an ion source configured to produce ions within an ion chamber, the ions having an ion path; and

an ion guide positioned downstream of the ion source in the ion path, the ion guide including a plurality of electrodes arranged around a central ion flow axis, each of the plurality of electrodes having a continuously rounded inner surface in a longitudinal plane extending through the ion flow axis, the plurality of electrodes being arranged into a plurality of electrode pairs, each electrode pair having two electrodes opposed across the central ion flow axis; and a radio-frequency (RF) voltage source for applying an RF voltage to the plurality of electrodes.