A charged particle emitting assembly comprises an emitter member (5) for emitting charged particles of one polarity. A tubular shield electrode (6) circumferentially surrounds the emitter member and is held in use at the same polarity as the charged particles. A tubular accelerating electrode (7) is positioned substantially coaxially with the shield electrode (6) and is held in use at the opposite polarity to the shield electrode. The arrangement is such that charged particles from the emitter member (5) initially spread laterally outwardly and then are focused into a beam which passes through the tubular accelerating electrode (7).