Separating the location of geometric and magnetic expansions in low-pressure expanding plasmas

A geometric expansion and sharply diverging magnetic field are typically colocated in low-pressure expanding plasma devices to enhance diffusion and drive ion acceleration through the creation of sharply decreasing axial potential profiles. In this study we assess the ion and electron transport for cases in which the geometric expansion is and is not colocated with the diverging magnetic field through the inclusion of a pyrex extension tube, displacing the geometric expansion further downstream. Measurements on axis demonstrate that a directional, accelerated ion population is still created when the the extension tube is included, however the beam has a lower velocity and density than the standard case. The change in beam characteristics is linked to variations in the source density profile which results in a smaller potential drop that is stretched over a larger axial distance. The modification in the source density is found to result from the increased residence time of hot electrons situated on radial magnetic field lines that would normally ionise high density conics downstream of the source, typically reported as a hollow density profile