Plasma stealth
From Biocrawler, the free encyclopedia.
Plasma stealth is a proposed process that uses ionized gas to reduce the radar cross section (RCS) of an aircraft. Interactions between EM radiation and ionized gas have been extensively studied for a variety of purposes, including the possible concealment of aircraft from radar that plasma stealth theorizes. While the theoretical possibility of reducing an aircraft's RCS by wrapping the airframe in ionized gas flow is not in question, the technological aspects of applying such methods represent considerable challenges. There are many possible means of accomplishing this effect, running from "simple" electrostatic discharges to complex and power-hungry plasma lasers.
Even though plasma theory is a highly complex topic with many unanswered fundamental questions, a plasma stealth device for combat aircraft was offered for export by Russia in 1999. In January of 1999, the Russian ITAR-TASS news agency published an interview with Doctor Anatoliy Koroteyev, the director of the Keldysh Research Center (FKA Scientific Research Institute for Thermal Processes), who talked about the plasma stealth device developed by his organization. The claim was particularly interesting in light of the solid scientific reputation of Dr. Koroteyev and the Institute for Thermal Processes, which is one of the top scientific research organizations in the world in the field of fundamental physics. [see "Russian scientists created revolutionary technologies for reducing radar visibility of aircraft", by Nikolay Novichkov, ITAR-TASS, January 20, 1999].
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. According to this June 2002 article, the Russian plasma stealth device has been tested aboard a Su-27IB fighter-bomber. The Journal also reported that similar research into applications of plasma for RCS reduction is being carried out by Accurate Automation Corporation (Chattanooga, TN) and Old Dominion University (Norfolk, VA) in the US; and by Dassault (Saint-Cloud, France) and Thales (Paris, France). ["Russia Working on Stealth Plasma", by Michal Fiszer and Jerzy Gruszczynski, Journal of Electronic Defense, June 2002].
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Introduction
A plasma is a quasineutral (total electrical charge is zero) mix of ions (atoms which have been ionized, thus losing their outer electrons), electrons, and neutral particles (possibly including unionized atoms). It should not be confused with full ionization. For example, if all particles in a given volume of gas lost all of their electrons, then this volume of fully ionized gas would have a strong electrical charge and would contain nothing but pure ions and, therefore, it would not be plasma but just a collection of electron-less atoms.
The vast majority of matter in the universe exists in the plasma state. Different types of plasma vary greatly in temperature, levels of ionization, density and chemical composition. The solar core and the Aurora Borealis are both composed largely of plasma. Plasmas are used in diverse situations, such as fluorescent lighting and magnetic confinement fusion. Near the Earth, plasma can be found in the form of the solar wind, the magnetosphere, and the ionosphere. The latter two plasma fields have the effect of blocking much of the Sun's radiation from reaching the Earth, acting as a natural form of a plasma stealth shield.
There are several types of oscillations in plasma: low frequency waves (sound waves in the ions), high frequency (oscillations of electrons relative to ions), spiral waves (in the presence of a magnetic field - "magnetosound"), and cross waves propagating along a magnetic field. A device for generating plasma is called a plasmatron. This device generates low-temperature plasma.
The most important physical property of plasma from an electromagnetic standpoint is its frequency. Plasmas have two other important physical properties: temperature (i.e. energy) and density. Plasmas cover a wide range of values in both properties; very cold plasma is close to absolute zero and very hot plasma has a temperature well beyond 109 kelvins (for comparison, tungsten melts at 3700 kelvins). Plasma stealth is possible because plasma is electrically conductive, meaning that plasma can carry electrical current and generate a magnetic field.
Glow discharge plasma
Extensive research has been carried out using plasma to reduce the aerodynamic drag of an aircraft, with hundreds of research papers written on the subject. Low-energy plasma is generated on an aerodynamic surface, such as a wing or fuselage, of a plane. This plasma exists at normal atmospheric pressure and serves as an electrohydrodynamic coupling layer between the artificially generated (by the aircraft's onboard systems) electric field and the electrically neutral boundary layer. This technique is called EHD coupling or EHD propulsion.
