Generally an assembly composed of: (i) an ionization chamber in which a stream of electrons flows from a hot filament across a stream of gas to collector (the potential between filament and collector is usually between 50 and 70 V); (ii) a device for the acceleration of these ions.
Source:
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2200 (https://doi.org/10.1351/pac199062112167)