Microwave electrothermal thrusters using waveguide heated plasmas

The absorption of microwave energy by a gas within a waveguide is being investigated for use in an electrothermal propulsion device. Plasmas created within a waveguide propagate towards the source of microwave energy and thus must be stabilized by some means. A very simple and highly efficient way of stabilizing the plasma is inserting a bluff body made out of dielectric material such as boron nitride into the flow. The plasma will then be stabilized in the recirculation region behind the blunt body. The coupling mechanism between microwave energy and waveguide plasmas was investigated to yield a quantitative understanding of the dependency of the coupling efficiency on such parameters as mass flow rate, power level and gas pressure. Maximum coupling efficiency values up to 90% could be obtained, being a strong function of the mass flow rate. So as to avoid erosion problems possible in the rectangular TE10-mode, a new circular waveguide applicator was designed and is currently under construction. Experiments with the circular waveguide are scheduled to start in the near future.