dc42:
What's the application? If you need pulses of 50kV at a few mA, it's easier to generate pulses at a lower voltage and step them up using a transformer. If you need to interrupt an otherwise continuous 50kV supply, you could derive the supply from an inverter and switch the inverter off to interrupt the supply, if you don't mind the supply taking a few milliseconds to turn on and off.
I am doing an experiment that involves the timed switching of ground terminals before the charge reaches the terminal (making it go in a circle until it runs down the voltage due to resistance). I need the high voltage for other applications, but a big plus to it is the electrostatic field it generates on uninsulated wire allowing me to measure it during the process.
Obviously the charge is going extremely quickly, so a switch with around a 100 ns response time would be good.