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Abstract:
Over the last decades, electric propulsion (EP) technologies have been playing a crucial role in the space market due to their ability to achieve higher specific impulse compared to chemical thrusters, resulting in improved propellant mass economy [1]. Although many EP thrusters are now highly mature, their lifetime is limited by plasma-induced erosion of critical components, such as electrodes and grids. Electrodeless plasma thrusters overcome this limitation by using electromagnetic interactions to energize the propellant, avoiding direct material contact [2]. The Microwave Electrothermal Thruster (MET) [3] is an electrodeless propulsion device consisting of a cylindrical resonant cavity excited by microwaves (MW). The cavity is designed to operate in the TM011 mode and is divided in half by a dielectric plate, which physically separates the MW excitation region from the discharge one. MWs form and sustain a plasma, which absorbs energy and subsequently transfer it to a swrling propellant. The heated propellant then expands through a solid nozzle, producing thrust. The MET has proven to be highly scalable and compatible with several atomic and molecular propellants, achieving performance comparable to that of arcjets but without the need for electrodes, which increases the operational lifetime.
Challenges in future MET development can be addressed through modeling and simulations [3-5]. Electromagnetic (EM) modeling of the resonant cavity can improve the understanding of how design choices (e.g., dimensions or material) affect the cavity response to the microwave excitatation [4]. Fluid models, on the other end, enable the investigation of discharge physics, its effect on the EM fields and coupling, as well as propellant expansion [5]. However, such analyses may become computationally expensive, especially when propellants with many species and complex plasma chemistry are considered. Global plasma models offer a compromise between physical fidelity, simplicity, and reduced computational costs, as they compute only volume-averaged quantities [3]. They are widely used to estimate key plasma parameters (e.g., composition, electron temperature) of plasma-based technologies (such as electric thruster) under various operating conditions.
This work present the numerical activitities carried out at the Nanolab, Politecnico di Milano, to design and optimize a 2.45 GHz Microwave Electrothermal Thruster and to investigate its behavior and performance under several propellants and operating conditions.
COMSOL Multiphysics ® is employed to design and optimize the resonant cavity, studying its response to MW excitation through the antenna. Considering the resonant frequency, the power reflection, and the electric field distribution as key parameters, the objective is to assess the impact of design choices such as dielectric plate thickness, plate and wall materials, and antenna insertion, using this data to support the design of a first prototype.
Finally, 0D multi-temmperature plasma models are employed to study the discharge and estimate thruster performance (e.g., thrust, specific impulse, and thruster efficiency) for several propellants and operating conditions. The set of particles and power balance equations are solved to get steady-state species densities and temeperatures. These models consider multiple temperatures, each governed by a dedicated power balance equation, because electrons and heavy species are generally far from translational equilibrium [3]. When molecular species are considered, rotational-translational equilibrium is assumed, whereas vibrational kinetics is modeled by assuming that vibrational states follow a boltzmann distribution characterized by a distinct vibrational temperature [6].
References:
1. Levchenko, I.; Xu, S.; Mazouffre, S.; et al., Phys Plasmas (2020), 27
2. Mazouffre, S, Plasma Sources Sci. Technol. (2016), 25
3. M Lauriola et al 2025 Plasma Sources Sci. Technol. 34 105017
4. Micci, M. M.; Bilén, S. G.; Clemens, D. E.; Progress in Propulsion Physics (2009) 1, 425-438
5. Lee, J.; Raja, L. L.; J. Appl. Phys. (2024) 135
6. M. Capitelli et al. Plasma Kinetics in atmospheric gases (2000) Spriger
