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In electron storage rings, positive ions are created by the ionization of the residual gas. Three main collision types including elastic collision, excitation collision and ionization collision are considered in this paper. In order to calculate the probability of the gas ionization using DSMC method, the total cross section, total inelastic cross section, elastic cross section, excitation cross section and ionization cross section are deduced separately. Once ions are created，PIC is adopted to trace the ion，s motion under the combined action due to externally applied field and self-field. The purpose of all study is to present how ion cloud gets to equilibrium little by little.
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