A Monte Carlo Simulation for Distribution of Cs Flux at Cs Nozzles of Negative Ion Sources

Radio frequency negative ion source is the best choice for neutral beam injection (NBI) in future nuclear fusion. Cs injection is an important means to increase the yield of negative hydrogen ions. In addition, the stability of the ion source is influenced by the distribution of Cs inside the source in the vacuum phase and plasma phase. Liquid Cs is heated into Cs vapor and then diffused into the source through a nozzle, the shape of which has a direct effect on the angular distribution of the Cs flux. This article studies the ballistic transport process of simulating three types of nozzles: cylindrical, bent, and edge-cut by developing a Monte Carlo program. The results show that the angular distribution of Cs flux from cylindrical nozzles approximates the cosine square distribution in the case of the same nozzle diameter. In comparison, the peak coverage density increases with a certain shift of the distribution state relative to the cosine square for bent and edge-cut nozzles. By analyzing the flux distribution of different nozzles, this study provides a reference for subsequent simulations of Cs and the design of Cs injection systems.