(Note: I'm not suggesting a Plasma Thrower, that's already in trello)
So in my mind the Plasma Thrower would be the first roof mounted weapon (If you don't count the tractor beam) and it would, as you guessed, literally throw balls of plasma. When a target is in range, it would spin (Like an Ftl drive) and gradually pick up speed. When the max speed is reached, it would hurl a ball of plasma, and whenever it hit something (this is in the trello) it would catch a large area on fire, but do no damage itself (aside from the fire).