This paper discusses the current research status of the MET (Microwave ElectroThermal) thruster. In the MET thruster, an electrodeless, vortex stabilized, plasma is produced in a microwave resonator cavity for the purpose of heating gaseous fuel to produce a high temperature rocket exhaust for space propulsion. The higher specific impulse (momentum transfer per unit weight) of these heated gases offers advantages over traditional chemical rockets in terms of reduced fuel mass. In MET devices, dense plasmas have been produced in various possible fuel gases, nitrogen, hydrogen, and ammonia, using 600 to 2200 Watts of microwave power at a frequency of 2.45 GHz. Ammonia has been found to give a specific impulse of 550 sec. It has been found that the plasma is a 98% absorber of microwave power leading to negligible reflection of power back to the microwave source and making the cavity operate at low Q. Taking advantage of this effect, it has been found that a very compact MET thruster design could be operated, with the magnetron microwave source and resonator cavity joined in one unit.
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