How do I charge a 2100v 0.95uF capacitor?
WARNING: A mistake with a high-voltage charged capacitor can lead to death, or a missing finger!!! Do NOT try this until you have a high-voltage probe rated 40 KV (Tektronix) The probe divides by 1,000 and cost a few hundred dolars.

You will need a step-up transformer that puts out 1,200 VAC maximum. The peak DC voltage will be 1,700 volts. Do NOT attempt to charge the capacitor anywhere near its maximum rating.

You will have to buy 2 transformers with 120/240 primaries and 600 VAC secondaries. Put the secondaries in series to get 1,200 VAC.

You must buy rectifiers rated 3 times the DC voltage on the capacitor, because the capacitor will have +1700 volts on it when the AC swing back in the negative direction, putting about 2,400 volts potential on this rectifier. So it needs to be rated about 5 KV and several amps. This gives you about a 50% or so safety margin with the rectifiers.

The way we did this at work was put many 6A10 diodes in series. Five of them in series with 3 each 120 K 1/2 W (360 K) bleed resistors across each diode will give you a 5 KV 6 AMP rating. The inrush current into an empty capacitor means you must use diodes that can tolerate such currents for a brief time.

You must buy high voltage bleed resistors of 1 M or so rated 20 KV from Caddock. Digikey carries them. About $50 USD to $75 USD each. Place them across the capacitor so it will always self-discharge if not used. Do NOT short it out with a screw driver. It will go BANG loud enough to make you deaf.

I don't think this will make much of a rail gun, which needs sustained power for several milliseconds. You may need a 100 of these capacitors in parallel. Something I do not want to think about.