Interactions between electromagnetic radiation and ionized gas have been extensively studied for many purposes, including concealing aircraft from radar as stealth technology.
In 1956, Arnold Eldredge, of General Electric, filed a patent application for an "Object Camouflage Method and Apparatus," which proposed using a particle accelerator in an aircraft to create a cloud of ionization that would "...refract or absorb incident radar beams."
[2] During Project OXCART, the operation of the Lockheed A-12 reconnaissance aircraft, the CIA funded an attempt to reduce the RCS of the A-12's inlet cones.
A series of high voltage spark gaps were used to generate UV radiation, which creates plasma via photoionization in a waveguide.
[7] Despite the apparent technical difficulty of designing a plasma stealth device for combat aircraft, there are claims that a system was offered for export by Russia in 1999.
[8] The Journal of Electronic Defense reported that "plasma-cloud-generation technology for stealth applications" developed in Russia reduces an aircraft's RCS by a factor of 100 (20 dB).
Almost all the matter in the universe is very low density plasma: solids, liquids and gases are uncommon away from planetary bodies.
In order to obtain substantial attenuation of radar signal, the plasma slab needs adequate thickness and density.
When electromagnetic waves, such as radar signals, propagate into a conductive plasma, ions and electrons are displaced as a result of the time varying electric and magnetic fields.
However, plasma stealth implies a substantial reduction of an aircraft's RCS, making it more difficult (but not necessarily impossible) to detect.
When faced with frequency hopping radar, it is possible, at least in principle, to change the plasma temperature and density to deal with the situation.
Last but not least, it is extremely difficult to produce a radar-absorbent plasma around an entire aircraft traveling at high speed, the electrical power needed is tremendous.
There have been several computational studies on plasma-based radar cross section reduction technique using three-dimensional finite-difference time-domain simulations.
Chung studied the radar cross change of a metal cone when it is covered with plasma, a phenomenon that occurs during reentry into the atmosphere.
The article is entitled "Radar cross sections of dielectric or plasma coated conducting spheres and circular cylinders" (IEEE Transactions on Antennas and Propagation, September 1963, pp. 558–569).
Naturally, an aircraft would have a far more elaborate shape and be made of a greater variety of materials, but the basic effect should remain the same.
The authors of the paper found that a dielectric (plasma) shell may either decrease or increase the echo area of the object.
This would require a large body of experimental data for the specific airframe, properties of plasma, aerodynamic aspects, incident radiation, etc.
So much has changed in science and engineering since 1963, that differences between a metal sphere and a modern combat jet pale in comparison.