| Neutral beam injector (NBI) is one of the main auxiliary heating methods in international thermonuclear experimental reactor (ITER); moreover negative ion (NI) source is the key factor in neutral beam system. Based on the design requirements for ITER, it’s essential to extract48A H-or40A D-from ion source. With regard to the huge negative ion current, it’s a great challenge for the present technology of ion source. German institute of plasma physics (IPP) has adopted magnetic filter field technology on its small scaled test-bed, the device of BATMAN, obtaining the extracted H-current of300A/m2and D-current of200A/m2. The extracted electron to the extracted negative ion ratio is less than1and pulse time of power is6s. All these excellent properties above prove the feasibility of using magnetic filter field technology on negative hydrogen ion source. Magnetic filter field is to refrigerate hot electron generated in the driver from more than lOeV to less than1eV in order to reduce the loss of negative ion. However, the mechanism of action for magnetic filter field is still uncertain so far, and the design of magnetic filter field strongly relies on the experiment. If the transport process for the plasma in magnetic filter field can be completely figured out, the specific theoretical basis can be put forward on the design of magnetic filter field to optimize the structure and improve the overall performance of the whole NI source by combination with optimization for the driver. At present, all the researches concerning magnetic filter field are experimental and IPP are still conducting some experiment on the device of BATMAN to seek for the optimal design for magnetic filter field. Huazhong University of Science and Technology (HUST) undertook the project,"the Key Technology and Engineering Research of Driver in the ITER Negative Ion Source", with the support of Ministry of Science and Technology (MOST) in September,2011, which targets developing a kind of single driver similar to BATMAN, the experimental facilities with short pulse working mode. Depending on present HUST work, it not only helps to get a deep understanding of the physical process about plasma passing through the magnetic filter field, but also helps to provide guidance and reference for the optimization and the design for the magnetic filter field by adopting numerical simulation technique of PIC/MC (Particle-in-cell/Monte Carlo). This article consists of four parts. Part one is about the algorithm research related to PIC/MC model and that’s what chapter two is all about. Part two is about the design for HUST cylinder shaped magnetic filter field, which is in the chapter three. Part three is about numerical simulation research on the magnetic filter field and this part includes chapter four. Part four is about the diagnosis on the HUST setup and this part includes chapter five.Chapter two introduces the general numerical method of PIC/MC, gives a brief introduction of general steps for the method, which includes the accumulation of the super particle model and the electric charge, particles movement and calculation method for space electric field, etc. Part of MC focuses on the introduction of particle sample, collision type between particles, and speed processing after collision, etc. In this part, part of calculation method is optimized to improve the efficiency of calculation. The introduction of the numerical scheme for particles movement based on linear multistep method makes the solving accuracy of movement locus close to4nd Runge-Kutta method under the condition that the calculated quantity stays the same to the Boris leapfrog algorithm. In the two-dimensional space electric field solution, it is greatly improved in the aspect of computation speed by using the conjugate gradient method compared with the traditional gaussian iteration method and successive over relaxation (SOR) method.Chapter three makes an analysis of finite element on the magnetic filter field of the device of BATMAN, based on which a new magnetic filter field pattern quite different from the track pattern for BATMAN is designed relying on HUST cylinder-shaped expansion chamber. After comparing the magnetic filter field of HUST and IPP, shape factor is presented to represent the characteristic parameter of the two magnetic filter fields.Chapter four conducts a research on the function of how magnetic filter field affects the transportation process of plasmas through one-dimensional and two-dimensional numerical simulation. The magnetic filter field divides the magnetic filter field into two regions obviously different from each other concerning the plasma parameter. One is called upstream region with high density of plasma and electron temperature; the other is called downstream region with relatively lower density of plasma and electron temperature. And then comes a conclusion that the main mechanism of action is collision by researching on the mechanism about electron passing through the magnetic filter field and adopting ne-dimensional numerical simulation.At the end of this chapter conducts a research on spatial plasma parameter in different magnetic filter field patterns. When the shape factor stays the same, the density of plasma and electron temperature in the downstream region will decrease with the increasing of the maximum value of the magnetic field. In the end it is concluded that integral value in the extraction path for the magnetic field has some influence on the plasma parameter in the downstream region. With the increasing of integral value, the density of plasma and electron temperature in the downstream region will decrease. The high density of plasma and the low electron temperature in the downstream region are beneficial to add negative current; however these two conditions can’t be achieved at the same time. Therefore, there exists one optimized value to balance for the design of the magnetic filter field.Chapter five gives a detailed introduction of diagnostic system on HUST high-power n egative hydrogen ion source and presents how the parameter of the plasma is changing with the RF power as well as how the plastma is distributed along extraction line. |
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