A team of researchers from the Wuhan University Institute of Technical Sciences proposed a prototype engine that uses air plasma induced by microwave ionization.
Currently, plasma thrusters are considered the simplest and most promising form of propulsion for next generation spacecraft. The news is a few months ago, mid-February 2020 of the latest Pulsar Fusion tests on a 160.000 km / h plasma engine.

A plasma engine simply uses air and electricity to produce pressurized high temperature plasma for jet propulsion.
Laboratory tests
The team used a self-made device that measures lift force and jet pressure at various microwave power and airflow rate settings. Carried out all the tests foreseen by the research, they have shown that, given the same energy consumption, the pressure of the plasma engine is comparable to that of traditional jet engines for aircraft that use fossil fuels.

This means that a carbon-free thruster could potentially be used in the atmosphere as a jet thruster.
Just like solids, liquids and gases, plasma is a normal state of matter.
The mechanism is not complicated: ionized air creates low-temperature plasma which is then moved through a tube by an air compressor. Moving through the tube, the plasma is affected by a microwave pulse that shakes the ions, causing them to crash back into the non-ionized atoms and further increasing the temperature and pressure.
This is what generates the push.
At this point, having ascertained the effectiveness of the system, the goal is to develop a mechanism that is increasingly compatible with installation on existing aircraft.
The next challenges concern the engineering and energy components: will they square the dimensions? Will it be energy sustainable?