Ionizer for gas cluster ion beam formation

A neutral beam ionizing apparatus for electron impact ionization of a substantially cylindrical neutral beam. The apparatus includes an electron source, and a circularly cylindrical ionizing region that is substantially free of magnetic fields. In one embodiment of the invention, the beam is a gas cluster beam, and the electron source includes a heated filament (1000) for emitting thermions, the filament including one or more direction reversals shaped to produce self-nulling magnetic fields so as to minimize the magnetic field due to filament heating current. In another embodiment of the invention, a neutral beam ionizing apparatus for electron impact ionization of a substantially cylindrical neutral beam includes at least one electron source (1402), and an elliptically cylindrical ionizing region (Fig.15). In one embodiment, the elliptically cylindrical ionizing region includes a pair of co-focal elliptically cylindrical electrodes biased so as to cause electrons emitted from the at least one electron source to orbit repeatedly through the axis of the beam to be ionized.