Octupole Ion Guide

Exhibit no. 960

RF-only multipole ion guides can be used for transporting ions from one part of a vacuum system to another in a differentially pumped system. They consist of metal rods arranged symmetrically around a central axis. The rods are separated into two pairs, alternating in such a way that each rod is neighbored by rods of the other pair. The high-frequency voltage is applied to one set of rods, while the same but phase-opposite voltage is applied to the other set of rods. RF-only multipole ion guides are typically used in higher-pressure regions (1 Torr - 1 mTorr), where ions undergo regular collisions.  Reducing kinetic energy makes it easier for the ion guide to focus the ion beam. The effective mechanical potential near the centerline is relatively flat in higher-order multipoles like octupoles. Consequently, such guides accept a widely diverging ion beams and the space charge forces are limited, but the beam focusing properties are worse compared to e.g., quadrupoles.