An ion thruster is a form of electric propulsion intended for spacecraft propulsion that generates thrust by accelerating ions. Ion thrusters are grouped by the way they accelerate the ions, utilizing either electrostatic or electromagnetic force. Electrostatic ion thrusters apply the Coulomb force and accelerate the ions in the direction of the electric field. Electromagnetic ion thrusters use the Lorentz force to accelerate the ions (Wikipedia).
At the same time as the Dawn spacecraft is traveling to the asteroids Vesta and Ceres, NASA Glenn has been working on the next generation of ion thrusters for future missions. NASA's Evolutionary Xenon Thruster (NEXT) Project has engineered a 7-kilowatt ion thruster which can offer the capabilities needed in the future.
Ion thruster produces small levels of thrust relative to chemical thrusters, but does so at higher specific impulse (or higher exhaust velocities), which means that an ion thruster has a fuel efficiency of 10-12 times greater than a chemical thruster. The higher the rocket's specific impulse (fuel efficiency), the farther the spacecraft can go with a given amount of fuel. Given that an ion thruster produces small levels of thrust relative to chemical thrusters, it needs to operate in excess of 10,000 hours to slowly accelerate the spacecraft to speeds necessary to reach the asteroid belt or beyond.
The NEXT ion thruster has been operated for over 43, 000 hours, which for rocket scientists indicates that the thruster has processed over 770 kilograms of xenon propellant and can provide 30 million-Newton-seconds of total impulse to the spacecraft. This illustrated the fact that its performance permits future science spacecraft to travel to varied destinations, such as extended tours of multi-asteroids, comets, as well as outer planets and their moons.