CONFINEMENT OF IONS INJECTED INTO A RADIOFREQUENCY QUADRUPOLE ION TRAP - ENERGY-SELECTIVE STORAGE OF PULSE-INJECTED IONS
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abstract
Energy-selective storage of a pulsed ion beam injected into a radiofrequency (r.f.) quadrupole ion trap of characteristic dimension ro = 0.54 cm is numerically evaluated. The calculation is performed for a well-collimated pulsed beam of singly charged ions of mass 100 u, injected along the gap between the electrodes or through an aperture at the mid-point of the ring electrode or at the apex of one of the endcap electrodes. Since the ion confinement property of the r.f. quadrupole trap for externally injected ions is energy dependent, phase-synchronized switching-on of the driving r.f. field can be employed to trap only a narrow energy portion of the ion beam, which is incident with a large spread. It is shown that the magnitude and the width of the accepted ion energy can be selected and that energy-selective storage of ions can be achieved in two different ways. The first way is to choose suitable trap operation parameters to achieve a very narrow time delay interval for ion confinement at theion energy of interest. The second way is to allow a field-free drift distance before ion injection and to take advantage of the energy spread of the ion packed. In both ways an energy window of less than 1 eV at an ion energy of 10 eV cam be obtained. When the ion trap is operated at the mass-selective tip of the stable region instead of at the center, an even higher energy-selectivity is possible. However, this goes at the expense of a lower ion confinement efficiency. 1981.
