Issues in the numerical modelling of positive ion extraction

Abstract The wide application of ion sources requires the optimization of the ion optics to achieve a balance between the performance, life cycle and size. A 2D hybrid model is represented to simulate the ion optics for positive ion extraction where adaptive particle management and parallel computing are considered. Then, misunderstandings and inconsistencies in the previous numerical models of positive ion extraction are discussed, and new methods for determining the key simulation parameters are proposed. The local model of positive ion extraction is found to be unable to form a presheath and simulate a Bohm sheath self-consistently, which indicates that the upstream surface of the numerical domain should be the sheath edge and the simulation parameters should satisfy the sheath characteristics. Accordingly, the methods for determining the length of the upstream domain, the potential of the upstream surface and the injection of ions are discussed, as well as the effects of key parameters on the simulation results. Further simulations demonstrate that the thermal velocities of ions can affect the perveance condition of the ion beamlet. By comparing the simulation results with earlier experimental results, it is shown that our 2D model can describe the beam divergence characteristics correctly and predict the perveance limit well with high computational efficiency.

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