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 q uadrupole potential. The influence of these perturbations on the motio n of a laser-cooled trapped ion cloud is manifested by couplings betwe en the different eigenfrequencies of the ion motion and by a disappear ance 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-li near resonances. We find evidence for two stable spatial ion density d istributions. This leads to an understanding of earlier, so far unexpl ained observations.