Plasma Beam

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, though in the Prime Series it appears as a short-range beam of intense heat that can be used to melt ice and weld metals. Samus's Gunship is also stated to be equipped with a "retractable rear-mounted plasma cannon".

In the Metroid Prime series
The Plasma Beam was the second to last beam acquired in Metroid Prime, and as such, besides the Phazon Beam it is perhaps the most powerful beam weapon in the game. When charged, it could instantly incinerate some enemies, such as Baby Sheegoths and Plated Beetles, and it could set fire to others, causing damage for a few moments after the shot hits them. The Plasma Beam's color scheme in Prime was red, and it represented the "element" of fire. The Plasma Beam is also capable of melting Frigidite casings. 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 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-based and burning effect as well as with its animation. However, it was the Annihilator Beam that used its Arm Canon shape from Metroid Prime.

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, as all beam weapons were stacked upgrades in Corruption, although it suffered a downgrade in firepower when compared to Metroid Prime to compensate for this heightened firing rate. It retains its ability to ignite enemies and melt objects, though its Arm Cannon shape was altered. No longer is the gun extended forwards; instead, it takes after the Ice Beam from Metroid Prime by being slightly elevated upwards, which is also 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 beam-focusing attachment, initially used to repair Aurora Unit 217's cables that Ghor damaged.

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 several 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.

In Metroid Fusion, the Nettori possessed this function and according to Adam, could pierce the SA-X's armor and annihilate it. Also at some point, Adam told Samus that the Federation completed the Plasma Beam Upgrade, but withheld it because they knew Samus would hunt the SA-X after acquiring it, preventing them from their studies.

In Super Smash Bros. Brawl, Zero Suit Samus has two attacks called the Plasma Whip and "Plasma Wire" that fire from her Paralyzer. Both also act as a tether, similarly to the Grapple Beam she would use with her armored Power Suit.

Metroid II manual
"This extremely powerful weapon fires a beam that can even penetrate the solid walls. It is the strongest and most destructive of all of the beam type items."

Super Metroid manual
"The most powerful of all the beams can destroy several targets at once."

Metroid Fusion manual
"This beam passes through enemies, hitting multiple targets."

Zero Mission Samus Screen data
"This powerful beam can blast through multiple enemies."

Trivia

 * Interestingly, the Plasma Beam in Metroid Prime is the only beam that increases in range when charged (though slightly).
 * Strangely, in the New Play Control! and Metroid Prime Trilogy versions of the game, embers no longer float off the Arm Cannon when charging. Theories suggest that this may be due to the way this and other visual effects were programmed in the original versions of the game, making it too difficult to reproduce the effects on the Wii.
 * In Corruption when Samus tests the Plasma Beam out during the pickup cinematic, the sound it makes is the same as in Prime, although it makes a different sound in Corruption.
 * Samus appears to have a green arm cannon in some parts of the Metroid: Other M trailer, it has been speculated to either be the Plasma Beam or the Nova Beam. However in recent demos, it appears that the green arm cannon is present when Samus is using the Charge Beam.
 * The Plasma Beam, along with Samus and several of her other abilities were slated to appear in Marvel: Ultimate Alliance, but they were removed due to Nintendo not giving consent. The Plasma Beam in Ultimate Alliance appears to be based off of Prime's iteriation due to its orange color.
 * In the first Metroid Prime, the symbol for the Plasma Beam is a hand print presumably combining its index finger with its middle finger, and its ring finger with its pinkey, and putting them together, with the thumb inward toward the hand. Using the X-Ray Visor reveals that Samus configures her hand this way to activate the beam.