Magnetohydrodynamic scenario of plasma detachment in a magnetic nozzle

Some plasma propulsion concepts rely on a strong magnetic field to guide the plasma flow through the thruster nozzle. The question then arises of how the magnetically confined plasma can detach from the spacecraft. This work presents a magnetohydrodynamic (MHD) detachment scenario in which the plasma flow stretches the magnetic field lines to infinity. Detachment takes place after the energy density of the expanding magnetic field drops below the kinetic energy density of the plasma. As plasma flows along the magnetic field lines, the originally sub-Alfvénic flow becomes super-Alfvénic; this transition is similar to what occurs in the solar wind. In order to describe the detachment quantitatively, the ideal MHD equations have been solved for a cold plasma flow in a slowly diverging nozzle. The solution exhibits a well-behaved transition from sub- to super-Alfvénic flow inside the nozzle and a rarefaction wave at the edge of the outgoing flow. It is shown that efficient detachment is feasible if the nozzle is sufficiently long.