Ion drive
You may also be looking for advanced ion propulsion.
An ion drive was a type of propulsion that used an electric field to accelerate charged particles and eject them at high velocities (in some designs, the particles approached the speed of light). This type of propulsion was used by many pre-warp civilizations to propel probes and lightweight spacecraft once they were in open space. The thrust produced was quite gentle, often requiring intervals from hours to weeks to reach full velocity.

The Ares IV was powered by a third-generation ion drive. ()

External links

FAQ about ion propulsion at NASA
Ion propulsion at NASA's Glenn Research Center
Ion-propelled moon probe at ESA - reached the moon in November 2004

de:Ionenantrieb
nl:Ionaandrijving
Propulsion technology