Noble Gas Propellants
Noble gases are commonly used in electric propulsion systems because they are non-reactive and allow the thrusters to function continuously for long durations without corroding. Examples include Xenon and Argon. (Back 2001)
Sources
Back, Dwight Douglas, and Charlie Ramos. "Noble gas storage and delivery system for ion propulsion." U.S. Patent 6,286,304, issued September 11, 2001.