Beam emitter

Beam Emitters are large delta-shaped, tower-like structures on Halo with a cleave down the approximate centre. They fire unknown beams upwards, in the case of the Halos to the centre, and in the case of the Ark up into space. They are likely used to transport energy for the activation process of the Halo rings or used to get rid of excess energy. They can be seen in the Halo: Combat Evolved level, Halo, and on the Halo: Combat Evolved cover. They are also found in the Halo: Combat Evolved multiplayer level, Infinity. Similar structures that most likely share the same purpose can be seen in the Halo 3 multiplayer map Valhalla and in the Halo 2 multiplayer level Relic.

If you manage to stand at the top of the chute in which the beam shoots from, the beam can either kill you, sending you over a hundred feet in the air, or completely pass through you. When you are killed by it, its says you were Killed by the Guardians

It is possible that the beams visible during the campaign of Halo: Combat Evolved are simulated firings of Alpha Halo.