This system is already working in the lab, and is currently the most promising propulsion system for long distance missions. It is currently produced by the Ad Astra Rocket Company.
Best case numbers: (Isp explained in Background section)
Isp ~1,000,000 s (assuming extensive R&D and >1MW engine)
Thrust ~ 5N
The Isp is twice the expected maximum Isp for the upcoming MW engine. The gas is energized by an RF antenna before being shot out of the craft (shown below). The thrust of 5N means that most of the travel time will be spent accelerating up to a top speed, not cruising. Also, this is a technology that is currently being pursued by NASA, so development will be easily streamlined with current research. Power can be generated for the RF booster by an on board nuclear reactor, the only source that is realistically able to generate the large power requirements of the thruster over extended missions. (currently 200kW, soon to be a more powerful engine requiring 1MW).
