Wow, that would be some project! 1 MHz pulse rate?
If I had to do something like that, I would probably investigate light controlled squibs as the switches. They can hold off a lot of voltage becasue they are basically insullators until a strong laser light pulse hits them, then they become electrical short circuits.
I would make up some sort of ring of storage capacitors, which charge up at a slower rate, and use the squibs to fire off one capacitor after another, clockwise, getting the fast pulses. You would need some interesting capacitors with very low esr to get anything like that pulse width. If you wanted longer pulses, maybe lengths of coaxial transmission line charged up to 10 KV each, again in a rotating ring.