Influence of anharmonicities of a Paul trap potential on the motion of stored ions

Real Paul traps contain higher-order contributions to the ideal pure quadrupole potential. The influence of these perturbations on the motion of a laser-cooled trapped ion cloud is manifested by couplings between the different eigenfrequencies of the ion motion and by a disappearance of laser-induced fluorescence at certain operating conditions of the trap. The observation can be explained by the model of anharmonic oscillations and by energy gain from the a.c. trapping field at non-linear resonances. We find evidence for two stable spatial ion density distributions. This leads to an understanding of earlier, so far unexplained observations.