Mechanism for the influence of the geometric parameters on the electrical characteristics of Penning ion sources

A steady-state Penning ion source is studied experimentally. Depending on the geometric parameter I, (the anode length to diameter ratio) and pressure, maxima are observed in the discharge current and in the ion beam current extracted from an aperture at the center of the cathode. It is shown that at pressures of the order of 10(-4) Torr, two maxima appear in these currents: one, for short discharge gaps with l(a) = 1-1.5, corresponds to a diverging ion beam, and the other, for longer anodes with l(a) = 4-5, to a collimated ion beam. At pressures of the order of 10(-5) Torr, only one maximum appears in these currents, for short anodes l(a) = 2-3 with a diverging ion beam. A physical explanation is proposed for these findings. (C) 1998 American Institute of Physics. [S1063-7842(98)00609-6].