Authors: Dan Ye, Jun Li, and Jau Tang
Abstract . . .
We propose 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. We used a home-made device to measure the lifting force and jet pressure at various settings of microwave power and the airflow rate. We demonstrated that, given the same power consumption, its propulsion pressure is comparable to that of conventional airplane jet engines using fossil fuels. Therefore, such a carbon-emission free thruster could potentially be used as a jet thruster in the atmosphere.
Similar to solids, liquids, and gases, plasma is a normal state of matter. Plasma naturally arises due to the ionization of molecules at high temperatures (such as in the sun) or in high electric fields (such as in lightning). In the laboratory, plasma can be generated using an electric arc, microwave cavity, laser, fire flame, or discharging high-voltage needle. Plasma has wide applications in many areas, including metal processing crystal growth, medical treatment, food processing, energy, and environmental industries.
Plasma jet thrusters have also been used in aerospace applications for many years. The jet thruster using xenon plasma in a spacecraft exerts only a tiny propulsion force and can only be used in outer space in the absence of air friction. Even though such a plasma engine has a very small propulsion force, after months and years of constant acceleration, the spacecraft can ultimately reach a high speed. However, this type of engine, like that of the NASA Dawn space probe is not useful in the atmosphere environment.
Recently, a research team from MIT demonstrated a plasma-powered glider that can operate in the air by using a needle-discharge array to generate air plasma to power the flight. This team demonstrated a continuous flight time of 12 s and a flight distance of 55 m. However, this Tesla type of plasma thruster has a lifting force and jet pressure of only 6 N/kW and 3 N/m2, respectively.
It is very challenging for this approach to become feasible for use as a powerful engine for actual air transportation. In this report, we consider a microwave air plasma jet thruster using high-temperature and high-pressure plasma generated by a 2.45 GHz microwave ionization chamber for injected pressurized air. We propose a simple prototype plasma jet thruster that can generate approximately 10 N of thrust at 400 W using 0.5 l/s for the airflow, corresponding to the lifting force of 28 N/kW and a jet pressure of 2.4 × 104 N/m2. At a higher microwave power or greater airflow, propulsion forces and jet pressures comparable to those of commercial airplane jet engines can be achieved.
Our experimental setup is shown in Fig. 1 and includes 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 length, width, and height of the waveguide are 600 mm, 90 mm, and 50 mm, respectively. The flattened part of the waveguide has a height of 25 mm. The flat area of the waveguide is designed to increase the electric field strength. The microwave generated by the magnetron passes through the circulator and the three-stub tuner and reaches the flattened waveguide. This flat part has a circular opening for the insertion of a quartz tube with an inner diameter of 24 mm, an outer diameter of 27 mm, and a length of 600 mm. The quartz tube passes vertically through the wall of the flattened waveguide tube and the central axis of the tube located at a quarter wavelength from the short end of the waveguide.
The igniter is used to ignite and generate a plasma jet. An industrial cooler is used to cool the circulator and the magnetron. We use an air compressor and an airflow meter to generate and condition the high-pressure air into the quartz tube. Air enters the quartz tube from the side, forming a vortex that keeps the plasma jet stable in the tube.
As shown in Fig. 2, variation in the microwave power affects the length of the air microwave plasma jet. Our observation indicates that the length of the flame increased with increasing power. In addition, changes in the injected airflow also affect the flame length.