Research On The Plasma Transport And Negative Ion Extraction In High Power Rf Negative Ion Source Based On PIC-MCC

High power radio-frequency(RF)negative ion source is a key component of the neutral beam injection(NBI)system for fusion.Its goal is to provide negative ion beam with high density,high current,high uniformity,low divergence angle,and low co-extracted electrons.In the negative ion source,the plasma is transported from the driver through the expansion chamber to the extraction region.The plasma parameters obtained in the extraction region determine the upper limit of the negative ion beam that can be extracted.The emission meniscus formed when the negative ions are extracted from the plasma influences the quality of the extracted beam.The above processes in the negative ion source are far more complicated than those in the positive ion source,which is essential for understanding the mechanism and optimizing the design of the negative ion source.However,the experimental study of the above process is not only limited by the lack of meniscus diagnostic means but also difficult to reveal the physical mechanism.Particle-In-Cell with Monte Carlo collision(PIC-MCC)simulation,for its self-consistent and accurate advantages,has become a powerful tool to detailedly study the transport process and extraction process.There are some problems that need to be solved in the research on the plasma transport process and extraction process of negative ion sources:(1)The PIC-MCC simulation takes huge computation effort and is time-consuming,and the correctness of the results is difficult to guarantee,so an efficient and accurate calculation method needs to be developed;(2)In terms of transport process research,the mechanism of plasma transport in the chamber is not fully clear,and the low-dimensional model used for reducing computation effort ignores the plasma loss to the side wall,resulting in incorrect calculation results;(3)In the study of the extraction process,the formation of the meniscus is not clear,and the mechanism that the meniscus profile and beam properties are affected by relevant factors remains to be explained.To solve the above problems,the following researches are carried out in this thesis:(1)This thesis studies the implementation of PIC-MCC parallel algorithm completely based on Graphics Processing Unit(GPU),which provides effective support for physical process simulation research.According to the parameters of the negative ion source and the characteristics of GPU calculation,the algorithm is optimized and improved,the appropriate data structure is adopted,and the particle sorting method and multi-grid method are used to improve the calculation efficiency.The GPU implementation achieves the same level of efficiency as international peers with low-cost hardware.The sheath model and charged particle drift model are used to test the PIC module and MCC module respectively to verify the correctness of the algorithm and program.(2)Based on the negative ion source platform in Huazhong University of Science and Technology(HUST),the plasma transport process in magnetic filter was experimentally investigated,and a one-dimensional(1D)PIC-MCC model considering plasma sidewall loss was proposed to simulate the transport process.The 1D sidewall loss model can be used to study the main physical processes in plasma transport with greatly reduced computation effort.The plasma transportation under different magnetic filters is diagnosed experimentally and the correctness of the proposed model is verified.The mechanism of highly electronegative plasma generated by strong negative ion surfaces emission under the influence of magnetic filter is simulated,which provides a reference for determining the plasma state of the extraction process.(3)A two-dimensional PIC-MCC model of single-aperture extraction is established,and the self-consistent emission meniscus is obtained.The coupling of PIC-MCC simulation and beam calculation is realized,and the influence of extraction related factors on the formation of the meniscus and the final beam quality is studied.The position and shape of the meniscus under different extraction voltages and plasma electronegativities are obtained by simulation.The difference in beam performance between the negative ions extracted directly from the grid surface and from the meniscus is studied.The mechanism of plasma grid bias and deflection magnetic field to reduce co-extracted electrons was explored,and a lower co-extracted electron ratio was obtained by increasing the intensity of the deflection magnetic field.The influence of different meniscus states on beam quality is analyzed,and the optimal extraction state is obtained,which can provide guidance for the optimization of the extraction system.