A robust, efficient surface ionisation source

A surface ionisation source and ion lens are described which produced resolved beams of ground state Ca+ and Ba+ ions with currents equal to the space charge limit. For example, stable ion currents of 10-7 A were produced for more than 200 hours in a beam 2 mm*3 mm in cross section with 3 keV energy. The source would also be suitable for other volatile metals with similar ionisation energies.