by Victor8o5 @ instructables.com:
This high voltage power supply has been designed to output a fixed voltage of around 50kV, it could easily be converted to an adjustable supply by connecting a variac in case of using transformers or by adding some extra circuitry to regulate the power going in. I initially thought about a high frequency PWM to regulate the power going into the capacitors, but I abandoned the idea. I found that adjusting the frequency is enough to make the voltage vary by a significant amount, allowing some control over it, this happens because the flyback must operate at a certain frequency in order to maximize the output.
50,000V High Voltage Power Supply – [Link]