| To address future energy needs and reduce dependence on fossil fuels,the International Thermonuclear Experimental Reactor(ITER)was proposed and implemented in 1985.The ITER project plans to use the Neutral Beam Injection(NBI)system to provide auxiliary heating for the high-temperature plasma of the nuclear fusion reactor,to achieve self-sustained nuclear fusion reaction in a large magnetic confinement fusion device.The NBI system uses ion sources to generate ion beams,which are then accelerated and neutralized to produce high-energy neutral beams(above 1 Me V)that are injected into the high-temperature plasma of the nuclear fusion reactor.However,as the beam energy increases to 1 Me V,the neutralization efficiency of positive ion beams gradually decreases to nearly zero,while the neutralization efficiency of negative ion beams remains above 60%.Therefore,negative ion sources will be the inevitable choice for future NBI systems in thermonuclear fusion devices.In recent years,high-power radio frequency(RF)negative hydrogen ion sources have been favored by researchers in the field of nuclear fusion both domestically and internationally.As an important starting point for the ITER NBI system,the RF negative hydrogen ion source has the advantages of large beam current,simple RF power source structure,long-term maintenance-free operation and etc.However,its structural principle is very complex,and the working mechanism is not fully understood.To deeply investigate the physical mechanisms behind the RF negative hydrogen ion source and explore methods for optimization design,this dissertation developed a parallel algorithm for the Scalar Magnetic Potential finite-difference Method(SMPM),and used SMPM combined with the Particle-In-Cell/Monte Carlo Collision(PIC/MCC)method to comprehensively and deeply simulate the production,transportation,and extraction processes of H~- ions on the RF negative hydrogen ion source BATMAN device from the Max Planck Institute for Plasma Physics in Germany under low pressure conditions,and optimized the magnetic field in its extraction region.The research work of this dissertation mainly includes the following four aspects:1.In the Cartesian coordinate system,according to the difference iterative formula of the Scalar Magnetic Potential finite-difference Method(SMPM),a parallel algorithm for the SMPM method was developed.The algorithm was used to efficiently and accurately calculate the magnetic system of the BATMAN device.An algorithm implementation program that combines SMPM and PIC/MCC methods was developed on the three-dimensional particle simulation software CHIPIC platform,and numerical calculations using a classic small ion source were carried out to verify the correctness of the algorithm.2.A numerical simulation study of the two types of H~- ion production processes on the BATMAN device was carried out using the PIC/MCC method.Since the ion volume production efficiency depends on the electron temperature being less than 2 e V,an electron energy function was set up to characterize the electron temperature,and onedimensional and three-dimensional numerical studies were conducted on the electron energy and velocity distribution functions.The accuracy differences and reasons for lowtemperature electron temperature calculation are analyzed.The conclusion that threedimensional velocity distribution of electrons is more accurate than one-dimensional one when using the electron energy function to characterize the electron temperature was obtained.Furthermore,the influence of the magnetic field type I and type II structures on the H~- ion distribution under the three-dimensional "point emission" electron beam model was numerically simulated to enhance understanding of the H~- ion production process and the magnetic field configuration of the BATMAN device.3.The transport process of H~- ion on the BATMAN device was numerically simulated using the PIC/MCC method.An expansion region model was established,and the SMPM method was used to calculate the magnetic filter field.The main collision reactions of electrons were fully considered under the H~- ion volume production mechanism,and the formation process of "cold electrons" under the magnetic filter field was numerically studied.It was revealed that the magnetic filter field could to some extent reduce the temperature of electrons on the extraction surface,and the excitation collision reaction was the main reason for the energy attenuation of electrons into "cold" electrons.In order to promote the excitation collision reaction and increase the proportion of lowenergy electrons,the influence of the gradually increasing electron beam temperature on the probability distribution of electron energy and the uniformity of H~- ion distribution on the extraction surface was explored.In order to better reflect the physical scenario,the main extinction reactions of H~- ion were further considered,and the H~- ion distribution was corrected.The simulation results promote the understanding of the characteristics of the expansion region of the BATMAN device and the mechanism of H~- ion transport.4.A PIC numerical simulation study was conducted on the H~- ion extraction process of the BATMAN device.A three-dimensional complex magnetic field topology was established using the SMPM method.The magnetic system consists of a magnetic filter field and a magnetic deflection field.A large-scale three-dimensional model of the complex extraction region with multiple apertures was also established.Based on the results of the expansion region research,an emission surface for a non-uniform electron beam was set up on a biased plate.A plasma sheath algorithm was developed and used to numerically calculate the H~- ion extraction process of the non-uniform electron beam and the H~- ion emitting from the plasma grid surface under the sheath structure.The calculated results of the biased current and the co-extracted electron current density under the sheath structure were compared with experimental data to validate the rationality of the extraction region model.The impact of the plasma sheath on the co-extracted electrons and the extracted H~- ions was explored,revealing the dynamic characteristics of electrons and H~- ions under the sheath structure.Finally,a magnetic layout was proposed to optimize the magnetic field in extraction region,successfully improving the uniformity of electrons in front of the plasma grid. |
