For a long time I tried figuring out what type of plasma the plasma beam was made of in the 2D Metroid games, and then I saw the description for it from the Metroid Fusion Web site, which reads, “The Plasma Beams three lasers can easily cut through enemies, continuing on to take out even more foes.” Here we have “plasma” and “laser”, which doesn’t seem to make any sense, unless we think of a laser-induced plasma channel. This works by firing a laser powerful enough to cause blooming, which is an atmospheric effect that causes the laser to disperse, thereby making the laser impractical.
The next thing I noticed is in Metroid: Other M, where the plasma beam produces an electrical discharge. With the properties of a laser, plasma, and an electrical discharge, I interpret this to be an electrolaser. I have been unable to find the muzzle velocity of electrolasers, but if they’re anything like the speed of lightning, then Samus’ plasma beam has a muzzle velocity of 100.18 kilometers per second. The only problem is electrolasers don’t cut through things, at least as far as I’m aware. This might be an association with plasma cutting.