The Particle Cannon is a long dormant but extremely powerful Forerunner weapon, built in the early stages of the Ark's construction when it was most vulnerable to a pre-emptive attack by the Flood.
Once the Ark's construction had been completed, the cannon lay dormant until its location was discovered in the Cartographer by Professor Anders. The cannon was designed to protect against orbital attacks, and draws its power directly from the Ark itself, through a series of conduits located underneath the Ark's surface. Ionized particles are charged until they reach a high-enough density, allowing the cannon to fire an incredibly long-range and continuous particle beam at its target.