Plasma stealth
Encyclopedia
Plasma stealth is a proposed process to use ionized gas (plasma
) to reduce the radar cross section
(RCS) of an aircraft
. Interactions between electromagnetic radiation
and ionized gas have been extensively studied for many purposes, including concealing aircraft from radar as stealth technology
. Various methods might plausibly be able to form a layer or cloud of plasma around a vehicle
to deflect or absorb radar, from simpler electrostatic or radio frequency
(RF) discharges to more complex laser discharges. It is theoretically possible to reduce RCS in this way, but it may be very difficult to do so in practice.
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 inlets. Known as Project KEMPSTER, this used an electron beam generator to create a cloud of ionization in front of each inlet. The system was flight tested but was never deployed on operational A-12s or SR-71s
.
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. In January 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.
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 Sukhoi Su-27
IB 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, Tennessee
) and Old Dominion University
(Norfolk, Virginia) in the U.S.; and by Dassault Aviation
(Saint-Cloud, France) and Thales
(Paris, France).
s (atom
s which have been ionized, and therefore possess a net charge), electron
s, and neutral particles (possibly including un-ionized atoms). Not all plasmas are fully ionized. Almost all the matter in the universe is plasma: solids, liquids and gases are uncommon away from planetary bodies. Plasmas have many technological applications, from fluorescent lighting to plasma processing for semiconductor manufacture.
Plasmas can interact strongly with electromagnetic radiation: this is why plasmas might plausibly be used to modify an object's radar signature. Interaction between plasma and electromagnetic radiation is strongly dependent on the physical properties and parameters of the plasma, most notably, the temperature and density of the plasma. Plasmas can have a wide range of values in both temperature and density; plasma temperatures range from close to absolute zero and to well beyond 109 kelvin
s (for comparison, tungsten melts at 3700 kelvins), and plasma may contain less than one particle per cubic metre, or be denser than lead. For a wide range of parameters and frequencies, plasma is electrically conductive, and its response to low-frequency electromagnetic waves is similar to that of a metal: a plasma simply reflects incident low-frequency radiation. The use of plasmas to control the reflected electromagnetic radiation from an object (Plasma stealth) is feasible at higher frequency where the conductivity of the plasma allows it to interact strongly with the incoming radio wave, but the wave can be absorbed and converted into thermal energy rather than reflected.
Plasmas support a wide range of waves, but for unmagnetised plasmas, the most relevant are the Langmuir waves, corresponding to a dynamic compression of the electrons. For magnetised plasmas, many different wave modes can be excited which might interact with radiation at radar frequencies.
. In particular, electrohydrodynamic coupling can be used to accelerate air flow near an aerodynamic surface. One paper considers the use of a plasma panel for boundary layer control on a wing in a low-speed wind tunnel
. This demonstrates that it is possible to produce a plasma on the skin of an aircraft. Xenon nuclear poison
isotopes when successfully suspended in the plasma layers or vehicle hull can be utilized to reduce radar cross-section and will shield against HMP/EMP and HERF weaponry.
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. The wave field gives energy to the particles. The particles generally return some fraction of the energy they have gained to the wave, but some energy may be permanently absorbed as heat by processes like scattering or resonant acceleration, or transferred into other wave types by mode conversion or nonlinear effects. A plasma can, at least in principle, absorb all the energy in an incoming wave, and this is the key to plasma stealth. However, plasma stealth implies a substantial reduction of an aircraft's RCS, making it more difficult (but not necessarily impossible) to detect. The mere fact of detection of an aircraft by a radar does not guarantee an accurate targeting solution needed to intercept the aircraft or to engage it with missiles. A reduction in RCS also results in a proportional reduction in detection range, allowing an aircraft to get closer to the radar before being detected.
The central issue here is frequency of the incoming signal. A plasma will simply reflect radio waves below a certain frequency (which depends on the plasma properties). This aids long-range communications, because low-frequency radio signals bounce between the Earth and the ionosphere and may therefore travel long distances. Early-warning over-the-horizon radars utilize such low-frequency radio waves. Most military airborne and air defense radars, however, operate in the microwave band, where many plasmas, including the ionosphere, absorb or transmit the radiation (the use of microwave communication between the ground and communication satellites demonstrates that at least some frequencies can penetrate the ionosphere).
Plasma surrounding an aircraft might be able to absorb incoming radiation, and therefore prevent any signal reflection from the metal parts of the aircraft: the aircraft would then be effectively invisible to radar. A plasma might also be used to modify the reflected waves to confuse the opponent's radar system: for example, frequency-shifting the reflected radiation would frustrate Doppler filtering and might make the reflected radiation more difficult to distinguish from noise.
Control of plasma properties is likely to be important for a functioning plasma stealth device, and it may be necessary to dynamically adjust the plasma density, temperature or composition, or the magnetic field, in order to effectively defeat different types of radar systems. Radars which can flexibly change transmission frequencies might be less susceptible to defeat by plasma stealth technology. Like LO geometry and radar absorbent material
s, plasma stealth technology is probably not a panacea against radar.
