Ion Propulsion System Hot Fire Test for Deep Space 1 (Credit: NASA/JPL)
The Dawn spacecraft uses ion propulsion to get the additional velocity needed to reach Vesta once it leaves the Delta rocket. It also uses ion propulsion to spiral to lower altitudes on Vesta, to leave Vesta and cruise to Ceres and to spiral to a low altitude orbit at Ceres. Ion propulsion makes efficient use of the onboard fuel by accelerating it to a velocity ten times that of chemical rockets. This efficiency is measured in terms of the specific impulse of the fuel (Isp).
Dawn's engines have a specific impulse of 3100 s and a thrust of 90mN. While a chemical rocket on a spacecraft might have a thrust of up to 500 Newtons, Dawn's much smaller engine achieves an equivalent trajectory change by firing over a much longer period of time. The figure below shows the specific impulse and thrust of different thruster types.
The ion thruster is powered by large solar panels. The power ionizes the fuel (Xenon) and then accelerates it with an electric field between two grids. Electrons are injected into the beam after acceleration to maintain a neutral plasma.