Abstract: Based on the particlein cell（PIC）method，a numerical simulation on iodine RF ion thruster optics system was carried out to analyze characteristics of ion beam.Besides substantial I⁺，ionization products of iodine contain a little multivalent ions including I₂⁺，I²⁺，I³⁺.Comparison of ion beam characteristics′ simulation results with and without addition of multivalent ions obtained spatial distribution of ions，potential distribution，phase space distribution，beam current and divergence angle.The results show that the program can simulate the motion of ions in optics system properly.With addition of multivalent ions，plasma potential increases and saddle point potential declines，though the variation range of the whole potential distribution is limited.The charge-to-mass ratio of the ion group increases slightly with addition of multivalent ions，therefore beam current increases and divergence angle declines a little.Both thrust and specific impulse increase slightly through theoretical analysis.When the plasma density in discharge chamber increases to about 2.4×10¹⁷m^-3，the optics system reaches the limit of beam extraction，and extracted beam current decreases instead of increasing with proceeding increase of plasma density.The simulation results can provide a reference for design of RF ion thruster optics system.

Key words: RF ion electric propulsion, iodine, optics system, PIC method, ion beam characteristics