Many aviation enthusiasts have heard about the so-called "plasma stealth": using ionized gas to reduce the radar cross section of an aircraft. Some consider this to be science fiction - a Romulan cloaking shield. However, sarcastic remarks aside, "plasma stealth" is quite real. At least in theory. Interactions between EM radiation and ionized gas have been extensively studied for a variety of purposes ranging from radio communications to astrophysics. Plasma is a highly complex topic with many unanswered fundamental questions. Nevertheless, a plasma stealth device for combat aircraft has been offered for export by Russia in 1999. 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 such methods represent considerable interest. How is it done? There are many possibilities running from "simple" electrostatic discharges to complex and power-hungry plasma lasers. The following collection of resources is unlikely to answer specific technical question, but it may provide you with a push in the right direction. Plasma stealth-related articles in scientific and industry publications Article listings by source: Institute of Electrical and Electronics Engineers American Institute of Aeronautics and Astronautics Jane's Information Group Aviation Week & Space Technology NOTE: these article collections will be updated more frequently than this page, so you may want to bookmark them separately. Foreword As I already mentioned, plasma is a complex subject. Not all researchers understand it equally well. This includes me. No single scientist understands it all. Physics is a very multifaceted science and plasma (being, after all, the fourth state of matter) is a big element of it. Most of the universe exists in the state of plasma, while we live in its tiny corner consisting of solids, gases and liquids. Different type of plasma vary greatly in temperature, levels of ionization, density and chemical composition. Different types of plasma are as far from each other in their physical properties as the fluorescent light in your kitchen is different from the solar core or as Aurora Borealis is different from magnetic confinement fusion. The fact that you found a relevant scientific article written by PhDs and published by a reputable source does not mean that you can take everything printed in the article for a fact. In January of 1999 Russian ITAR-TASS news agency published the interview with the director of the Keldysh Research Center (FKA Scientific Research Institute for Thermal Processes), Academician Anatoliy Koroteyev who talked about the plasma stealth device developed by his organization for use on combat aircraft. The claim was particularly interesting due to the reputation of Dr. Koroteyev and the Institute for Thermal Processes - one of the top scientific research organizations in the world in the field of fundamental physics. I thought this was a significant development in the view of the recently revealed Mikoyan MiG 1.44 fighter prototype and persistent rumors of a novel LO technology developed for the aircraft. So I wrote a short article about plasma and its applications for reducing RCS of military aircraft. This article was published on my site just a few days after the article by ITAR-TASS in late January. Two months later Jane's Information Group reported that a plasma stealth device has been offered by Russia for export. In early summer of 2002 I was reviewing research papers presented at the 33rd Plasmodynamics and Lasers Conference that took place in May of 2002 in Maui, Hawaii. After going through a few dozen boring technical reports I finally came across a paper entitled "Weakly ionized gas characterization and applications in shear flow control by microwave". The article was written by four researchers from the University of Memphis, NASA/Marshall Space Flight Center and the Alabama A&M University. The paper contained a very good general description of various types of plasma and the possible applications of plasma in military aviation. One thing bothered me: the article looked very familiar. I knew I could not have read it before because it was just published and, yet, it seemed like I've already seen it somewhere. After scratching my head for a few minutes I ran a search in Google using a sentence from the article and found... my own Web site and the article about plasma stealth that I wrote three years earlier. The article presented at the 2002 Plasmodynamics and Lasers Conference was in large part copied word for word (including illustrations and punctuation errors) from my article. Most people would have been upset but I felt proud: NASA was plagiarizing my Web site - what better compliment can one hope for? An article written by me during commercial brakes while watching Star Trek marathon was good enough to pass for NASA research and to be presented in front of hundreds of experts in plasmadynamics. The four "researchers" who copied my article: Dr. Alan Chow, the aerospace technologist at NASA's Marshall Space Flight Center in Huntsville, Alabama and the recipient of a NASA Administrator's Fellowship; Dr. Xiujun Tang, visiting Professor of Mechanical Engineering at the University of Memphis; Dr. J.D. (Jiada) Mo, Associate Professor of Mechanical Engineering and the coordinator of undergraduate and graduate programs at the University of Memphis; and Dr. K.X. He, Associate Professor of Physics at the Alabama A&M University. As I see it, the problem is not plagiarism but the fact that unverified information found through Google is being passed for serious scientific research; that it is being presented at an international scientific conference and none of the experts in the field raise any questions. Needless to say, the article I wrote for my Web site was not a research paper or even a serious essay - I didn't even bothered to run it through a spell checker. Therefore, when you read articles in this collection keep this incident in mind and don't assume too much. In the June 2002 issue of the Journal of Electronic