Plasma

In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. After sufficient heating a gas dissociates its molecular bonds, rendering it into constituent atoms. Further heating leads to ionization (a loss or gain of electrons), thus turning it into a plasma, containing charged particles: positive ions and negative electrons. Plasmas show a marked difference in electrical and thermal conductivity compared to non-plasmas. Plasmas are highly responsive to electromagnetic fields due to the mobilization of charge carriers (electrons, cations, anions). Plasma is the most common state of matter in the universe.

Weaponry
Covenant military forces, both Loyalist and Separatist, use plasma in weapons to great effect. These weapons contain and guide the plasma 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 the main armament of their warships, dominating almost their entire weapon arsenal. Small arms are designed to deplete shielding or melt infantry armor, while heavier weapons can destroy vehicles or buildings. The largest are mounted onto capital ships and 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, a high quantity of beta particle radiation is emitted.

The UNSC has yet to develop its own plasma weaponry, using more conventional projectile-based weapon systems. However, the UNSC has been seen to operate captured Covenant plasma weapons, and in one case, UNSC forces were able to dramatically improve the lethality of a capture Covenant starship's plasma turrets without significant technical modifications. There was an attempt to produce a tank with plasma-based weaponry for the UNSC armed services, but it was abandoned, falling back on projectile weaponry.

The only known Forerunner constructs to use plasma weaponry are the Sentinels.

Complete armor protection is generally difficult to accomplish due to plasma's high damage ratio. The armor used by the UNSC Marine Corps and UNSC Army usually provides good protection from plasma, but only to a certain extent. A series of hits from a plasma weapon will literally melt or burn the armor, rendering it useless. The SPARTAN-II armor variant is similar the Marines' armor, in that it will eventually give way to the plasma. Some newer versions of the Spartans' armor boast energy shields, which provide additional protection from plasma, although shielding systems can be completely depleted by sustained plasma fire, or a single, overcharged bolt.

The armor of neither Covenant nor UNSC warships provides significant protection against plasma, which will melt through the hulls of both. Covenant plasma torpedoes have a temperature of approximately 3,000 degrees Celsius, while titanium, a key component of UNSC ship armor, has a melting point of 1668 degrees Celsius. In all encounters with Covenant ships, plasma-based weaponry has melted through and disabled UNSC ships in a matter of seconds. However, it is possible that since the UNSC uses Titanium-A, an augmented form of titanium, in its ship armor, the melting point of their starship armor is much higher than suspected.

Other uses
The UNSC uses plasma as an energy source in fusion reactors, and to cut heavy metals in factories.

Trivia

 * 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 instances of plasma easily burning through both armor and flesh. This disparity is due to gameplay mechanics, rather than being a canon contradiction.