Measurements of current-free double layers in two expanding helicon plasma sources

This work concerns measurements of parallel ion flow in expanding plasmas generated by two different helicon plasma sources. The measurements indicate the existence of a current-free double layer in a region of diverging magnetic field. With laser-induced fluorescence (LIF), the double layer structure in both helicon plasma sources was investigated through measurements of the bulk parallel ion flow speed. Both double layers have a total potential drop of 3-4\textit{kT}$_{e}$. A stronger double layer, with a potential drop of $\sim $ 6\textit{kT}$_{e}$, was created in a uniform magnetic field region with a plasma limiting aperture plate.