A Hollow-Cathode Ion Source in Glow Discharge Mass Spectrometry

Abstract

The paper proposes a new glow discharge ion source based on a hollow cathode. The proposed source produces a denser plasma than the previously used glow discharge sources. It is shown experimentally that the maximum rate of cathode sputtering of a conducting solid sample in a hollow cathode ~25 mm in diameter is achieved when the sample has the shape of a rod with a diameter of 2–4 mm and a length of 10–15 mm and is placed along the axis of the cathode cavity. If a cover with a hole is placed on the cathode cavity, a plasma bunch is formed near the hole (a “plasma plug”), which resists the flow of argon from the hollow cathode. The pressure in the discharge chamber decreases, and the glow discharge is concentrated in the cathode cavity. This results in a decrease in the background level in the glow discharge source by approximately two orders of magnitude. The mechanism of formation and extraction of ions from a glow discharge ion source is considered. The ions formed in the glow discharge plasma form two oppositely directed ion flows in the source, one of them with an ion energy of 100 eV or higher. These ions, accelerating in the region of the cathode potential drop, are directed to the cathode, bombard the analyzed sample, and spray it. The second flow of low-energy ions is drawn from the same region of negative luminescence and is transported in the opposite direction, that is, to the surface of the discharge chamber-anode due to ambipolar diffusion. These ions are extracted from a hole in the anode chamber of a conventional ion source by an accelerating voltage and used for mass spectral analysis. The use of a hollow cathode with a plasma plug, the effect of ambipolar diffusion, and a modified Pierce lens makes it possible to increase the luminosity of the glow discharge source by more than one order of magnitude.