Analysis -> The Plasma Rifle, like the Laser Rifle, uses a micro fusion cell as its power supply. It taps into the cell, essentially a small fusion reactor, to produce a toroid of plasma that is ejected down a superconductor barrel. A typical micro fusion cell can supply the rifle with around eight shots. Further research needs to be done to determine exactly how the plasma retains its density while traveling through the air toward its target.

http://fallout.wikia.com/wiki/Fort_Independence_terminal_entries#Research_Note_-_Plasma_Rifle