Discharge characteristics of a saddle-field ion source

Abstract

The discharge characteristics of a cylindrical, saddle-field ion source, operating with nitrogen in the oscillating mode, have been measured using a Langmuir probe capable of being traversed along the axis of the source. Measurements made closer than 30 mm from the anode plane cause the source to become unstable. Outside this limit the discharge is found to have an electron density of approximately 2*1014 m-3, rising to approximately 5*1014 m-3 close to the cathode end surface. The electron temperature rises from 2.5*105 K to 4*105 K in the same distance range. The space potential falls monotonically from 580 V, 20 mm from cathode, to 220 V in the cathode aperture. The potential distribution is similar to the vacuum potential predicted by solution of Laplace's equation, but is about 200 V higher at any given point. Measurement of the ion energy distribution shows only two peaks, one at 200 eV, the other at 63% of the anode potential, essentially exactly that predicted for the saddle-point potential by solution of Laplace's equation, It is shown that this is not an effect of cathode geometry. A model is proposed to account for these effects based on spatially selective neutralisation of the ions within the source. This can arise because the electron density along the device axis is lower than anywhere else in the ion-forming region. The spatially selective neutralisation of ions by electrons within the source proceeds with a higher cross section for N2+ than N+ leading to an ion output largely comprising N+.