EVIDENCE OF RADIAL-AXIAL MOTION COUPLINGS IN AN RF STORED ION CLOUD

The dynamics of an rf stored ion cloud is a demonstrative experimental illustration of non-linear equations such as those which are obtained when additional terms are considered in the Mathieu equation. One man ifestation of the non-linearity is frequency locking leading to ration al ratios between the frequencies of the ion motion. Macro-micro motio n coupling modes were often observed during the last few years. We rep ort here on radial-axial coupling modes, which were investigated from the variation of the number of stored ions, in absence of a buffer gas , as a function of the working point in the stability diagram. A pseud opotential built to take into account the additional terms permits the ion trajectory to be modeled with forced Duffing equations, whose spe ctral properties allow an understanding of the experimental results.