If you wanted to double ac voltage, you could use one of those capacitor diode voltage multipliers. The thing I'm wondering is can you use it on mains? I've only seen people use them with transformers. For example, if you have a european light bulb and wanted to use it, could you use the voltage multiplier? You wouldn't need a massive transformer, so would it work?
Yes, it would work But be aware that voltage multipliers are severely limited in the amount of CURRENT they can provide. They are typically found in applications that need very little current (like bug-zappers or electrostatic gadgets, etc.) And, of course, you are producing DC and not AC if that is important to your (undisclosed) application.