This method allows one to change the aerodynamic properties of the aircraft without changing the physical geometry of the airframe by modifying the electric field. For example, the pilot could send more power to the trailing edge of the wing, producing a plasma "flap" lowered for landing. If the plane took an Igla in the horizontal stabilizer, the problem could be fixed by turning up the electric generator to modify the airflow in the tail section.
This is all theoretically possible and has been tested to a certain extent in lab experiments, but practical applications are another question. No aircraft currently flying is known to use this technology, although it is rumored that the B-2 Spirit does.
EHD coupling is also usable as a method of propulsion, as it can accelerate the boundary layer and outer flow, and working models have been constructed that utilize this method of propulsion. In addition to having the ability to control its aerodynamic properties, the aircraft using this technology would also get additional thrust. Therefore, the energy expended on creating plasma is not spent entirely on smoothing out the airflow. For a military aircraft, EHD methods could mean fewer control surfaces, higher angles of attack, extra thrust, greater speed and fuel efficiency, and high combat survivability. However, it is not entirely clear if this plasma cloud would also produce a lower radar cross-section, despite the many papers written on the subject.
Absorption of EM radiation
When electromagnetic waves, such as radar signals, encounter plasma the two will interact, usually reducing the energy of the EM wave. Since plasma is electrically conductive, an electromagnetic field will be formed in presence of an external EM signal. Creating this field requires energy, which comes from the incoming signal. The more energy used in this process, the better it is at lowering the aircraft's RCS.
The key issue here is frequency of the incoming signal. With low-frequency signals such as radio waves, a plasma field may act as a mirror. This aids long-range communications, as the radio signal bounces between the Earth and the ionosphere and travels long distances, but it will not help a stealth plane. This range of frequencies is used by early-warning over-the-horizon radars. The effect, however, depends on the properties of plasma and not just on the radar's frequency. Most military airborne and air defense radars, however, operate in the microwave band. At these frequencies, the ionosphere will absorb the radiation; if a plane's plasma cloud could do the same, it would not be directly visible on the radar. The only way to locate the aircraft would be to detect its habit of blocking more distant objects.
An EM wave going though a plasma field will change its own properties in a process called mode conversion. Because of this, plasma offers endless possibilities for manipulating EM waves. An effective plasma stealth device would offer control over the frequency of plasma, which would be adjusted depending on the frequency of the hostile radar signal. Since there is no obvious way of controlling the chemical composition of the plasma stealth "shield", all that can be changed is the level and density of ionization.
This, in turn, means that radars can employ anti-plasma techniques by rotating their transmission frequency. Just as with LO geometry and radar absorbent materials, plasma stealth will not be a panacea against radar. Moreover, the plasma itself emits EM radiation, and it takes some time for plasma to be re-absorbed by the atmosphere, so a trail of ionized air would be created behind the moving aircraft. Whether this would be a problem is a subject for computer analysis and experimentation.
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 of aircraft, or of plasma-microwave radiation interactions.
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 year 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 traveling inside of a plasma shell.
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 itself 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 previous 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. On the other hand, 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.
Active radar cancellation
A few years ago there were rumors that France's Dassault Rafale fighter may be using some form of active radar cancellation. Put simply, a receiver aboard the aircraft picks up the radar signal, a computer analyses its base frequency and modulations, and an out-of-phase signal is generated by onboard systems to cancel out the enemy radar signal.
This is easier said than done, but theoretically it is possible. The main problem is that the incoming signal is complex and the reflection off the surface of the aircraft is even more complex. How do you cancel it out? How do you process so much information so quickly? But most importantly, how do you position transmitting antennae aboard the aircraft to cover the entire aircraft (since the enemy radar signal is reflected from a multitude of points on the airframe and it's reflected differently from every one of them)? However, an aircraft surrounded by an artificial plasma shell that is created by an electric field, which is controlled by onboard computers, would not be subject to this problem.
