Plasma detachment from a magnetic nozzle

Abstract

Magnetic nozzles are used to convert plasma pressure into directed flow, thereby providing thrust for propulsion. The detachment of the flow from the magnetic field must be highly directed or the thrust efficiency will be low. A simple model of flowing, cold plasma in an axisymmetric nozzle is considered to determine the fundamental physics and scaling of separation in the limiting case that it is determined by plasma inertia. Ion and electron motions are coupled by the ambipolar electric field. In the limit of negligible meridional electric current, separation from the field is determined by the hybrid (electron-ion) Larmor radius evaluated at the initial magnetic flux and flow velocity. It is shown that the detachment is thereby significantly constrained, but that significant improvement can be obtained if electric currents can flow across the magnetic field either in walls or as plasma currents, or if dissipative effects are sufficiently strong.