How much resistance does the capacitor itself contribute to an RC circuit?
You want a rapidly rising magnetic field at ns timescales, and you worry about cap esR?? ESL is more likely to be the gotcha, but even then....

You may want to start by thinking about the inductance of your magnet, and then how much voltage you will need to make the current change quickly in that inductor (Inductors really do not do step changes in current).

By the time you are playing in ns speeds you really want to be thinking EM not just E or H as often your circuit geometry means you have to think fields and waves, not lumped element.