A cross-section of DS9's central core, the fusion reactors at the lower part of the core.
A fusion reactor or fusion powerplant is a power generation device.
Inside such a reactor, two deuterium atoms are fused together to form a single helium atom. This reaction generates a massive amount of energy, which can be used to power various systems.
Starship impulse engines are powered by fusion reactors. In emergencies, these reactors could be used to power the ship as a whole. (citation needed • edit)
The Cardassian built Federation starbase Deep Space 9 was powered by six fusion reactors, located at the base of the core section. (citation needed • edit) Cardassian built reactors were based on a series of carbon reaction chambers, which uses laser-induced fusion. (DS9: "The Forsaken")
According to the specifications for Cardassian fusion reactors, the operating efficiency was acceptable at as low as twenty percent peak efficiency. In 2369, Chief O'Brien conducted a diagnostic of DS9's fusion powerplant. The results were found to be thirteen percent below normal, well with Cardassian specifications, but not O'Brien's.
To increase operating efficiency, O'Brien increased the deuterium flow to the carbon reaction chamber by five percent, while at the same time observing the exhaust fusion temperature. (DS9: "The Forsaken")
The fusion reactor in the Uxbridge's house on Delta Rana IV would last for 5 more years in 2366 before it would run out of energy. (TNG: "The Survivors")
One way to shut down DS9's reactor involves manually disengaging the laser fusion initiators in the reactor room, or one of the stations control junctions found on Level 34. (DS9: "Civil Defense")
The self-destruct sequence of DS9 involves the computer ordering the station's main fusion reactor to disengage the reaction stabilizers, causing the reactor to overload, and the station to destroy itself. (DS9: "Civil Defense")
In 2372, the USS Voyager nearly drained its fusion reactors while attempting to land safely amidst a massive electrical storm, produced by the Sky Spirits. (VOY: "Tattoo")