Plasma

In physics and chemistry, Plasma is a partially ionized gas, in which a certain proportion of electrons are free rather than being bound to an atom or molecule. The ability of the positive and negative charges to move somewhat independently makes the plasma electrically conductive so that it responds strongly to electromagnetic fields. Plasma therefore has properties quite unlike those of solids, liquids or gases and is considered to be a distinct state of matter. Plasma typically takes the form of neutral gas-like clouds; for example, in the case of stars.

Function
Though difficult to control, the Covenant military and Forerunner Constructs use plasma in a weapon form for a great effect, containing and guiding it in the form of a single bolt, beam, or continuous stream. Plasma-based weapons are the most common armament of Covenant infantry and vehicular forces, and are the main armaments of their warships. Smaller weapons are designed to deplete shielding or melt infantry armor, while heavier weapons can destroy vehicles or buildings. The largest are mounted to capital ships, are used for orbital bombardment and ship-to-ship combat, and can reduce a ship's hull to a pile of molten metal floating in space. When a plasma weapon is charging, high quantities of beta particle radiation are emitted. The UNSC uses plasma as an energy source and to cut heavy metals in factories, and attempted to produce it for use in their armed services, but apparently chose to abandon the project, instead falling back on projectile weaponry.

Protection
Complete protection from plasma is generally hard to obtain due to the plasma's high damage ratio. Armor used by Marines usually has a good protection from plasma, but only to an extent. The armor will be literally melted or burned to uselessness. That's why when a Marine is constantly hit in a certain place numerous times by plasma, the armor will give way and the person will have severe to life threatening injuries. SPARTAN-II armor is much like the Marines armor, being that it will eventually give way to the plasma. The added bonus Spartans have is the shield which protects from plasma.

Trivia

 * Humans can contract Boren's Syndrome from prolonged contact with the radiation emitted by plasma grenades.
 * For gameplay reasons, plasma is less lethal to unshielded NPCs in the Halo games than in the novels. In the games, Marines can take a barrage from a plasma weapon without being incapacitated, but in the books there are several incidences of plasma easily burning through both armor and flesh. This disparity is due to gameplay mechanics, rather than being a canon contradiction.
 * It seems that the power of the plasma depends on what color the plasma is. For example the Plasma Pistol shoots out green plasma but is not very powerful, even though the extremely powerful main cannon of a scarab shoots green plasma, the Plasma Rifle shoots blue plasma and is more powerful, the Brute Plasma Rifle shoots red plasma which is slightly more powerful than blue plasma, while the forward turret on a Phantom in Halo 3 shoots out purple plasma and is the most powerful. It should be noted that in Halo: Combat Evolved and Halo 2, the plasma on some of the plasma based weaponry can be seen even when Active Camouflage/Invisibility has been activated.
 * If the "plasma hierarchy" is true, the purple Energy Sword being wielded by an Elite in Second Sunrise Over New Mombasa should be more powerful than the one used in-game. However, this is probably due to aesthetic differences.

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