Analysis and particle-in-cell simulation on the similarity relation during an ion extraction process

Abstract

Ion extraction time is one of the key parameters for an ion extraction process. Particle-in-cell (PIC) simulation can provide a detailed description on the charged-particle behaviours during the ion extraction process in a decaying plasma. However, the PIC modelling is a very time-consuming task with very small space step (~ Debye length) and time step (~ inverse of plasma frequency), as well as a massive number of macro-particles, especially for the cases in multi-dimensions and large geometrical sizes. In this paper, based on the sheath expansion and ion-acoustic rarefaction wave propagation model, a similarity relation of ion extraction time with different geometrical sizes of the ion extraction regions is established. The theoretical analysis shows that, by changing the magnitude of the externally applied voltage to keep the ion extraction flux equal, the ion extraction time is proportional to the geometrical size ratio. Then, the PIC simulations on the ion extraction process are conducted, which show that there exists a good consistency with the theoretical analysis and previous experimental data. This research is helpful for promoting numerical simulations facing actual ion extraction processes with large geometrical sizes and provides theoretical guidance for improving the ion extraction efficiencies in applications.