Modulated plasma potentials and cross field diffusion in a Helicon plasma

Abstract

As the input rf power to a helicon plasma is increased there is a discontinuous increase in the density which is identified with a change from a capacitive to an inductive/wave coupling to the plasma. The radial distribution of the density is much narrower in the inductive (high) mode than in the capacitive (low) mode. Although the time average plasma potential (Vp) decreases markedly at this mode change, the radial profile of Vp is flat in both modes. Measurements with an emissive probe show that in the capacitive mode Vp is strongly modulated (ΔV is order of Vp) at the rf drive frequency indicating the presence of large rf fields in the source. There is also a large radial gradient in the amplitude of this modulation. In the inductive mode the modulation decreases significantly but can still be observed in measurements of the ion energy distribution. It is suggested that in the capacitive mode, the radial diffusion of electrons is driven by the large gradients in the rf fields. As the density increases, the fields are screened out, diminishing the radial electron loss and consequently, the radial ion loss. The reduced radial diffusion of both charged species produces a still higher density as the major loss surface is now reduced to the ends only.