Influence of space charge on the computed statistical properties of stored ions cooled by a buffer gas in a quadrupole rf trap

We use an improvement of a three-dimensional model [Phys. Rev. A 27, 2321 (1983)], taking space charge into account for ions confined in a quadrupole rf trap. This model allows us to calculate the spatial and energy probability densities of strongly confined ions in the presence of a light buffer gas and shows the evolution of these properties as the total number n of ions increases. The ions are Ce⁺ in the presence of dilute helium. For the chosen working point, the distributions remain Gaussian when n does not exceed (2-3) ×{} 10⁵ ions at room temperature. The influence of the temperature on the importance of space-charge phenomenon is also studied.