Defense - a respected industry publication - the following article has appeared: Russia Working on Stealth Plasma Russia is working to develop plasma-cloud-generation technology for stealth applications and achieved highly promising results, reportedly reducing the radar cross-section (RCS) of an aircraft by a factor of 100. Russian research into plasma generation is spearheaded by a team of scientists led by Anatoliy Korotoyev, director of Keldysh Research Center. The institute has developed a plasma generator weighing only 100 kg, which could easily fit onboard a tactical aircraft. For the system to work, there has to be an energy source on the aircraft that ionizes the surrounding air,probably at the leading surfaces. Since the resulting ions are in the boundary layer of the aircraft, they follow the airflow around the plane. But the system is not without drawbacks. First, the amount of power required is quite high, so it will likely only be activated when an enemy radar is detected. The other is that the plasma also blocks the radar of the aircraft being protected, necessitating holes in the plasma field to look through it. The plasma generator was tested first on flying models and then on actual aircraft. The new Su-27IB strike aircraft (known in export - certainly without the plasma generator - as the Su-32) utilizes the system and is likely the first production combat aircraft with this critical technology. Work on plasma generation is not the purview of Russia alone, though. In the US, for example, research in this field is being conducted by Accurate Automation Corporation (Chattanooga, TN) and Old Dominion University (Norfolk, VA). French companies Dassault (Saint-Cloud, France) and Thales (Paris, France) are jointly working in the same area as well. - Michal Fiszer and Jerzy Gruszczynski, Journal of Electronic Defense, June 2002 I don't have the faintest idea where the authors got their information. Probably from Usenet, Google and similar sources. In this case rumors and newsgroup discussions are being passed for professional analysis. Nevertheless, it's interesting and, who knows, it even might be true. Introduction Before discussing anything related to plasma and its interactions with EM radiation, it is important to review some relevant textbook definitions. Even physicists sometimes cannot give a correct definition of plasma, instead describing a particular type of plasma. Consider the following definitions: Ion: an atom or group of atoms that has acquired a net electric charge by gaining or losing one or more electrons Ionization: the formation of or separation into ions by heat, electrical discharge, radiation, or chemical reaction Plasma: an electrically neutral , ionized gas composed of ions, electrons, and neutral particles The common point of confusion is the electrical neutrality of plasma: how can something consisting of ions be electrically neutral. It can and it is, so just take this for granted. Plasma is a quasineutral (total electrical charge is zero) mix of various particles and 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. To summarize: plasma is a mix of ions, electrons, and neutral particles. Therefore, plasma flow is a flow of this quasineutral mix. Ion is an electrically charged particle or group of atoms. Plasma cloud is a quasineutral collection of free charged particles electrons, and neutral particles. The vast majority of matter in the universe exists in plasma state. Near the Earth plasma can be found in the form of solar wind, magnetosphere and ionosphere. The main property of plasma (for our purposes) is its frequency, which is equal to a square root of a ratio of 4 * Pi * square of ion charge * concentration of ions to the mass of ion: where e is electron or ion charge, n is the concentration of ions per volume of plasma and m is the mass of ion. There are several types of oscillations in plasma: low frequency (ion-sound waves), 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 plasmatron. This device generates the so-called low-temperature plasma. Now that we are done with basic definitions the next step is to consider physical properties of plasma. Generally speaking, plasmas have two main properties: temperature and density. In both regards plasmas cover a huge range of values. Very cold plasma is close to absolute zero and very hot plasma has a temperature well beyond 10^9 degrees Kelvin (for comparison, tungsten melts at 3700 degrees Kelvin). Oh yeah, did I mention that plasmas are electrically conductive? They are and that's why plasma stealth is possible. Plasmas carry electrical currents and generate magnetic fields. Earth is surrounded by plasma - it's magnetosphere - which shields us from cosmic radiation. You can say that we live inside of a plasma stealth shield. Glow discharge plasma Extensive research has been carried out with using plasma for reducing aerodynamic drag of aircraft (and not just aircraft). Hundreds of research papers have been written on the subject and it is not my desire to go into boring details (you can do it on your own by following the links at the top of this page). To summarize: low-energy (low-temperature) plasma is generated on the aerodynamic surface such as a wing or fuselage of a plane. 