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I thought the original generators were fusion and powered some kind of ion engines?

The ostensible explanation for the later casimir generators was that they were pulling energy from vacuum fluctuations in a similar way that armature-linked buoys can generate power from ocean waves, but they were pretty vague on how that could provide more than a minuscule amount of power. Instead it was portrayed as being able to supply orders of magnitude more energy than fusion, which seems implausible. But at that point we're like 2 books in and already pretty invested...