Selective radial release of hot, magnetised electrons downstream of a low-pressure expanding plasma

Abstract

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.