From Halopedia, the Halo wiki
A particle cannon is a type of Forerunner directed energy particle beam weapon that fires a hyper-dense beam of negatively charged ions. Ranging from a few meters to hundreds of meters long, particle cannons were installed on a multitude of ecumene worlds and used in the defense of important Forerunner installations.
The Z-8060 High-Impact Particle Weapon, as it is designated by the United Nations Space Command, is a model of particle cannon almost five-hundred meters in length which consists of a spherical firing assembly surrounded by three curved "arms." Several Z-8060s were used to defend the gravity well generator of the Forerunner shield world, Requiem.
Smaller particle cannons identical in appearance to the Z-8060s were installed on the Didact's vessel, Mantle's Approach, and used for point-defense against other ships. In 2557, after being dormant for one-hundred millennia, the weapons were reactivated for use against the UNSC Home Fleet. Four were destroyed by John-117 with an F-41 Broadsword fighter to enable to UNSC Infinity to get a clear shot on the Mantle's Approach, blasting open a hole for John to enter the ship with a HAVOK tactical nuclear weapon to destroy the ship.
Multiple several-meter-long particle cannons similar in appearance to Z-8060s were positioned around an access point to the Domain on an artificial Forerunner world, known as Kamchatka to humanity. The cannons were reactivated during an invasion by Jul 'Mdama's Covenant in October 2558 and fired bursts of particle energy at the attacking aircraft and spacecraft.
Particle cannons were also positioned across the surface of Installation 00, also known as the Ark. The firing mechanisms of these cannons were embedded in the surface of the Ark and fired a blue particle beam upwards between three "prongs." In 2559, Professor Ellen Anders of the UNSC Spirit of Fire learned the location of one of these particle cannons from the Ark's Cartographer. As part of Isabel's plan, Anders assumed control over the particle cannon in order to help destroy the Banished CAS-class assault carrier Enduring Conviction though the cannon by itself was not powerful enough to destroy the ship. Instead, the cannon was used to immobilize the ship and drain its energy shields. However, even with Anders drawing as much power as she could into the cannon, it still took around half an hour to fully drain the carrier's shields during which time Douglas-042 and UNSC forces were forced to fend off a massive Banished attack. Once the carrier's shield was down, the UNSC forces abandoned the cannon to the Banished. With the shields down, Jerome-092 was able to board the Enduring Conviction where Isabel provoked the Ark's defending Sentinels to destroy the ship.
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