Abstract: An electron impact ion beam source is provided with a pressure chamber to confine a specific high pressure area within excited gas to a small enough volume that the source can be operated at relatively high pressure and still achieve substantial brightness of the extracted ion beam. In particular, the area is configured such that the overall linear dimension along the beam path is less than the mean free path of the ions and the electrons within the chamber. If pressure is increased, the linear dimension must be correspondingly decreased to maximized brightness. By keeping linear dimensions sufficiently small, both incident electrons and extracted ions are enabled to transit the source region without significant energy loss. The new source design allows operation at pressures at least an order of magnitude higher than other known ion sources and thus produces an order of magnitude higher brightness.