From Halopedia, the Halo wiki
The EMP rifle is a support weapon seen in concept art for Halo Wars. The weapon is an Insurrectionist infantry weapon that generates an Electromagnetic Pulse. To use the rifle, one must wear a charging backpack that features a large, metal dish. The dish collects and focuses static charge and transmits this to the weapon. The charge is converted into an EMP beam, which is then shot out from the rifle and used to disable electrical systems.