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 is so-called EHD coupling or EHD propulsion. The method allows one to change the aerodynamic properties of the aircraft without changing the physical geometry of the airframe. You control the electric field around the aircraft - you control the basic aerodynamic properties of the aircraft. Let's say, you send more "juice" to the trailing edge of the wing and, voila, you have a plasma "flaps" lowered for landing. Another example: your plane took an Igla in the horizontal stabilizer. No problem: turn up your electric generator to modify the airflow in the tail section. This is all theoretically and even technologically possible and has been tested to a certain extent in lab experiments. Practical applications are another question. We don't know for a fact that any aircraft currently flying uses this technology. The B-2 might, rumor has it, but there's no way to know for sure without kidnapping and torturing some people or stealing the strategic bomber and flying it to your secret base in Siberia. There is more. EHD coupling is also a method of propulsion as it can accelerate the boundary layer and outer flow. Working models have been constructed that utilize this method of propulsion. So in addition to having the ability to control the aircraft's aerodynamic properties you also get some extra thrust. In turn, this means that the energy you expend on creating plasma is not wasted entirely on just smoothing out the airflow. For a military aircraft, in our case, EHD methods could mean fewer control surfaces, higher angles of attack, extra thrust, greater speed and fuel efficiency, high combat survivability. Would it also mean lower radar cross-section? Maybe :-) The topic of propagation of electromagnetic signals in the atmosphere is a bottomless pit of numbers of formulas of monstrous proportions. Just reading the abstracts of all the scientific papers published on the subject can be a full-time job and will last well into your retirement. This, however, should not stop you from reading some of the articles collected on this site. Endless possibilities Generally speaking, when electromagnetic waves encounter plasma they interact with plasma and, as the result of this interaction, energy of the EM wave is partially exhausted. This is stealth for you. To be more specific, since plasma is electrically conductive an electromagnetic field will be formed in presence of external EM signal. Creating this field requires energy and the this energy comes from the radar signal. The more energy is used in this process the better it is for lowering the aircraft's RCS. The key issue here is frequency of the incoming signal. For example, for low-frequency signals a plasma field may act as a mirror. This is good for long-range communications, as the radio signal bounces between the Earth and the ionosphere and travels long distances, but this is bad for our stealth plane. This range of frequencies are used by early-warning over-the-horizon radars. the effect, however, also depends on the properties of plasma and not just on the radar's frequency. Fortunately for us, most military airborne and air defense radars operate in the microwave band. An EM wave going though a plasma field will also change its own properties. This is called mode conversion. 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 way of controlling chemical composition of the plasma stealth "shield" (I, sort of, assume that. But it may be possible, although I don't see how) what can be controlled is the level and density of ionization (see the plasma frequency formula above). This, in turn, means that radars can also employ some sort of anti-plasma techniques by rotating transmission frequency. Just as LO geometry and radar absorbent materials plasma stealth will not be a panacea against radars. Not to mention that plasma itself emits EM radiation and that it takes some time for plasma to be re-absorbed by the atmosphere, which would create a trail of ionized air behind the moving aircraft. Whether this would be a problem depends on too many things to be seriously discussed even in theory. It is a subject for computer analysis and experimental approach. Flying in the Null This is all talk. The real difficulty is with finding experimental studies of plasma's effect on the radar cross section of aircraft or studies of plasma-microwave radiation interactions. This is hardly surprising as the subject has obvious military applications of great importance. One of the most interesting articles related to the effect of plasma on the RCS of aircraft was published back 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 launched the first artificial satellite. While trying to track the 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. You know what the Sputnik looked like and it's simple shape ideally serves as an uncomplicated illustration of plasma's effect on the RCS of an aircraft. Naturally, an actual aircraft has a far more elaborate shape and it made of a greater variety of materials, but the basic effect remain the same. To summarize the article, in the case with the Sputnik flying through the ionosphere at high velocity and surrounded by a naturally occurring plasma shell we deal with two separate radar reflections: the one from the conductive surface of the satellite itself and the second one from the dielectric plasma shell. The authors of the research found that a dielectric shell (a plasma shell) may decrease or increase the echo area of the object. If either one of the two reflections (from the object itself or from the plasma shell) is considerably greater, then the other weaker reflection will not contribute much to the overall effect. The authors also stated that the EM signal that penetrated the plasma shell and reflected off the object's surface will drop in intensity while traveling through plasma. This is self-explanatory. 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., etc., 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. This brings us to another old topic - 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. To put this in the most primitive way, an air defense radar is transmitting at a certain frequency; the signal is bouncing off the aircraft; a receiver aboard the aircraft picks up the signal and 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. You can read more on one of my old pages here. 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). An aircraft surrounded by an artificial plasma shell that is created by an electric field every aspect of which is controlled by onboard computers... This opens a range of possibilities for radar signal cancellation that may go well beyond the system developed by the Keldysh Institute.