So check the citations for papers:
Besides, the whole ship would have maybe 15kg of fuel. That means that there can only be a maximum 15kg of fission products. It would burn fuel that for several years, spreading the exhaust out over the distance from here to Pluto and back.
In fact, even though the fuel pellets are only about 1µm across, they still bump into enough atoms on their way out that the pellets heat up significantly. A major engineering consideration of the engine would be to absorb or deflect the heat radiated by the >1000°C fuel pellets without letting that heat quench your superconducting magnets.
On the other, I'm not sure where your 15 kg came from.
This matters, because fancy fuels matter a lot more for higher-mass or high-Δv payloads than smaller ones.
A fission fragment rocket can be Isp of 1,000,000[1] depending on the exact details — thrust is proportional to momentum (mv), not energy (0.5(mv^2)), and that means four million times the energy density is two thousand times the momentum and thrust, so that 15 kg is like 30 tons of conventional propellant: a nice saving, but you'd use a lot more than that for e.g. a manned mission to Mars.
For missions where the payload rather than the speed is critical, fuel is also a small fraction of total mass, so you also get a performance boost from being able to approximate the Tsiolkovsky rocket equation as linear.
But that's perhaps another factor of 10, which is still roughly 25% of a Starship upper stage, so even then I'd expect at least 60 kg even if the engine itself can be considered negligible in both cases.
And that's likely to be burned through much sooner than Pluto, though it depends on the details of the design. The ship would likely melt if you tried to thrust at 1 gee, but I think it would still be comparable to the Earth-Moon distance, give or take a factor of 3.
If you want something that burns from here to Pluto, then… huh, I was going to say you're likely back in Tsiolkovsky's realm, but apparently still not, and also still sub-relativistic (~ 1 milli-c for the specific values I was using).
Which is still safe, I just don't think it's quite as trivial as you say.
Which, if you do the math, is definitely the case. Ain’t nobody walking away from getting one of those to the face.
Larry Niven’s Known Space stories coined the phrase Kzinti Lesson when a peaceful Human crew turned their photon drive on an attacking Kzinti ship, slicing it in half. The Kzinti had acquired an anti–gravity drive from aliens (who they then enslaved and ate) so they didn’t have a good visceral sense of the energy required to visit the stars the hard way. Their telepaths kept reporting that the Humans were peaceful and didn’t have any weapons on their ship right up until the ship flipped over and sliced them in half. That kind you can point.
At the accelerations most sci-fi happens at (even fractions of 1G), the energy coming out the business end of any sort of drive based on extrapolations of currently known physics are going to be in the TJ+ range, and often with relativistic particle velocities.
Probably not as focused as a laser type drive, but in a vacuum such a fusion type drive would melt any known materials at multi-km distances and likely cause extreme x-ray emissions from whatever it was vaporizing when it came in contact with it. In atmosphere, it would likely just vaporize the space craft along with anything in a several mile radius.
It’s basically carrying around a directable, continuously exploding hydrogen bomb.
So is that at first an electrical force that results in kinetic acceleration?
Given this, I'm guessing that, for direct conversion to be at all efficient here, a significant fraction of this energy would have to be converted into electrical potential energy rather than be dissipated as heat in collisions between these nuclei and any part of the apparatus. Are there any nascent technologies of this sort?
[1] https://en.wikipedia.org/wiki/Nuclear_fission