Fractional frequency collective parametric resonances of an ion cloud in a Paul trap

ion cloud ina Paul trap driven at simple fractions of twice the secular frequency of the trap by an additionally appliedquadrupole field. The fractional resonances are observable only if the excitation field surpasses a criticalstrength. Odd-even staggering of the thresholds is observed.@S1050-2947~98!51307-8#PACS number~s!: 32.80.Pj, 07.75.1h