Plasma acceleration in a magnetic arch

Abstract

When two magnetic nozzles with opposite polarity are placed side by side, a ‘magnetic arch’ (MA) is formed, which connects the field lines of each nozzle into a closed-line configuration. The plasma expansion and acceleration in this magnetic topology are relevant for clusters of electrodeless plasma thrusters, as well as novel, non-cylindrical thruster architectures. A collisionless, quasineutral, two-fluid model of the plasma expansion in a MA, is introduced. The plasma properties (density, electron temperature, electrostatic potential, ion velocity, electric currents) in the 2D planar and zero plasma-beta limit are analyzed, and the magnetic thrust density is discussed. It is shown that the ions coming out of the two nozzles meet on a shock-like structure to form a single beam that propagates beyond the closed lines of the applied magnetic field, generating magnetic thrust. A small magnetic drag contribution comes from the final part of the expansion. The plasma-induced magnetic field is then computed self-consistently for non-zero plasma-beta expansions, showing that it stretches the MA in the downstream direction and helps reduce that drag contribution. Finally, the limitations of the present model are discussed.