Class-1 directed energy cannon
From Halopedia, the Halo wiki
The Plasma Cannon (Called the Big Needler in Halo 2's code) is a new statonary turret for Halo 2. Unlike the Shade, this turret fires slow shots that do a lot more damage, except on top of the shadow, where the turret fires impossibly quick. To help protect the gunner, the turret is equipped with shields, similar to the Jackal shield.
http://server2.uploadit.org/files/devilmingy-cov_turret.jpg