Plasma stealth technology also faces various technical problems. For example, the plasma itself emits EM radiation. Also, it takes some time for plasma to be re-absorbed by the atmosphere and a trail of ionized air would be created behind the moving aircraft. Thirdly, plasmas (like glow discharges or fluorescent lights) tend to emit a visible glow: this is not necessarily compatible with overall low observability. Furthermore, it is likely to be difficult to produce a radar-absorbent plasma around an entire aircraft traveling at high speed. However, a substantial reduction of an aircraft's RCS may be achieved by generating radar-absorbent plasma around the most reflective surfaces of the aircraft, such as the turbojet engine fan blades, engine air intakes, and vertical stabilizers.
At least one detailed computational study exists of plasma-based radar cross section reduction using three-dimensional computations.
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 phasor
s 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.
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
) to reduce the radar cross section
Radar cross section
Radar cross section is a measure of how detectable an object is with a radar. A larger RCS indicates that an object is more easily detected.An object reflects a limited amount of radar energy...
(RCS) of an aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
. Interactions between electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
and ionized gas have been extensively studied for many purposes, including concealing aircraft from radar as stealth technology
Stealth technology
Stealth technology also termed LO technology is a sub-discipline of military tactics and passive electronic countermeasures, which cover a range of techniques used with personnel, aircraft, ships, submarines, and missiles, to make them less visible to radar, infrared, sonar and other detection...
. Various methods might plausibly be able to form a layer or cloud of plasma around a vehicle
Vehicle
A vehicle is a device that is designed or used to transport people or cargo. Most often vehicles are manufactured, such as bicycles, cars, motorcycles, trains, ships, boats, and aircraft....
to deflect or absorb radar, from simpler electrostatic or radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
(RF) discharges to more complex laser discharges. It is theoretically possible to reduce RCS in this way, but it may be very difficult to do so in practice.
First claims
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." It is unclear who funded this work or whether it was prototyped and tested. U.S. Patent 3,127,608 was granted in 1964.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 inlets. Known as Project KEMPSTER, this used an electron beam generator to create a cloud of ionization in front of each inlet. The system was flight tested but was never deployed on operational A-12s or SR-71s
SR-71 Blackbird
The Lockheed SR-71 "Blackbird" was an advanced, long-range, Mach 3+ strategic reconnaissance aircraft. It was developed as a black project from the Lockheed A-12 reconnaissance aircraft in the 1960s by the Lockheed Skunk Works. Clarence "Kelly" Johnson was responsible for many of the...
.
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
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
in 1999. In January 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.
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 Sukhoi Su-27
Sukhoi Su-27
The Sukhoi Su-27 is a twin-engine supermanoeuverable fighter aircraft designed by Sukhoi. It was intended as a direct competitor for the large United States fourth generation fighters, with range, heavy armament, sophisticated avionics and high manoeuvrability...
IB 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, Tennessee
Chattanooga, Tennessee
Chattanooga is the fourth-largest city in the US state of Tennessee , with a population of 169,887. It is the seat of Hamilton County...
) and Old Dominion University
Old Dominion University
Old Dominion University is a state university located in Norfolk, Virginia, United States, and is accredited by the Southern Association of Colleges and Schools...
(Norfolk, Virginia) in the U.S.; and by Dassault Aviation
Dassault Aviation
Dassault Aviation is a French aircraft manufacturer of military, regional and business jets, a subsidiary of Dassault Group.It was founded in 1930 by Marcel Bloch as Société des Avions Marcel Bloch or "MB". After World War II, Marcel Bloch changed his name to Marcel Dassault, and the name of the...
(Saint-Cloud, France) and Thales
Thales Group
The Thales Group is a French electronics company delivering information systems and services for the aerospace, defense, transportation and security markets...
(Paris, France).
Plasma and its properties
A plasma is a quasineutral (total electrical charge is close to zero) mix of ionIon
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s (atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s which have been ionized, and therefore possess a net charge), electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s, and neutral particles (possibly including un-ionized atoms). Not all plasmas are fully ionized. Almost all the matter in the universe is plasma: solids, liquids and gases are uncommon away from planetary bodies. Plasmas have many technological applications, from fluorescent lighting to plasma processing for semiconductor manufacture.
Plasmas can interact strongly with electromagnetic radiation: this is why plasmas might plausibly be used to modify an object's radar signature. Interaction between plasma and electromagnetic radiation is strongly dependent on the physical properties and parameters of the plasma, most notably, the temperature and density of the plasma. Plasmas can have a wide range of values in both temperature and density; plasma temperatures range from close to absolute zero and to well beyond 109 kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
s (for comparison, tungsten melts at 3700 kelvins), and plasma may contain less than one particle per cubic metre, or be denser than lead. For a wide range of parameters and frequencies, plasma is electrically conductive, and its response to low-frequency electromagnetic waves is similar to that of a metal: a plasma simply reflects incident low-frequency radiation. The use of plasmas to control the reflected electromagnetic radiation from an object (Plasma stealth) is feasible at higher frequency where the conductivity of the plasma allows it to interact strongly with the incoming radio wave, but the wave can be absorbed and converted into thermal energy rather than reflected.
