Fusion power is nothing new: life on Earth-That-Was, or any planet, has always looked to free energy from the sun and other stars—the biggest damn fusion reactors in the ‘Verse. Harnessing all that energy into a power supply without blowing everyone up was the joker that taunted four generations of high-energy physicists back around the last millennium. What finally did the trick was artificial gravity, which let scientists recreate the intense pressures and temperatures in the center of a star, where fusion happens on its own.
Fusion power plants consist of an electro-magneto-gravitic “bottle” that contains plasma (protons from hydrogen) at super high temperature under tremendous pressure. The protons fuse together to produce helium, releasing energy in the process. The hot plasma is tapped off the bottle and run through a magneto-hydrodynamic generator to produce electricity. Other designs employ nanotech materials to convert heat energy directly into electricity (and cool the power plant in the process).
Because there are almost no moving parts, a fusion power plant can run for many decades without a major overhaul. All you have to do is occasionally replace the containment jacket (a sandwich of lithium alloys, boron, and plastics) that surrounds the core and absorbs the stray neutrons produced in the interior; else the jacket gets brittle and breaks down over time. Without core containment, these high-energy neutrons would shoot right through you, poisoning you and turning the ship into a glow-in-the-dark deathtrap through secondary radiation effects.
Contrary to what you read in books, fusion plants don’t explode, even when severely beat up. Despite the extremes of temperature and pressure in the core, the density of plasma is surprisingly low. The amount of hydrogen used to produce power is likewise close to none. Not that a plasma leak won’t still barbecue anyone unlucky enough to be caught in the same compartment, of course. Which is why it’s important to have a backup power supply for when the main fusion plant is offline.
Hydrogen-oxygen fuel cells, which combine the two gases to produce electricity without “burning” them in the ordinary way, are popular aboard ship because they are cheap to run and the exhaust is clean, potable water. The vapor condensers have other uses as well, for the imaginative. Many ground vehicles also use hydrogen-fueled motors, due to their high power-to-weight ratio and the ease of manufacturing. Power cells—loops of high- temperature super conducting wire storing loads of electrical energy—make good backup aboard ship, as well as being the primary power source for small electric vehicles and hand lasers. They are too finicky to bet your life on entirely, however, as they sometimes run out of juice before they ought to.
Along with power goes thermal management: all that power-using equipment generates heat, and dumping it into the perfect insulating vacuum of space is a problem. Big ships, with a generally lower ratio of surface to volume, have an even harder time with heat, and are designed with a lower power budget per ton than smaller ones. Spaceships ordinarily have nanomaterial heat pump wiring running through their structure into radiator grids embedded in parts of their hull plating.