A team of researchers at the Institute of Technological Sciences at Wuhan University has demonstrated a prototype design of a propulsion thruster that utilizes air plasma induced by microwave ionization. Such a jet engine simply uses only air and electricity to produce high temperature and pressurized plasma for jet propulsion.

An open-access paper on their work is published in the journal AIP Advances.

The motivation of our work is to help solve the global warming problems owing to humans’ use of fossil fuel combustion engines to power machinery, such as cars and airplanes. There is no need for fossil fuel with our design, and therefore, there is no carbon emission to cause greenhouse effects and global warming.

—Jau Tang, a professor at Wuhan University

Beyond solid, liquid and gas, plasma is the fourth state of matter, consisting of an aggregate of charged ions. It exists naturally in places such as the sun’s surface and Earth’s lightning, but it can also be generated. The researchers created a plasma jet by compressing air into high pressures and using a microwave to ionize the pressurized air stream.

This method differs from previous attempts to create plasma jet thrusters in one key way. Other plasma jet thrusters, such as in NASA’s Dawn space probe, use xenon plasma—which cannot overcome the friction in Earth’s atmosphere—and which are therefore not powerful enough for use in air transportation. Instead, the authors’ plasma jet thruster generates the high-temperature, high-pressure plasma in situ using only injected air and electricity.

Their experimental setup included a magnetron with the power of 1 kW at 2.45 GHz, a circulator, a flattened waveguide, an igniter, and a quartz tube. The magnetron is the microwave source, the circulator is used to absorb reflected microwaves, and a three-stub tuner is used to optimize the power inside the air ionization chamber.

The microwave generated by the magnetron passes through the circulator and the three-stub tuner and reaches the flattened waveguide. The igniter is used to ignite and generate a plasma jet. An industrial cooler is used to cool the circulator and the magnetron.

A schematic diagram of a prototype microwave air plasma thruster and the images of the bright plasma jet at different microwave powers. This device consists of a microwave power supply, an air compressor, a compressed microwave waveguide and a flame ignitor. Credit: Jau Tang and Jun Li.

The prototype plasma jet device can lift a 1-kilogram steel ball over a 24-millimeter diameter quartz tube, where the high-pressure air is converted into a plasma jet by passing through a microwave ionization chamber. To scale, the corresponding thrusting pressure is comparable to a commercial airplane jet engine.

By building a large array of these thrusters with high-power microwave sources, the prototype design can be scaled up to a full-sized jet. The authors are working on improving the efficiency of the device toward this goal.

Our results demonstrated that such a jet engine based on microwave air plasma can be a potentially viable alternative to the conventional fossil fuel jet engine.

—Jau Tang

Resources

• Dan Ye, Jun Li And Jau Tang (2020) “Jet Propulsion By Microwave Air Plasma In The Atmosphere,” AIP Advances doi: 10.1063/5.0005814.