Numerical investigation of the electric field distribution and the power deposition in the resonant cavity of a microwave electrothermal thruster

Abstract

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.

Characterization of a radio-frequency inductively coupled electrothermal plasma thruster

“…37 Figure 11 shows measurements of the RF antenna current, power transfer efficiency, and stagnation temperature as a function of mass flow rate (blue open markers), as well as estimates of the thrust, specific impulse, and thruster efficiency. The thrust cannot be directly measured in the current experiment, so it is instead 6), (7), and (11), and using the measured stagnation pressure and temperature, and the nozzle dimensions. Although the nozzle is over-expanded due to the vacuum chamber pumping speed (see Sec.…”

“…Since there are no immersed electrodes, no erosion occurs and significantly higher lifetimes are possible. While METs have been extensively studied in the literature, [6][7][8][9][10][11][12] they have only recently flown in space. METs require high frequencies and are resonant devices where the thruster size and operating frequency are strongly coupled.…”

Design and Construction of a 2.45 GHz Microwave Electrothermal Thruster

“…On the other hand, Yildiz and his colleagues numerically studied the electric field distribution and power absorption in the cavity to intensify a microwave electrothermal thruster. Using a two-dimensional symmetric model, they investigated the effect of cavity length, antenna length, and separator plate thickness on wave power absorption in the MET thruster at the Bogazici University Space Technology Laboratory [6]. They also geometrically optimized the cavity to intensify a 2.45 GHz microwave electrothermal thruster.…”

Plume diagnostics of BUSTLab microwave electrothermal thruster using Langmuir and Faraday probes

“…In MET systems, electromagnetic energy is transmitted into the cavity and a standing wave is formed. The free electrons in the propellant gas are coupled to the electric field of the wave and they are energized [16,17]. New electrons are produced after successive collisions between electrons and the neutral atoms.…”