Properties and limitations of an ion guide

Abstract

A flexible electrode hose of 1 m length and with a diameter of 18 mm, consisting of two helically wound wires has been built. Its ability to guide a low-energy Ar ion beam of 1 keV and 100 μA has been tested with electrode potentials up to 500 V. The Smooth Approximation of the equation of motion leads to a focussing effective boundary potential and to a critical glancing angle for ion beam reflection. Similarities with light and neutron guides are discussed. Measurements of the guided ion beam show that the beam is attenuated by atomic charge transfer along the 1 m path due to gas flow from the ion source, that beam losses occur due to fringing fields at the entrance and exit of the guide, which could be prevented in an improved version, and that the defocussing potential caused by the space charge of the low-energy heavy-ion beam diminishes the critical glancing angle severely. With thermal electrons the ion beam cannot be charge-neutralized within the two-wire structure.