Impact of Microwave Coupling and Magnetic Field Topology on Thrust and Beam Parameters in Electron Cyclotron Resonance Thrusters with Magnetic Nozzles

Abstract

Conventional ion thruster technologies face challenges, such as electrode and grid erosion and the requirement of separate neutralizer devices for operating in space. Electric propulsion concepts that utilize electron cyclotron resonance (ECR) for plasma generation can eliminate the need for internal electrodes within the plasma. Furthermore, ECR concepts that employ magnetic nozzles to accelerate the entire plasma for thrust generation remove the need for both, neutralizers and acceleration grids. A new thruster concept, called DEEVA, developed at the German Aerospace Center (DLR), implements a magnetic nozzle for plasma acceleration and employs ECR for plasma ignition and heating. Given its novel design, in this study we explore how the interaction between microwave coupling and the necessary magnetic field topology influences plasma properties, thrust generation, and thruster efficiency. A series of experimental investigations is conducted to address this question. Alongside the development of three iterations of the DEEVA prototype, a reference thruster - the MINOTOR prototype, developed at the French National Aerospace Research Center (ONERA) - is examined. This reference thruster operates at a similar microwave frequency and within comparable power ranges. The presented investigations employ a range of experimental setups, vacuum test chambers, and plasma diagnostics. Tests are carried out in three vacuum facilities of different sizes. Diagnostic tools include a thrust balance, Faraday cups, retarding potential analyzers, and Langmuir probes. Results indicate that the divergent magnetic field topology and coaxial coupling, realized by the MINOTOR thruster, produce higher ion currents and ion energies in the plasma plume than the first DEEVA prototype. By progressively adapting the DEEVA thruster's magnetic field topology in each prototype, allowing for larger ECR zones and higher mag- netic field gradients, the latest DEEVA prototype (DEEVAv2-repulsive) achieves perfor- mance levels approaching those of the MINOTOR prototype. While MINOTOR remains more efficient in the lower power range, findings suggest that adapting the magnetic field in case of the DEEVA concept can balance the power coupling effects. Additionally, DEEVA demonstrates greater flexibility in operating with various propellants and at higher power ranges, as it avoids exposing electrodes to the plasma, unlike the MINOTOR prototype, which uses a rod antenna inside the plasma to couple power. These advantages provide strong motivation to continue refining the DEEVA concept.