'The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is a type of propulsion system that produces a plasma exhaust at temperatures similar to those in the interior of the sun. The system may generate rocket thrust with performance hundreds of times higher than that of present chemical rockets. The increased performance could mean dramatic reductions in fuel requirements. While conventional rocket nozzles would melt under the extreme temperatures, VASIMR uses magnetic force fields to control and direct the plasma exhaust jet.'(link)
Sounds neat. Don't know how viable it is though. Chang-Diaz seems pretty convinced of this technology's future:
'A NASA astronaut and scientist for 25 years and a veteran of a record seven Space Shuttle flights, Chang-Diaz retired from NASA in July 2005 to continue work with the development of the VASIMR engine with Ad Astra Rocket.
'Chang-Diaz conceived VASIMR in 1979 while at The Charles Stark Draper Laboratory in Cambridge, Mass.