Comparative Study of Electron Temperature and Ion Energy in Two Different Magnetic Nozzle Thruster Designs

Abstract

Typical obstacles when operating conventional ion thruster technologies are erosion of electrodes and grids and the need for additional neutralizer devices. An electrode-less concept for plasma generation, e.g., by employing an electron cyclotron resonance (ECR) excitation approach, is therefore easily motivated. The same holds for employing a magnetic nozzle concept for accelerating the entire plasma to produce thrust and, thus, obviating the necessity for a neutralizer. Part of the project 'Decentralized Energy Electric Propulsion' (DEEP) jointly conducted by various institutes of the German Aerospace Center is to develop an ECR thruster which overcomes both obstacles. In this paper, we present a new thruster concept which allows electrode-less plasma generation by microwaves, fulfilling the ECR condition. Plasma acceleration is achieved by the diverging magnetic field of a magnetic nozzle. This new thruster is developed by the German Aerospace Center within the DEEP project. It is compared with a prototype of the well-known thruster concept developed by the Office national d’études et de recherches aérospatiales under the project 'Magnetic Nozzle Electron Cyclotron Resonance Thruster' (MINOTOR). Both thruster concepts employ a magnetic nozzle, are of similar size, and are designed to operate within a similar frequency and power range. However, they differ in terms of the concept for microwave coupling into the plasma. The microwave coupling in case of MINOTOR is achieved by a coaxial coupling structure, where the inner conductor is directly exposed to the plasma. The new DEEP thruster concept pursues a different approach. It uses an electrode-less coupling by an annual waveguide (ring cavity) through two resonant coupling slots into the plasma discharge chamber made of quartz. Comparing both thrusters under the same operating conditions allows us to carry out detailed studies of the impact of the microwave coupling method used on thrust and plasma parameters. We vary the operation conditions of the thrusters and conduct thrust balance, Faraday cup as well as Langmuir probe measurements to map the thrusters’ parameters and to obtain a direct comparison. In terms of operation parameters, we discuss the dependence on input power, excitation frequency and propellant flux. Correlating the results of the different diagnostic approaches and the direct comparison of the performance of both thrusters allows us to identify the impact of the two different excitation schemes on performance. This in turn can be correlated with the plasma and beam properties. By examining trends arising from variations in operational parameters, we can also draw conclusions about the influence of microwave coupling on plasma parameters and, consequently, thrust generation. Considering the early development stage of the new DEEP thruster, it is not clear yet whether the performance parameters of the optimized MINOTOR thruster are achievable in the future. However, if this were the case, the major obstacle limiting the lifetime of ECR thrusters, i.e., electrode erosion, would be overcome. The novel concept of microwave coupling into the plasma introduced by the DEEP project may pave the way for long-term space missions employing ECR thrusters.