SOME OBSERVATIONS ON HIGHER-ORDER NONLINEAR RESONANCES IN A PAUL TRAP

We present some detailed measurements on instabilities of ion trajecto ries in a Paul trap caused by deviations of the ideal quadrupole poten tial by trap imperfections. Numerous lines of reduced stability have b een observed. Their position in the stability diagram can be well expl ained by theory. The identification of a non-linear resonance can be p erformed by a measurement of the ions' macrofrequencies omega(r), omeg a(z). We have observed space-charge shifts of the resonances. From the strength and shape of a non-linear resonance, one may possibly determ ine the contribution of a particular perturbation to the quadrupole po tential, and thus characterize the specific trap.