PIC/MCC Simulations For Production Of Highly Charged Ions In Electron Cyclotron Resonance Ion Sources

Electron cyclotron resonance(ECR)ion source,as one of the most effective tools in the world for the production of highly charged ions,has attracted much attention in the field of accelerators due to its excellent performance.However,the research on the physical mechanism of ECR ion source lags far behind the development of its technology.Because of the complexity and variability of the ECR plasma,it is difficult to understand all the physical mechanisms only by experimental study.Therefore,numerical simulation has gradually become an important supplementary means to study ECR ion sources.The production of highly charged ions is one of the most important physical processes of ECR ion sources.However,the current computational power is not enough to match the complexity of its simulation,especially considering the self-consistent solution of the nonlinear multi-physical coupling.And so far there is still no numerical simulation research that can perfectly describe the related physical mechanism.Therefore,in this dissertation,a high-performance 3D particle-in-cell plus Monte Carlo Collision(PIC/MCC)algorithm for the production of highly charged ions in an ECR ion source was developed.The algorithm reasonably combined the advantages of a variety of simulation models and numerical optimization schemes,basing on the key physical characteristics of the ECR plasma.It broke through the limitation of spatial and temporal scale in the conventional PIC/MCC simulation of ECR ion sources,and realized the dynamic simulation of the production of highly charged ions.With the simulation results,the key physical processes involved were analyzed,which provided a scientific basis to further understand the working mechanism of ECR ion sources.The research content and innovation of this dissertation are mainly divided into the following five parts.(1)Research on the basic algorithm for ECR ion source simulations.According to the different targets of ECR ion source simulations,the corresponding single particle method and electrostatic PIC/MCC method were developed respectively.The fully 3D simulation algorithms in the cylindrical coordinate can apply to any magnetic field structure of ECR ion sources,and the motion of charged particles was solved through the local transformation of the Cartesian coordinate.The PIC/MCC method was adopted to obtain a coupled nonlinear solution of several key physical problems of ECR ion sources,including the collective action of plasma and the collision action.After code implementation and numerical verification,the simulation algorithm showed high computational accuracy and efficiency,which laid a foundation for ECR ion source simulation research.(2)Development of the advanced algorithm for ECR ion source simulation.According to the physical characteristics of the ECR plasma,a precalculated PIC/MCC method for the ion production simulation of ECR ion sources was developed by combining the single particle model and the PIC/MCC method.The static and dynamic temperature equilibrium models were further established based on the simulation problem caused by the involved physical simplification.Meanwhile,a variety of numerical optimization schemes were implemented to improve the computational efficiency,and the numerical validations were carried out respectively.The advanced algorithm broke through the limitation of spatial and temporal scale in the PIC/MCC simulation to some extent,and accurately described the self-consistent evolution of the ECR plasma under the effects of multiple physics.It was able to reduce a lot of computational consumption,while ensuring the key physical essence.(3)Simulation for the confinement and heating of electrons in ECR ion sources.The electron confinement and heating processes in an ECR ion source under the effects of the minimum-B magnetic field and input microwave power were studied by using a fully 3D single particle model.The characteristics of electron motion and distribution were analyzed,and the effects of Coulomb collision,microwave power and biased disk voltage were compared and discussed.The research shed more light on the understanding of electron magnetic confinement and ECR heating.(4)Simulation for the production of highly charged ions in ECR ion sources.The production of high charge ions in an ECR ion source was simulated by using the fully 3D precalculated PIC/MCC method,where the static and dynamic temperature equilibrium models were applied respectively.The simulation provided the characteristics of particle motion and plasma potential distribution of the corresponding conditions in an ECR ion source.From the perspective of ambipolar diffusion,some key physical problems in the production of highly charged ions in ECR ion sources were analyzed,and the effects of microwave power and RF diffusion on the potential distribution and ion extraction were discussed.The research helped to further understand the mechanism of the production of highly charged ions in ECR ion sources.(5)Research on the self-consistent feedback algorithm for time-varying electromagnetic fields.In order to solve the ECR plasma feedback to time-varying electromagnetic field which was ignored in the current simulations,a quasi-3D hybrid theoretical simulation algorithm based on the modal expansion method in a cylindrical cavity was developed,together with the analytical approximation method for the mode selection in an actual simulation.Its feasibility and correctness were validated in an electron beam model,including the effective saving of computational consumption by the analytical approximation method.With the ablity to solve the nonlinear interaction between charged particles and time-varying electromagnetic field,the hybrid algorithm can either work independently,or couple into the single particle simulation and the PIC/MCC simulation.