This circuit runs very smoothly and gives an impressive output. You may adjust both the duty cycle and frequency for best performance.
The base of Q1 is neatly driven by positive pulses (watch out D1, if it burns, say goodbye to Q1...). You may use other NPN transistor for Q1 instead of a 2N3055, just keep in mind to replace de varistor (MOV) with one rated slightly less than the Q1's Vceo (to protect it from voltage spikes).
The OPAMP should be one with high input impedance, high slew rate and high gain-bandwith product.
Flyback primary is about 5 turns of hook-up wire on the ferrite core of an old flyback); usually less than 10 turns.
For higher frequencies replace the capacitor on the inverting input of the IC1, for example 1 nF.
If you have a pulse generator of your own, you may use just the power side of the circuit, from point A to the right. Inject the signal at point A (IC1 & IC2 won't be used then).
I've obtained wonderful 2 inch RF arcs from a little flyback drawing about 2 A at 20 kHz.