Plasma plume symmetrization by electron rotation and ion detachment properties in a diverging magnetic nozzle

The ion flux direction in a rapidly diverging magnetic field and the role of Earth's magnetic field in plasma dynamics are investigated experimentally using a low power electron cyclotron resonance thruster. The comparison between ion trajectories measured with a directional Faraday cup and the magnetic field geometry allows us to assess some of the ion flow properties. The results show that the ion trajectories follow the magnetic field lines in the center part of the plasma plume but not on its edges, defining the detachment location. Once detached, ions follow straight line trajectories, and their detachment has a divergent nature. The effect of the thruster operating condition on these properties is explored. Plasma potential measurements carried out with an emissive probe reveal the presence of a large radial electric field, pointing outward at the detachment point, that explains the divergent nature of the ion detachment. The amplitude of the induced E×B drift rotation of the electrons in the nozzle damps the disymmetrizing influence of Earth's magnetic field on the electron dynamics and leads to an axisymmetric plasma plume. This mechanism is demonstrated using single particle simulations.