Fusion beams were fielded on starships of the Mirak Star League and the Hydran Kingdom. Its power was derived from a stream of excited deuterium and tritium nuclei projected to the target. The nuclei were then fused into helium, releasing tremendous energy at the moment of contact. (TOS video game: Starfleet Command (game))
Although it could potentially deal a lot of damage, fusion beams had a major drawback: these weapons had a very short range. Additionally, the Mirak fusion beam technology was quite crude, limiting their arcs. Also, finding a sufficient amount of both deuterium and tritium to fuse in order to fire the weapon was a logistical nightmare. (RIS Bouteina: "Distress Call")
- In the TOS video game Starfleet Command (game) fusion beams were attributed to the Hydrans, but in RIS Bouteina, they were attributed to the Mirak. It was likely that the Hydrans gave the Mirak their older carrier designs as well as weapons during the Dominion War.