Confinement of pulse-injected external ions in a radiofrequency quadruple ion trap

Abstract

The phase-space method is used to evaluate the ion confinement properties of a radiofrequency (r.f.) quadrupole trap with phase-synchronized switching-on of the driving r.f. field for pulsed ion injection from an external source. Singly charged ions of mass 100 u and energy 10 eV are considered. The calculation is performed for a well-collimated pulsed ion beam with a circular crow-section 1 mm in diameter, injected into an r.f. quadrupole trap with characteristic dimension r o = 5.4 mm. Ion injections along the gap and through an aperture in one of the electrodes, including off-axial- and off-equatorial-plane incidence directions, are considered. The time delay interval, determined by the intersection of the unperturbed ion trajectory with the critical volume for ion confinement, is shown to be a measure of the ion confinement property. The most favorable initial r.f. voltage phase-angle corresponds to the value when the time delay interval is greatest. Ion confinement efficiency depends on theion pulse duration. In general a shorter pulse duration leads to a higher trapping efficiency. Off-axial and off-radial injections of the ion beam are superior to the corresponding axial and radial injections. Trapping of the injected ions is shown to be effective, with the efficiency reaching unity for many possible incidence directions and pulse durations of the pulsed ion beam.