The Dual-Stage 4-Grid (DS4G) is an electrostatic ion thruster design developed by the European Space Agency,[1] in collaboration with the Australian National University.
[2] The design was derived by D. Fern from Controlled Thermonuclear Reactor experiments that use a 4-grid mechanism to accelerate ion beams.
With that energy input rate, the thruster could produce a thrust of 2.5 N. The specific impulse (a measure of fuel efficiency), could reach 19,300 s at an exhaust velocity of 210 km/s if xenon propellant was used.
These characteristics facilitate the development of ion thrusters that can result in extraordinary high-end velocities.
To provide 250 kW with Stirling radioisotope generators would require roughly 17 tonnes of plutonium-238 (for which the US stockpile as of 2013 was no more than 20 kg[4]), and so a nuclear thermal reactor would be needed.