Theoretical Work With Sputnik
Due to the obvious military applications of the subject, there are few readily available experimental studies of plasma's effect on the radar cross section (RCS) of aircraft, but plasma interaction with microwaves is a well explored area of general plasma physics. Standard plasma physics reference texts are a good starting point and usually spend some time discussing wave propagation in plasmas.
One of the most interesting articles related to the effect of plasma on the RCS of aircraft was published in 1963 by the IEEE. 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). Six years earlier, in 1957, the Soviets had launched the first artificial satellite. While trying to track Sputnik it was noticed that its electromagnetic scattering properties were different from what was expected for a conductive sphere. This was due to the satellite's traveling inside of a plasma shell: the ionosphere.
The Sputnik's simple shape serves as an ideal illustration of plasma's effect on the RCS of an aircraft. 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. In the case of the Sputnik flying through the ionosphere at high velocity and surrounded by a naturally-occurring plasma shell, there are two separate radar reflections: the first from the conductive surface of the satellite, and the second from the dielectric plasma shell.
The authors of the paper found that a dielectric (plasma) shell may either decrease or increase the echo area of the object. If either one of the two reflections is considerably greater, then the weaker reflection will not contribute much to the overall effect. The authors also stated that the EM signal that penetrates the plasma shell and reflects off the object's surface will drop in intensity while traveling through plasma, as was explained in the prior section.
The most interesting effect is observed when the two reflections are of the same order of magnitude. In this situation the two components (the two reflections) will be added as phasors and the resulting field will determine the overall RCS. When these two components are out of phase relative to each other, cancellation occurs. This means that under such circumstances the RCS becomes null and the object is completely invisible to the radar.
It is immediately apparent that performing similar numeric approximations for the complex shape of an aircraft would be difficult. This would require a large body of experimental data for the specific airframe, properties of plasma, aerodynamic aspects, incident radiation, etc. In contrast, the original computations discussed in this paper were done by a handful of people on an IBM 704 computer made in 1956, and at the time, this was a novel subject with very little research background. So much has changed in science and engineering since 1963, that differences between a metal sphere and a modern combat jet pale in comparison.
A simple application of plasma stealth is the use of plasma as an antenna: metal antenna masts often have large radar cross sections, but a hollow glass tube filled with low pressure plasma can also be used as an antenna, and is entirely transparent to radar when not in use.
Read more about this topic: Plasma Stealth
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