The properties of an ordinary ion guide can be improved by inserting a grid between the nozzle and the skimmer. When positive ions are transported, an electric potential lower than +10 V is connected to the grid and one of the order of - 10 V to the skimmer. Efficient focusing is achieved between the nozzle and the grid by a combined action of the viscous helium flow field and the weak electric field. The focusing device is called a "squeezer". It can be extended by adding more grids, a second skimmer, and differential pumping. Contrary to the first grid, the additional elements act more or less like electrostatic lenses in medium vacuum. With the squeezer ion guide, ions created in a helium chamber can be brought into high vacuum in less than 1 ms without accelerating them to kinetic energies higher than the order of 10 eV. The width of the kinetic energy distribution is typically 2.5-3.5 eV (FWHM). Consequently, problems caused by ion scattering are greatly reduced as compared to ordinary ion guides. The squeezer ion guide was developed and tested with 215Po ions from a radioactive 227Ac source at JYFL as part of a quadrupole mass spectrometer. The ion currents were of the order of 100 ions/s. A typical yield through the first skimmer was 75% and through the second skimmer 65% of that through the nozzle. Later on a similar device was installed in the GARIS-IGISOL mass separator at RIKEN. It was tested with Ar and Xe ions froma gas discharge. Separated ion currents up to several nA were used. The mass resolution was improved by more than a factor of two. A squeezer ion guide tested at the JYFL IGISOL gave promising results in on-line experiments with fission products.
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