Capacitive discharge generators are very easy to build and work very fine. The circuit is very similar to the trigggering coil circuit of a strobo (without the xenon lamp) or an electronic ignition of a car.

When supplied with 30 volts, this model delivers a HV output of 40-60 kV @ <5 kHz with a PRR (Pulse Repetition Rate) of 850 Hz (max). At higher PRR values it delivers quite hot and fat sparks, so watch out were you put your hands! (Remember the PRR frequency is very low: 850 Hz, so painful shocking is is present).

Do you want to know more about the circuit? Technical calculations? Construction tips?
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The Circuit & Capacitive Discharge Theory

Here we have a beautiful example of the machine working at medium power (power supply @ 14 volts DC). The ECT reference shown in the photo is what I've called a Transparent Condenser Electrode (ECT). Further description of this type of electrode and its construction is found in my Kirlian pages: Condenser Electrodes

In this one the supply voltage was increased to 25 volts DC. Now a lot of streamers come out from the leaf, spraying out over the surface of the ECT.