What's the magic for a cap's AC voltage rating vs. DC? I see 630VAC caps that are rated at 2kVDC -- that seems to be more than just sqrt(2)*630. Is there some consideration of current at 60Hz, an estimate of the size of spikes you'd see on an AC line? Some regulatory razz-matazz?
I may be helping out on a CDI circuit, and it looks like choosing a robust and cheap storage cap is going to be a good part of the magic.