Here are some applications of the Cockroft-Walton voltage doubler circuit for generating high voltages from a low voltage AC source. The circuit is quite practical for generating thousands of volts (or more) or just a few volts at high current. Since the circuit is AC-coupled it may be connected to a power supply’s secondary transformer winding to generate an additional or opposite polarity voltage.

Source:
http://www.techlib.com/files/voltmult.pdf