A cold-cathode source of low-energy low-divergent broad ion beams

Abstract

This article presents a study of the effect of a magnetic field (5–30 mT) and the potential difference between the discharge plasma and the screen electrode of the accelerating–decelerating ion optics on the performance of the ion plasma emitter and on the parameters of the ion beam in a glow-discharge-based ion source. Used in the experiment was a modified Penning ionization gauge electrode system in which ions are extracted along the magnetic field and the discharge operates stably with a current of ∼1 A at low gas pressures (∼10−2 Pa). It has been shown that the changes in the radial distribution of the ion beam current density (1–10 mA/cm2) with increasing magnetic field are due to an increase in the radial gradient of the electron temperature in the plasma. With an optimal B field strength of ∼15 mT, a nearly uniform (within ∼10%) current density distribution has been obtained over an area of ∼50 cm2. It has been demonstrated that the angular divergence of low-energy (∼0.5–1 keV) ion beams can be reduced with a simultaneous increase in the potential difference between the plasma and the screen electrode and in the electric field in the acceleration gap of the ion optics; however, the maximum beam current density, which is achieved thanks to the increase in the plasma density under conditions when the electric field in the accelerating gap approaches the breakdown value, drops with growing potential difference. The experimental results obtained are explained qualitatively based on the changes of the position and shape of the plasma boundary on varying the parameters of the ion space charge sheath between the plasma and the screen electrode of the ion optics.