concentrator

noun

1. One who concentrates.

   “Experienced classroom teachers are well acquainted with the attention-seeker, the shy girl, the aggressive boy, the poor concentrator, the slow student […]”

2. A hub.
3. An apparatus for the separation of dry comminuted ore, by exposing it to intermittent puffs of air.
4. A frame or ring of wire or hard paper fitting into the cartridge case used in some shotguns, and holding the shot together when discharged, to secure close shooting; also, a device for slightly narrowing the bore at the muzzle for the same purpose.