Editing Plasma stealth (section)

== Plasma and its properties ==
{{Main|Plasma (physics)}}

A plasma is a ''[[Plasma (physics)#Plasma potential|quasineutral]]'' (total [[electrical charge]] is close to zero) mix of [[ion]]s ([[atom]]s which have been ionized, and therefore possess a net positive charge), [[electron]]s, and neutral particles (un-ionized atoms or molecules). Most plasmas are only partially ionized, in fact, the ionization degree of common plasma devices like fluorescent lamp is fairly low ( less than 1%). Almost all the matter in the universe is very low density 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 electron temperature and plasma density.

*'''Characteristic electron plasma frequency''', the frequency with which electrons oscillate ([[plasma oscillation]]): 
:<math>\omega_{pe} = (4\pi n_ee^2/m_e)^{1/2} = 5.64 \times 10^4 n_e^{1/2} \mbox{rad/s} = 9000 \times n_e^{1/2} \mbox{Hz} </math>

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 10<sup>9</sup> [[kelvin]]s (for comparison, tungsten melts at 3700 kelvins), and plasma may contain less than one particle per cubic metre. Electron temperature is usually expressed as electronvolt (eV), and 1 eV is equivalent to 11,604 K. Common plasmas temperature and density in fluorescent light tubes and semiconductor manufacturing processes are around several eV and 10<sup>9-12</sup>per cm<sup>3</sup>. 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. Low-frequency means it is lower than the characteristic electron [[Plasma parameters|plasma frequency]]. The use of plasmas to control the reflected electromagnetic radiation from an object (Plasma stealth) is feasible at suitable frequency where the conductivity of the plasma allows it to interact strongly with the incoming radio wave, and the wave can either be absorbed and converted into thermal energy, or reflected, or transmitted depending on the relationship between the radio wave frequency and the characteristic plasma frequency. If the frequency of the radio wave is lower than the plasma frequency, it is reflected. if it is higher, it is transmitted. If these two are equal, then resonance occurs. There is also another mechanism where reflection can be reduced. If the electromagnetic wave passes through the plasma, and is reflected by the metal, and the reflected wave and incoming wave are roughly equal in power, then they may form two phasors. When these two phasors are of opposite phase they can cancel each other out. In order to obtain substantial attenuation of radar signal, the plasma slab needs adequate thickness and density.<ref name=Chung1 />

Plasmas support a wide range of waves, but for unmagnetised plasmas, the most relevant are the [[Langmuir wave]]s, 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.