Plasmas support a wide range of waves, but for unmagnetised plasmas, the most relevant are the Langmuir waves, corresponding to a dynamic compression of the electrons. For magnetised plasmas, many different wave modes can be excited which might interact with radiation at radar frequencies.
Plasmas on aerodynamic surfaces
Plasma layers around aircraft have been considered for purposes other than stealth. There are many research papers on the use of plasma to reduce aerodynamic dragDrag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
. In particular, electrohydrodynamic coupling can be used to accelerate air flow near an aerodynamic surface. One paper considers the use of a plasma panel for boundary layer control on a wing in a low-speed wind tunnel
Wind tunnel
A wind tunnel is a research tool used in aerodynamic research to study the effects of air moving past solid objects.-Theory of operation:Wind tunnels were first proposed as a means of studying vehicles in free flight...
. This demonstrates that it is possible to produce a plasma on the skin of an aircraft. Xenon nuclear poison
Nuclear poison
A neutron poison is a substance with a large neutron absorption cross-section in applications, such as nuclear reactors. In such applications, absorbing neutrons is normally an undesirable effect...
isotopes when successfully suspended in the plasma layers or vehicle hull can be utilized to reduce radar cross-section and will shield against HMP/EMP and HERF weaponry.
Absorption of EM radiation
When electromagneticElectromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
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. The wave field gives energy to the particles. The particles generally return some fraction of the energy they have gained to the wave, but some energy may be permanently absorbed as heat by processes like scattering or resonant acceleration, or transferred into other wave types by mode conversion or nonlinear effects. A plasma can, at least in principle, absorb all the energy in an incoming wave, and this is the key to plasma stealth. However, plasma stealth implies a substantial reduction of an aircraft's RCS, making it more difficult (but not necessarily impossible) to detect. The mere fact of detection of an aircraft by a radar does not guarantee an accurate targeting solution needed to intercept the aircraft or to engage it with missiles. A reduction in RCS also results in a proportional reduction in detection range, allowing an aircraft to get closer to the radar before being detected.
The central issue here is frequency of the incoming signal. A plasma will simply reflect radio waves below a certain frequency (which depends on the plasma properties). This aids long-range communications, because low-frequency radio signals bounce between the Earth and the ionosphere and may therefore travel long distances. Early-warning over-the-horizon radars utilize such low-frequency radio waves. Most military airborne and air defense radars, however, operate in the microwave band, where many plasmas, including the ionosphere, absorb or transmit the radiation (the use of microwave communication between the ground and communication satellites demonstrates that at least some frequencies can penetrate the ionosphere).
Plasma surrounding an aircraft might be able to absorb incoming radiation, and therefore prevent any signal reflection from the metal parts of the aircraft: the aircraft would then be effectively invisible to radar. A plasma might also be used to modify the reflected waves to confuse the opponent's radar system: for example, frequency-shifting the reflected radiation would frustrate Doppler filtering and might make the reflected radiation more difficult to distinguish from noise.
Control of plasma properties is likely to be important for a functioning plasma stealth device, and it may be necessary to dynamically adjust the plasma density, temperature or composition, or the magnetic field, in order to effectively defeat different types of radar systems. Radars which can flexibly change transmission frequencies might be less susceptible to defeat by plasma stealth technology. Like LO geometry and radar absorbent material
Radar absorbent material
Radar-absorbent material, or RAM, is a class of materials used in stealth technology to disguise a vehicle or structure from radar detection. A material's absorbency at a given frequency of radar wave depends upon its composition...
s, plasma stealth technology is probably not a panacea against radar.
Plasma stealth technology also faces various technical problems. For example, the plasma itself emits EM radiation. Also, it takes some time for plasma to be re-absorbed by the atmosphere and a trail of ionized air would be created behind the moving aircraft. Thirdly, plasmas (like glow discharges or fluorescent lights) tend to emit a visible glow: this is not necessarily compatible with overall low observability. Furthermore, it is likely to be difficult to produce a radar-absorbent plasma around an entire aircraft traveling at high speed. However, a substantial reduction of an aircraft's RCS may be achieved by generating radar-absorbent plasma around the most reflective surfaces of the aircraft, such as the turbojet engine fan blades, engine air intakes, and vertical stabilizers.
At least one detailed computational study exists of plasma-based radar cross section reduction using three-dimensional computations.
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
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
.
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
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
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 phasor
Phasor
Phasor is a phase vector representing a sine wave.Phasor may also be:* Phasor , a stereo music, sound and speech synthesizer for the Apple II computer* Phasor measurement unit, a device that measures phasors on an electricity grid...
s 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
IBM 704
The IBM 704, the first mass-produced computer with floating point arithmetic hardware, was introduced by IBM in 1954. The 704 was significantly improved over the IBM 701 in terms of architecture as well as implementations which were not compatible with its predecessor.Changes from the 701 included...
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.