NONLINEAR COLLECTIVE OSCILLATIONS OF AN ION CLOUD IN A PAUL TRAP

In an experiment using a Paul trap, we create a H-2(+) ion cloud by el ectron ionization of the background gas at 10(-9)-mbar residual pressu re. Exciting the ions parametrically at twice the frequency of the sec ular motion of ions in the r or z direction, we observe a narrow reson ance at some distance from the motional resonance center if the amplit ude of the exciting field exceeds a threshold value. The threshold val ue decreases with increasing ion number. Since the narrow resonance do es not shift with ion number, we interpret it as a collective resonanc e of the center of mass of the ion cloud. The resonance shape exhibits the typical form of a driven anharmonic oscillator. The conclusions d rawn from the experiments are supported by detailed analytical and num erical computations. [S1050-2947(97)05211-6].