high voltage

This is probably the most well known flyback transformer driver circuit. It uses two 2N3055 power transistors in a push-pull oscillator configuration. The value of the resistors is not that critical. In fact you can even use a 1/2W for the 27 ohm resistor, but it may burn out if it is left to run [...]

If you need about 12,000 volts DC for an ion generator this circuit might be the ticket. It draws power from the 120vac power line but it uses a small 6KV camera flash trigger coil. The output signal is isolated from the power line. Although the circuit can only deliver about 5uA of current it [...]

This is circuit high voltage Power Supply DC Adjustable Voltage 0-300V 0.1A max. It is a simple circuit which can provide an Variable voltage source of 0 to 311V by R3-500K
The supply is short ciruit proof and the current is limited to about 100mA.
Source: http://www.geocities.com/tjacodesign/300vsup/300vsup.html