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A phase cannon was a phase-modulated energy weapon, a type of particle weapon which served as a successor to plasma cannons and as a precursor to the phaser of the 23rd and 24th centuries. The prototypes of ship-mounted phase cannons were first introduced by Starfleet in the mid-22nd century, designed as the primary defense of NX-class starships. The first NX-class vessel to test and actively use phase cannons in battle was the NX-01 Enterprise.
Designed as a starship-based version of the hand-held phase-pistol, the phase cannon was rated for a maximum power output of five hundred gigajoules. The cannons were mounted on retractable turrets which extended from the ship's hull when deployed and rotated as they were being targeted. Like phase-pistols, they emitted a concentrated beam of energy that could be set at different yields. The phase cannon assembly was equipped with multiphasic emitters and had a maximum yield of eighty gigajoules. Aboard an NX-class vessel, the assembly was located on F Deck and could be accessed through a hatch near the torpedo room.
The cannons were first installed aboard Enterprise in 2151. They were generally more powerful than spatial torpedoes and were therefore more effective in a firefight. Although they were not always powerful enough to penetrate the defenses of enemy vessels, they proved to be an integral part of the ship's defenses throughout the early 2150s.