I think I like the recent work on pulsed fission-fusion better. Basically you have a normal z-pinch fusion system but surround the fusing hydrogen with U238. It absorbs the fast neutrons from the fusion, fissions, and creates more heat driving the fusion. There's some radioactive waste flying out the nozzle but if you're already in space that's not a huge problem.

On the upside, people aren't going to get nervous about nuclear weapons in space since you can't trigger the fuel pellets outside the engine. And you don't need a massive pusher plate, you just direct the charged plasma with magnetic fields. And that means you can wait to turn this on until you're in orbit and avoid creating any fallout on Earth.

https://www.nasa.gov/content/pulsed-fission-fusion-puff-prop...