Editing Plasma stealth (section)

== Absorption of EM radiation ==

When [[Electromagnetic radiation|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. 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 [[Radar cross-section|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 (characteristic electron plasma frequency). This is the basic principle of short wave radios and 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 (typically lower than 50&nbsp;MHz). Most military airborne and air defense radars, however, operate in VHF, UHF, and microwave band, which have frequencies higher than the characteristic plasma frequency of ionosphere, therefore microwave can penetrate the ionosphere and communication between the ground and communication satellites demonstrates is possible. (''Some'' frequencies can penetrate the ionosphere).

Plasma surrounding an aircraft might be able to absorb incoming radiation, and therefore reduces signal reflection from the metal parts of the aircraft: the aircraft would then be effectively invisible to radar at long range due to weak signals received.<ref name=Chung1>{{cite book|author1=Shen Shou Max Chung|editor1-last=Wang|editor1-first=Wen-Qin|title=Radar Systems: Technology, Principles and Applications|date=2013|publisher=NOVA Publishers|location=Hauppauge, NY|isbn=978-1-62417-884-9|pages=1–44|edition=1|chapter-url=https://www.novapublishers.com/catalog/product_info.php?products_id=42399|chapter=Chapter 1: Manipulation of Radar Cross Sections with Plasma|doi=10.13140/2.1.4674.4327}}</ref> 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 like density and temperature is important for a functioning plasma stealth device, and it may be necessary to dynamically adjust the plasma density, temperature, or combinations, or the magnetic field, in order to effectively defeat different types of radar systems. The great advantage Plasma Stealth possesses over traditional radio frequency stealth techniques like low-observability geometry and use of [[radar-absorbent material]]s is that plasma is tunable and wideband. 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. The greatest challenge is to generate a large area or volume of plasma with good energy efficiency.

Plasma stealth technology also faces various technical problems. For example, the plasma itself emits EM radiation, although it is usually weak and noise-like in spectrum. 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, but at present there is no method to detect this kind of plasma trail at long distance. Thirdly, plasmas (like glow discharges or fluorescent lights) tend to emit a visible glow: this is not compatible with overall low observability concept. 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. However, a substantial reduction of an aircraft's RCS may be still 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, vertical stabilizers, and airborne radar antenna.

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.<ref name=Chung2>{{cite journal|last1=Chung|first1=Shen Shou Max|title=FDTD Simulations on Radar Cross Sections of Metal Cone and Plasma Covered Metal Cone|journal=Vacuum|date=Feb 8, 2012|volume=86|issue=7|pages=970–984|doi=10.1016/j.vacuum.2011.08.016|bibcode = 2012Vacuu..86..970M }}</ref> Chung simulated the radar cross section of a generic satellite, and also the radar cross section when it is covered with artificially generated plasma cones.<ref name=Chung3>{{cite journal|last1=Chung|first1=Shen Shou Max|title=Simulation on Change of Generic Satellite Radar Cross Section via Artificially Created Plasma Sprays|journal=Plasma Sources Science and Technology|date=Mar 30, 2016|volume=25|issue=3|pages=035004|doi=10.1088/0963-0252/25/3/035004|bibcode = 2016PSST...25c5004C |s2cid=101719978 }}</ref>