with many advantages in aspects of stable operating,adjustable intensity and countinuous beam.Thus,cyclotrons have been selected in 62%of the operating pronton therapy facilities.Superconducting cyclotron has the advantage of compact structure,light weight and low power consumption.The stable running superconducting magnets keep the machine parameters fixed and fast to boot.Due to the significant advantages of the superconducting cyclotron,development of the proton radiotherapy equipment based on superconducting cyclotron is supported during the 13th five years plan in China,and this is also the first time we combine the superconducting technology with the cyclotron design.Studing the beam dynamic in the cyclotron is the base of the design.Proton beam motion inside the cyclotron is divided into 3 stages:Injection,accelerating and extraction.Injection and extraction determines the beam quality.Since there are many complex electromagnetic structures in the compact superconducting cyclotron working in high magnetic field strength,which are requested for low beam loss,the design of injection and extraction system of superconducting cyclotron face many challenges:1)Injection in the complex electromagnetic structure of the central region,2)Extraction beam quality contral,3)High extraction efficiency.Recently,only a few commercial superconducting cyclotrons are operating internationally,and they are designed by experiences and experiments in the operating of conventional cyclotron,which causes problem in extraction efficiency and stability.In order to optimize the design and achieve high efficient beam injection and extraction in the compact superconducting cyclotron under strong magnetic field,it is necessary to study the physical processes of injection and extraction and to develop new methods and structures for design of the injection and extraction.Meanwhile,the development of the superconducting cyclotron is in early stages in China,which adds more difficulties for the design.This thesis focuses on problems of beam injection and extraction of the superconducting cyclotron for proton therapy,which aimed to achieve beam injection and extraction with high efficiency.In the thesis,combined beam dynamics theory analysis,and numerical algorithm for cyclotron design,ion source and electromagnetic structure of central region,resonance extraction method theory and fringe field parameters,injection and extraction have been researched and designed.Some theoretical methods and engineering designs are proposed.The main research content and innovation are:1)Orbits stable conditions of the spiral sector cyclotron is derived using similar equilibrium orbits theory,on basis of which beam dynamics of the main magnet of Huazhong University of science and technology superconducting cyclotron is evaluated.The magnetic field distribution of the main magnet,beam transportation,phase slipping and the work points are studied.Then the important horizontal oscillations are studied in analytical approximate expressions.2)To study the superconducting cyclotron numerically and provide design and beam simulation tools,algorithms used to design the magnetic field and track particles are designed,which contains searching algorithm of the equilibrium orbits,searching algorithm of the accelerated reference orbit,parallel particle movement simulation algorithm based on heterogeneous computing and the topology optimization for the main magnet.3)Two components of the injection system are analyzed:ion source and Central region.A new method to establish a 2D model of one kind of ion source is given based on field analysis method.The effects of the plasma emission face on the injected beam are analyzed using the virtual ion electrode method.Structure and the phase selection characteristics of the central region are evaluated.4)Physical designs of the extraction system are given based on precessional extraction which uses the first homarnic to obtain a large turn separation in the extraction region,optimization of the phase and amplitude of the first homarnic has been evaluated which achieves 5mm turn separation for the extraction orbit.Method to produce the magnetic field with the corresponding amplitude and phase is given.Effects on the beam extraction orbits by the electrostatic deflector voltage has been evaluated by numerical analysis and particle tracking method,the extraction beam orbit with designed electrostatic deflector voltage is given.Along the beam orbit compensation scheme is calculated by an equivalent lumped parameter method,and a layout of magnetic channels is designed to meet the beam quality requirements,the twiss parameters of the beam through the designed magnetic channels are evaluated using beam optics calculation program MADX.5)Engineering design of the three main components of the extraction system are given.First device is the Bump coil,engineering parameters,magnetic parameters and power supply requirements are designed.Second device is the electrostatic deflector,its structure design and thermal calculation are conducted.A high voltage experimental platform for electrostatic deflectors is designed,which contains the structure design,measurement system design and power supply design,a new power supply used to simulate beam heating is designed for the experimental platform.For the last device,namely magnetic channel,has been designed with two parameters,reasonable arrangement of soft iron is given,and the magnetic field is verified using FEA software. |
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