The Plasma Beam is a beam which appears in all of the Metroid games except for the original Metroid, Metroid Prime: Hunters and Metroid Prime 2: Echoes. In 2D incarnations, it is a thin ray capable of piercing multiple enemies, however, in the Prime series, it appears as a short-range beam of intense heat, used to melt ice and weld metals.
In the Metroid Prime series
In Metroid Prime the Plasma Beam's Beam Combo, the Flamethrower was a continuous Beam Combo, a stream of flame with an exceedingly short range, useful for killing Plasma Troopers and cold-based enemies, such as Sheegoth. The Plasma Beam's color scheme in Prime was red, and it represented the "element" of fire. The Plasma Beam was the second to last beam in Metroid Prime, and as such, besides the Phazon Beam it is perhaps the most powerful beam weapon in the game.
The Magmaul in Metroid Prime: Hunters takes over some of the Plasma Beam's functions.
In Metroid Prime 2: Echoes the Light Beam was very similar to the Plasma Beam with its heat base and burning effect.
In Metroid Prime 3: Corruption the Plasma Beam is acquired after defeating the hunter Ghor. The beam was no longer short ranged and the rate of fire remained the same as the Power Beam (all beam weapons were stacked upgrades in Corruption). It retains its ability to ignite enemies and melt objects, however its gun "position" was altered. No longer is the gun extended forwards, instead, it takes after the Ice Beam from Metroid Prime by being slightly elevated upwards and it also takes the shape of the Dark Beam from Metroid Prime 2: Echoes. New also to the Plasma Beam, in Corruption, is its ability to repair damaged cables and circuitry with a welding-type attachment.
In Other Games
In Metroid: Zero Mission, the Plasma Beam was gained as an unknown item and could not be used until the Power Suit was regained in Chozodia. When charged up, it has the potential to destroy many enemies, especially when all of them are standing in the line of fire.
In Super Metroid, the Plasma Beam was incompatible with the Spazer. If both were simultaneously selected with a variety of other factors, the game would glitch with results such as the Murder Beam, Spacetime Beam and the Chainsaw Beam.