Similar to an Energy Pack, unlike them, they do not store fuel, they instead hold raw plasma.
Effectively a Closed Energy Circuit, through the use of Plasma Coils, it powers its own containment device. these coils generate the magnetic field(s) that contain the plasma and stop it from exploding.
They are placed in weapons like Blasters for rapid fire.
While they vary from manufacturer, they all have a middle where the plasma is visible to the naked eyes behind its containment. They range from football size, to that of a battery.