IX-6315 "Dawn" Electric Propulsion System/Box
This is a very efficient piece of Kerbal tech, it provides very low thrust much like the atomic motor. It should mostly be used on satellites. However, one downside is that they require huge amounts of electric charge, but low amounts of xenon gas, the propellant. Xenon gas comes as a large supply in the 2 tanks, and will take a while to run out. Note all of the parts for an ion propulsion system are in the utility part of the VAB.