The European Space agency has done tests on a new design for an Ion propulsion engine which they expect will be 10 times more efficient then existing designs. The Ion electric engine are ideal for long deep space missions because the fuel source is much smaller than with conventional chemical rockets. They work by using electricity to generate an electromagnetic field that can propel charged particles (ions) at extremely high speeds.
The new engine can create an ion stream out of the engine at a speed of 210 km/sec. That’s 10 times faster than the engine on the SMART-1 probe, and 4 times faster than other current prototypes.
Unfortunately, we may not see this engine in practice because of the long time that it takes for an engine to be fully tested.
Read more about the engine design at New Scientist here