Plasma torpedo

Were you looking for the closely-named Plasma Cannon, the Covenant portable cannon?

"The Covenant cruiser came about to face them. Its plasma turrets glowed like angry red eyes."

- Halo: First Strike

The Plasma turret is a Covenant starship weapon placed on many of their ships, including frigates and corvettes.

Description
The plasma turret fires a spear of Plasma and can strike at almost any range. Upon contact it causes severe damage to the target. The weapon can be used in conjunction with Plasma Torpedoes, and Energy Projectors to decimate an entire Human fleet. The armament may be used against infantry as well.

The turret itself is antenna-like in appearance and is located along the sides of the ship. The beam is red or blue and short with a ball-like head. The turrets, from afar, usually appear as short spikes on the sides, the upper sides, and the bow of the vessel.

Before the weapon fires, it first has to charge. When the weapon charges, a ball of superheated plasma forms in front of the turret, then after about 3 seconds, it fires outward at its target. Because of the sheer power of the weapon, Covenant fleets have usually won all space encounters against the UNSC Navy. The closest weapon that the UNSC has that is anywhere near as powerful as the Plasma turret is the MAC gun, which is difficult and costly to make. The magnetic coils used in a MAC gun can be used to shape and guide the plasma fired by a Plasma Turret. Yet, when this happens the plasma is far denser than it would be if fired normally.

Physics
Inside the turret, gas is superheated by the ship's reactor to produce the charge of plasma. Magnetic coils shape and guide the plasma, acting as a sophisticated focusing lens.

When the plasma charge impacts its target, it super heats the material which boils through shields and melts through armor.