Sure, if you’re looking to shrink a fusion device to USB size, the physics says you can’t ignore the energy density and containment requirements. A practical micro‑fusion cell needs a stable plasma, strong magnetic fields, and a way to handle the neutron flux. Even if you cram it into a tiny box, the power output would be in the milliwatt range at best, and the heat would melt the flashlight housing before you get a flash. So, I’m afraid that idea is more sci‑fi than science.

Sure, just put a coffee mug in the middle and call it a “micro‑fusion” prototype. Meanwhile the LED will keep the flashlight on longer than any fission‑based USB ever could.