Unfortunately magnets put forces on themselves, which means you’re limited to storing up to about as much energy as it would take to break the thing making the field; or, to put it another way, it’s about the same range of energy densities as springs or rubber bands.

Earlier discussion: https://news.ycombinator.com/item?id=36954783

Does that mean you would get equal(but probably insanely more dangerous) performance with a giant miles long spring instead of the proposed superconducting loops?

Do you mean superconductors create super capacitors, or how do you want to trap the charge?

Or store in coils, as "magnetic energy"(forgot the name for it)