A geometric expansion and sharply diverging magnetic field are typically colocated in lowpressure 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.

In situ and ex situ techniques have been used to measure directional ion beams created by a sharp axial potential drop in low pressure expanding plasmas. Although Retarding Field Energy Analysers (RFEAs) are the most convenient technique to measure the ion velocities and plasma potentials along with the plasma density, they are bulky and are contained in a grounded shield that may perturb the electric potential profile of the expanding plasma. In principle, ex situ techniques produce a more reliable measurement and Laser Induced Fluorescence spectroscopy (LIF) has previously been used to characterise the spatial velocity profile of ion beams in the same region of acceleration for a range of pressures. Here, satisfactory agreement between the ion velocity profiles measured by LIF and RFEA techniques has allowed the RFEA method to be confidently used to probe the ion beam characteristics in the regions of high gradients in plasma density and DC electric fields which have previously proven difficult.

Axial plasma density measurements in a 1.5 m long plasma chamber are presented for when the regions of high magnetic field and radio frequency heating are progressively separated using a movable solenoid pair. The results show that the operating regime changes based on the degree of ion magnetisation under the antenna. When ions are magnetized, electrons heated under the antenna are efficiently transported to the solenoids along a column defined by the magnetic field lines which connect to the antenna region. The cross section of this column decreases due to the converging magnetic field geometry, thereby increasing the density of electrons on the axis. This results in a density profile which is singly peaked and centered on the location of maximum magnetic field strength. When the ions are unmagnetised under the antenna, the flux of positive charges to the wall there is increased. Electrons streaming along field lines that intersect the radial wall in the antenna region are then more attracted to the antenna region to balance this flux. This affects the equilibrium conditions along the entire magnetic field line and results in less efficient transport of electrons heated by the antenna to the region of high magnetic field strength. In this regime, there is a global decrease in plasma density and the axial density profile is doubly peaked.

A thorough understanding of particle transport in expanding plasma devices under different geometric and magnetic configurations is necessary for future thruster and reactor design. Recently, we have shown that the development of high density conics via hot electron transport along the most radial field lines escaping the source region of an expanding plasma device is crucial for the development of a high velocity ion beam on axis. In this study, a gap between the source region and a movable extension tube inserted in the downstream of an expanding plasma device is used to create a geometric window of variable size. Only those electron populations situated on field lines that pass through this window can be transported to the region where high density conics normally form. Critical field lines for the plasma behaviour are identified by increasing the size of the geometric window while concurrently measuring the conics density and ion beam characteristics. A step change in the beam velocity is observed when high density conics are able to fully reform. The field lines responsible for full conics formation are found to be those that pass within  ∼2 skin depths of the source chamber wall. As such, the downstream plasma behaviour is dominated by electrons situated on field lines that are most effectively heated by the radio-frequency antenna and escape the source.

Field-aligned Boltzmann electric triple layer in a low-pressure expanding plasmaTo cite this article: A Bennet et al 2019 Plasma Sources Sci. Technol. 28 06LT01 View the article online for updates and enhancements.