Home-made transformer 20-25,000V. Powered by an estimated 1800V DC using an over-volted 1600V DC bug zapper that is normally powered with 2 AA battery's except with 3 AA's making a slight increase in voltage. I am surprised the bug zappers capacitor can survive the increase in voltage.
The transformer is wound on a PVC pipe with a ferrite core inserted. There is about 1000 turns of 40SWG insulated copper wire for the secondary. The primary is 7 turns of a core from a house wiring cable.
The arc can be stretched to about an inch before the transformer will start arcing over.