Fusion power

"The power of the sun, in the palm of my hands!"

— Some mad scientist near the beginning of the 21st century

Fusion power is nuclear fusion. Essentially, small atoms, like Hydrogen, its isotopes, or helium, are fused into larger atoms, and the process releases energy. Stars are powered by this. A form of nuclear energy, the opposite of fission, the more well-known and actually existant method.
In the real world, fusion is not yet a proper thing. Energy-positive reactors are yet to be developed. Still, there are many consepts and prototypes. From the tokamak desing, which uses superheated plasma confined with electromagnets, to ones where small pellets of gold are heated by lasers. The tech is advancing pretty well, and I expect it within the century.

Utility

Power generation, for vehicles, robots, cities. Has also been applied to make plasma weapons. Most reactors draw in hydrogen directly from the atmosphere of the environment, and either vent out the product, or recycle it for, say, oxygen on a spaceship.

Manufacturing

A mid-tier assembler can build a functioning reactor. The reactors do need an initial jolt of power to get started, but from there, they are self-sustaining.

Social Impact

Massive. Nearly free fuel, next to no enviromental impact, and massive power generation capacity all made fusion the go-to power source for everything for nearly 400 years, until antimatter power was developed, and is still used as a go-to in most places. Antimatter reactors are often overkill for even, say, a small city of a few million.

Access & Availability

Extremely accessible. The standard for smaller spaceships, cities, robots, vehicles, almost anything with a reactor is going to have one of these, unless it's big enough to warrant antimatter reactors.

Complexity

Complex for our standards today, but not very complex for the people of the 2870s. Can be manufactured in assemblers, albeit somewhat more advanced ones. The most common desing is the tokamak, as it is somewhat easy to build and only requires hydrogen to power it. Other designs exist, but are less common.

Discovery

Consept derived in the late 20th century, and energy-positive reactors were finally achieved in the 2050s.

Related Materials

• Hydrogen