The story is here and it’s actually quite exciting. The key is VASMIR. Money quote:
Engine work has been underway for more than 25 years, and is based on NASA and U.S. Department of Energy research and development in plasma physics and space propulsion technology. Commercializing the VASIMR electric propulsion engine is the flagship project of Ad Astra, which has been in business for nine years and has invested $30 million to date to mature the concept. [Superfast Propulsion Concepts (Images)]
The driver for this engine is electric power which could be generated either by solar or a nuclear reactor. First practical application may well be the periodic boosts of the International Space Station required to maintain its low-earth orbit.