Ion beam apparatus with separately replaceable elements

In an ion beam apparatus wherein ions are generated by the collision of electrons from a filamentary emitter with gas atoms in the vicinity of an electron-attracting grid structure, with subsequent extraction of the ions so generated from the vicinity of the grid structure and the focusing of these ions into a beam by an electrostatic lens system, a new way of mounting the ion extractor and the electrostatic lens system is disclosed which facilitates the removal and replacement of the filament and the grid structure. A configuration of the grid structure is also disclosed which optimizes the profile and the current density of the ion beam