[Greysard]

Why couldn't they create a controlled EMP inside a containment vessel, a sort of shielded rotating cylinder, which has slots or windows in it that alternately "flashes" the radiation at a transducer/antenna?

A nuclear EMP is a geophysical effect on scale of the entire planet. Perhaps E1 components can be duplicated in a lab using a highly energetic gamma ray source; but far more energy could be recovered by not doing any of that :-)

[ROCKLOBSTER]

far more energy could be recovered by not doing any of that

But that's not going to happen again...is it?