How can I control high voltage signals? (1 to 15kV.)
15kV is a very large voltage.  Given any current path it will arc over the air. You may need to use conformal coat to prevent moisture on the surface of components causing this to happen.  Needless to say 15kV is very dangerous.  Place high value blead resistoes over any part of the circuit that is capacitive.  Even a few pF at 15kV will give you a nasty shock.

You can use cascode, but the biasing will be a real headache.  The bias tree has to be at many kV end to end.  Any leakage here will wipe out your PSU which I expect can't deliver much continuous current.

Transformer gate drive isolation is better (though you need a really good primary/secondary insulation)  But once you have that the gates of each FET are only at a few Volt above their source.

You stack FETs, Source to Drain e.g. 20 fets each 750V.  Buffer and bias each FET gate with another small FET and a rectifier (the extra FET gives better turn off times by draining gate charge).  This rectifier receives current from a small transformer.

This transformer's primary is a large very insulated wire driven with a large AC current, e.g. 10-15A

You can even do push-pull this way using two trees of FETs. (No need for P channel).  Drive each side with an anti-phase signal (wind transformers in anti-phase) with electronics ensuring a turn off gap to prevent shoot through.

This will easily switch 15kV at a reasonable current at a few kHz.

This document gives some of the info you require. Their example goes to 5kV.

I once worked on a system where we had several stages, each switching 1kV and used isolated DCDC converters to pass power between stages (so each no more than 1kV from the on next to it) and an optical fibre passing the trigger signal which magnetically coupled to the tree of FETs.  This switched 60kV at 10's of kHz and hundreds of amps!!!