Explanation of plasma densities high above critical density in a cylinder excited by an rf helix

We show that the density characteristics of plasmas created by an rf helix are completely determined by the low-power resonance features of the bounded system, which is constituted by the plasma column and the surrounding helix of finite axial length. Whether a magnetic field is present or not, plasma densities high above critical density are obtained in agreement with the general philosophy previously developed. Experiments already performed without magnetic field support the theoretical predictions.