A numerical investigation of the micronozzle geometry effect on the performance of microwave electrothermal thrusters

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The Microwave Electrothermal Thruster (MET) is a promising electrothermal micro-thruster for small satellite propulsion. The design of the MET utilizes a TM011 microwave resonant mode in a circular waveguide cavity, which creates regions of high electric energy density for plasma generation and propulsion. The nozzle attached to the cavity also plays a critical role in determining the MET performance. However, as the size of the MET resonant cavity is scaled down to fit micro- and nanosatellites, the nozzle attached to the cavity becomes smaller, presenting challenges for experimental investigations into the behavior of the plasma discharge and the gas flowing through the micronozzle. This paper presents a numerical study encompassing the multi-physical phenomena of electromagnetic field wave, plasma, and flow dynamics to fully understand the behavior of plasma discharge and gas flow through a micro nozzle attached to a scaled-down MET resonant cavity. The thrust and specific impulse of the MET are being analyzed by comparing three different types of nozzles - extended convergent-divergent (CD), standard CD, and divergent nozzles - that have the same throat diameter and exit area. The aim of this analysis is to identify design enhancements that could improve the performance and dependability of the system. The results indicate that the geometry of the nozzle has a significant impact on the resonant cavity's performance in terms of the amplification factor and plasma discharge properties. The divergent nozzle shows potential benefits for the MET thruster due to concentrated electron absorbed power close to the nozzle inlet. However, during plasma gas flow operation, it results in a lower specific impulse than the other two cases due to significant heat transfer to the cavity wall near nozzle inlet, leading to a loss of energy source for heating gas. Further investigation is suggested to improve specific impulse by reducing heat conduction to the cavity wall and enhancing the Joule heating effect in that case, which could significantly improve overall MET efficiency.