I have setup a HV probe made it myself to check the voltage generated from the flyback transformer and the result is still ambiguous to me. The sine wave is smooth and looks like to good to be true, not sure if I'm missing something in my probing setup. From the oscilloscope reading, the maximum voltage is 7.6kV at almost 34kHz.
Visually I can see the plasma arc start to arcing around 0.5cm between the brass spark gap, in which I guess the voltage is more than 10kV based on air dielectric breakdown of 30kV/cm (please correct me if I'm wrong).
My poor's man HV probe is using simple resistor divider consist of 10 x 1 MOhm and 1 x 1 kOhm. One tip on the flyback output wire and the ground have been left to touch the table ground.