A microwave thermal thruster (MET) is a type of electric thruster where the propellant is heated via microwave energy inside a properly designed resonant cavity and then the gas is expanded through a conventional converging-diverging nozzle to produce thrust. Thus, in a microwave electrothermal thruster a microwave resonant cavity acts in a similar fashion to the combustion chamber of a chemical propulsion system. Inside the resonant cavity a free floating plasma, generated by the microwave energy deposition into the gas, heats the gas before the gas is expelled. This study presents a two fluid global model which aims to provide a better understanding of the propellant heating mechanism in the microwave resonant cavity of a MET. COMSOL Multiphysics, a finite element software, is used to solve the appropriate equations to determine the thruster performance and plasma parameters, and the results are presented.

Numerous types of in-space propulsion systems have been developed since the 1960s for satisfying the propulsive needs of satellites and spacecraft. Electrothermal thrusters, a subclass of in-space propulsion systems, produces thrust by the thermal expansion of the propellant heated using electrical energy. Resistojets and arcjets are the kinds of electrothermal thrusters that have been used on various space platforms to date. Resistojets use heater elements, and arcjets use DC or AC arc to heat the gas. On the other hand, Microwave Electrothermal Thruster (MET) concept eliminates certain shortcomings of arcjets and resistojets by using a free floating plasma instead of a resistant heater or electric arc for the heating process. In microwave electrothermal thrusters, the goal is to convert a microwave resonant cavity to a heating chamber of a propulsion system and thus heating the propellant to be expelled using free floating microwave induced plasma. This study presents the preliminary experimental results of the prototype microwave electrothermal thruster developed at the Bogazici University Space Technologies Laboratory (BUSTLab). For the presented experiments, Helium gas is used as the propellant for the thruster operating at 2.45 GHz frequency. In the experiments, chamber pressure, propellant mass flow rate and power delivered to the thruster are directly measured, and the chamber temperature, specific impulse, and the thrust values are evaluated by using appropriate equations. For the tests at a delivered power level of 500 W , a maximum Isp level of 347 s and thrust level of 266 mN is evaluated.

Microwave electrothermal thruster (MET), an in-space propulsion concept, uses an electromagnetic resonant cavity as a heating chamber. In a MET system, electromagnetic energy is converted to thermal energy via a free floating plasma inside a resonant cavity. To optimize the power deposition inside the cavity, the factors that affect the electric field distribution and the resonance conditions must be accounted for. For MET thrusters, the length of the cavity, the dielectric plate that separates the plasma zone from the antenna, the antenna length and the formation of a free floating plasma have direct effects on the electromagnetic wave transmission and thus the power deposition. MET systems can be tuned by adjusting the lengths of the cavity or the antenna. This study presents the results of a 2-D axis symmetric model for the investigation of the effects of cavity length, antenna length, separation plate thickness, as well as the presence of free floating plasma on the power absorption. Specifically, electric field distribution inside the resonant cavity is calculated for a prototype MET system developed at the Bogazici University Space Technologies Laboratory. Simulations are conducted for a cavity fed with a constant power input of 1 kW at 2.45 GHz using COMSOL Multiphysics commercial software. Calculations are performed for maximum plasma electron densities ranging from 1019 to 1021 #/m3. It is determined that the optimum antenna length changes with changing plasma density. The calculations show that over 95% of the delivered power can be deposited to the plasma when the system is tuned by adjusting the cavity length.

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This study presents the Langmuir and Faraday probe measurements conducted to determine the plume characteristics of the BUSTLab microwave electrothermal thruster (MET). The thruster, designed to operate at 2.45 GHz frequency, is run with helium, argon and nitrogen gases as the propellant. For the measurements, the propellant volume flow rate and the delivered microwave power levels are varied. Experiments with nitrogen gas revealed certain operation regimes where a very luminous plume is observed. With the use of in-house-built Langmuir probes and a Faraday probe with guard ring, thruster plume electron temperature, plasma density and ion current density values are measured, and the results are presented. The measurements show that MET thruster plume effects on spacecraft will likely be similar to those of the arcjet plume. It is observed that the measured plume ion flux levels are very low for the high volume flow rates used for the operation of this thruster.