My vote for best fusion device in fiction goes to the thermocuclear device in “The Fourth Bear: A Nursery Crime” by Jasper Fforde. WARNING! SPOILERS! MAJOR PLOT POINT REVEALED! From the book:
…there is a theory that might explain the sort of damage we saw at Obscurity and on the Nullarbor. It was first postulated in the 1950s but was so far-fetched that even the hard-core pseudoscience elite dismissed it as nonsense. It was called Cold Ignition Fusion and was a way of building a small thermonuclear device using a deuterium/tritium fuel that could be self-extracting from the heavy hydrogen found in groundwater, and then mass-induced organic trigger to set it off.
Yes. A thermocuclear device. Just read the book. Critical mass at 50kg.
I liked the “fusion reactor overload” in Aliens II, which is probably the thing that got me seriously thinking about fusion back in the ’80’s, when a physics-major friend of mine explained how hokey that concept was. When I learned that sustained, runaway reactions were impossible with a fusion reactor, it made me wonder why we weren’t putting more resources into its development.