Experimental measurements of ion trajectories and plasma detachment in the magnetic nozzle of an ECR thruster*

Several electric propulsion devices currently under development use a diverging magnetic field, called magnetic nozzle (MN), to accelerate and eject plasma and produce thrust. The processes at stake in the demagnetization phenomenon, called detachment, of the plasma expanding in the MN are a key to optimize the design of those devices. We introduce a new electrostatic probe, the directional Faraday probe, inspired from the classic Faraday cup design. This probe is able, thanks to the geometry of its collectors, to measure the local angular distribution of ion current density. Therefore, it gives access to the ions trajectories in the plume of electric propulsion devices. Measurements made with this diagnostic in the MN of an ECR thruster are compared with the magnetic field topology. This analysis yields valuable information on the location of the plasma detachment which can be confronted to theoretical models. Preliminary results showed that several centimeters away from the thruster source, the ion trajectories remain collinear to the magnetic field lines at the center of the plume while a converging detachment occurs at the edges of the MN. Moreover, plasma parameters seem to have a strong influence on the plasma expansion and detachment.