Double layers in low pressure expanding magnetised plasmas

Abstract

Summary form only given as follows. Electric double layers, cliffs of potential within a plasma, exist in the plasma environment of the Earth and the stars and can cause phenomena as diverse as aurorae, or electromagnetic radiation from rotating neutron stars called pulsars. They may also play an important role in supplying and accelerating plasma in coronal funnels at the surface of the Sun. In the laboratory, double layers spontaneously form in low pressure laboratory expanding plasmas of various geometry and density. In Chi Kung, our helicon plasma system, there is a magnetic-field-induced transition from a simple plasma expansion to a current-free double layer containing expanding plasma. The double layer separates a high potential region in the helicon source, from a low potential region in the expansion region and is the source of particle acceleration: a large area supersonic ion beam is measured in the low potential region for a variety of gases. In the high potential region, low energy electrons trapped in the double layer potential well are Maxwellian with a temperature of 8 eV and the high energy free electrons which flow downstream have a temperature of 5 eV. Although a single upstream plasma source is required to maintain the double layer, the double layer estinguishes when the plasma expansion is forcibly restrained. The current-free double layer in CHI KUNG is being developed as a new electrode- less space plasma engine with infinite lifetime, the Helicon Double Layer Thruster (HDLT), with potential applications to satellite altitude control or deep space missions.