Or it could be something other than plasma powers their circuits. We'll have to agree to disagree here. Technically, the Constellation's impulse engines would create a "fusion explosion" when overloaded in "The Doomsday Machine". "Thermonuclear" missiles were used against the Enterprise in "Patterns of Force". I think it is pretty safe to say nuclear fusion technology exists in TOS. What Delta Vega produced we have no idea other than it was a Lithium cracking station. You can safely assume it cracked lithium for ore ships to pick up. You can safely assume that some of its power packs can be modified for use on the Enterprise's warp engines. But I don't agree you can safely assume that it produces materials essential for nuclear fusion... lithium for the warp engines, yes, but nuclear power, not enough evidence. I am curious though, how did you arrange the 22 decks (minimum) of the ENT Defiant?