Antimatter is matter that is composed of negatively charged particles. Thus, while a hydrogen atom consists of a positively charged proton and a negatively charged electron, an antihydrogen atom consists of a negatively charged antiproton and a positively charged positron (the opposite of an electron). By definition, all antimatter is annihilated if it is brought into contact with normal matter. Through this annihilation, energy is released, usually in the form of photons. It is this process that is exploited in the antimatter reactor.
Antimatter is created naturally through particle collisions and other events all the time. However, since almost everything in the universe is made of normal matter, almost all antiparticles are annihilated instantly. Thus,
Most antimatter in the M’rowan Commonwealth is produced in large space-based particle accelerators. Due to the high energy requirements of these facilities, they are combined with giant solar panels and placed in a close orbit around a star, typically in systems that are also home to major military depots or trading centres.
It is important to note that antimatter production is extremely energy-intensive, as well as inefficient. Since the actual amount of energy contained in the resulting antiparticles is only a small fraction of that necessary to produce and store them, a great deal of energy is wasted in the process. This means that antimatter reactors are only useful in situations where constraints of space are a greater concern than efficiency, such as especially on starships. An antimatter reactor of a given size will produce about twice the amount of energy that a fusion reactor of equal size would, and over four times that of a traditional fission reactor. This allows the ship to save space that can be used for other systems, or often to carry extra reaction fuel.
Antimatter can also be collected in space, especially near planets with strong magnetic fields that can capture and secure the anti-particles before they are annihilated. (The very best candidates are gas giants with large, Saturn-like ring systems). Even then, the amounts that can be collected in this way are too small to support an infrastructure of any size, but can be useful for individual ships refuelling ‘in the wild’.
Storage of antimatter particles is a particular problem, as they must of course be kept completely separate from any form of positive matter. This is usually achieved by cooling the particles to near-absolute zero and keeping them suspended in strong magnetic fields inside a so-called ‘trap’. By manipulating the fields, the particles can be manipulated and fed into the reactor in a controlled manner.
Since even a few grammes of antimatter (though itself a considerable amount) can keep a spaceship poweredd for months, such traps tend to be relatively small and easily portable. In fact, most of their volume is usually made up partly of the magnetic coils and partly of an independent set of power cells – since any power failure would immediately release all antimatter in the trap, they should never depend on outside power sources.
See also: Wikipedia: Antimatter