Too powerful to be an ion drive. Most likely something like this: https://www.nasa.gov/pdf/718391main_Werka_2011_PhI_FFRE.pdf

First you take a dozen nuclear warheads, then you take them apart, and grind the cores into nano-scale dust. Radioactive decay will make this dust very highly electrically charged, which means you can - in low gravities - suspend it in magnetic fields. So you take a magnetic bottle, expose it to vaccum, surround it with a moderator, and pour in bombdust until it goes critical. Since this reactor core is mostly vaccuum, each atom that splits mostly doesn't turn into heat - instead the halves of the former fissile atom try to leave the magnetic bottle going at 4-5 percent of the speed of light. Use more magnets to point this stream of particles out the back, and it's a rocket. An absurdly good rocket. I mean, you can't take off from anything bigger than ceres using it, and you should avoid pointing the exhaust directly at planet earth (as long as you point it even slightly away from any planets, the exhaust will be leaving the solar system shortly. So, no this isn't a polluting engine...) but.. the isp's and the masses it allows you to move are very impressive.
Space exploration fueled by disarmament, what's not to like?