Microwave waveguide helium plasmas for electrothermal propulsion

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 toward 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 of dielectric material, such as boron nitride, into the flow. The plasma is then stabilized in the recirculation region behind the blunt body. The coupling mechanism between microwaveenergy 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. Spectroscopic measurements yielded values for the electron temperature of the stabilized plasma between 12,000 and 12,800 K.