Around Installation 00's Citadel, three barrier towers resembling beam-emitter towers are used to project an energy barrier that surrounds the Citadel. In Halo: Combat Evolved Anniversary, the beam-emitters also have a spinning fan/wheel on the side that appears to charge the beam like a dynamo would, spinning faster as the point of firing approaches. They also seem to act as cooling vents like those of the plasma rifle, opening immediately after firing and closing again after a few seconds.