SPACE-CHARGE EFFECTS AND ELECTRONIC BISTABILITY

The excitation of metastable states in an atomic beam apparatus by mea ns of electron collision is a widespread technique. We have observed a large bistable behaviour in our apparatus designed to provide an inte nse and collimated beam of metastable helium by excitation with orthog onally impinging electrons. This bistable behaviour largely affects th e efficiency of the apparatus and is therefore worth of being carefull y investigated. The apparatus has an electrode configuration equivalen t to that of a tetrode valve with large intergrid distances. The bista bility consists in a hysteresis cycle in the curve of the anode curren t vs. grid voltage. Experimental measurements, supported by a simple t heoretical model and by numerical simulation, stress out the crucial r ole played by space charge effects for the onset of bistability. A com parison with previous observation of this phenomenon is given. Spontan eous current oscillations with various shapes have been recorded in on e of the two curves of the hysteresis cycle.