Development Of Single Particle Simulation System And Its Application At The SUNIST Spherical Tokamak

Single particle motion has an important role in understanding toroidal plasma physics. The guiding center method, where the gyro-motion is averaged, is not valid in some cases, for example, when the gyro-radius is large enough or when the orbit of gyro-motion is distorted. In this dissertation, a single particle simulation system was developed by employing the Lorentz method. Using this code, the behavior of single particle motion in the SUNIST spherical tokamak is investigated and, especially, results are applied to analyze the experimental conditions for Alfven wave current driving in the SUNIST.Computation methods are considered throughout in the dissertation. In particle simulation by the Lorentz method, time step is a very important parameter to be chosen, which concerns the validity as well as the efficiency of computation. However, in previous codes, the time steps are chosen empirically. Boris method and Runge-Kutta method are compared both in the slab and toroidal geometry and it is found that Boris method is not a good choice in simulating toroidal plasma. Moreover, the criterion of time step for Boris method obtained previously leads to wrong orbits in complicated fields. Finally, a general method of choosing the time step is given in the dissertation. Based on the single particle code programmed, a comprehensive simulation system is developed which provides effective database management and a friendly interface.Employing the code, the single particle motion of hydrogen ion is investigated for the SUNIST. At typical discharge parameters, regardless of the ripple of the toroidal field, particle orbits have similar behaviors as in conventional tokamaks, and, therefore, can be approximately described by analytical results from the guiding center method in the standard model magnetic field. It is partially because the poloidal field is less than the toroidal field even outside the torus due to relative low plasma current (<50 kA) in typical discharge at SUNIST, and partially because the plasma temperature is so low (< 100 eV) that the finite orbit effect is not significant. If the plasma current increases to 150 kA, magnetic islands will appear in the low field side and then thin banana orbits and local trapped orbits will appear as well as known in other spherical tokamaks. Another notable point is that there is a large ripple (±20%) of the toroidal field in the low field side of the SUNIST due to its compact design. Due to the ripple effect, magnetic moment is not a good adiabatic invariant any longer. The ripple results in some complex orbits of particle, such as ripple-trapped-particles which are limited in very small toroidal range. The ripple of the field also induces the enhancement of ion loss, although ion ripple loss at the SUNIST is not serious at current discharge parameters (ion temperature < 100 eV). However, when ion temperature increases to 1 keV, ion orbit loss become substantial and the stochastic ripple banana diffusion enhances the ion loss dramatically.Alfven wave current drive experiment is scheduled at the SUNIST. The ratio of trapped electrons and their bounce frequency are simulated at experimental parameters. The simulation results are compared to the analytical results from the guiding center method in the standard model magnetic field as well. It is found that, at the expected Alfven resonance layer, the trapped ratio of electrons is very high, so it might be a good platform to investigate the role of trapped electrons in Alfven wave current drive. By comparing the average bounce frequency to the effective collision frequency, the regime where electrons are effectively trapped and not distorted by collisions is identified, which is determined by plasma parameters such as density, temperature and plasma current. These results provide scenarios for the Alfven wave current drive experiment at the SUNIST.