I posted this vid last night from some clips I shot right after I made that tube. I have more but they're all unorganized so for. I'll post more later.

Here is one:
YouTube - Gray Tube Demonstration Proof of Concept

That shows the HV moving backwards through the diode back to ground.

Jetijs, you can do this very, very simple test. On ANY spark plug circuit you make..plasma or not... from the ground of the spark plug, which goes back to ground....in between the plug ground and ground of the circuit...place a HV diode right there with the cathode touching the ground of the plug... some would think it would never run but if you do that, you'll see that the HV has no problem getting to ground going backwards through the diode.

Anyway, enjoy the vid and I hope it is useful. I originally did that particular vid because I saw comments in other forums about Luc's spark method claiming that the HV jumps the gap first, ionizes the air then the caps discharge over the gap. They didn't realize that actually it jumps backwards through the diode from the caps seeking ground since it is a lower resistance path...then the diode shuts off and it has nowhere to go but the spark gap on the plug.

Even in the Suckewer and NASA patents, etc... and other plasma patents with the mixing of HV and LV, it is apparent none of them understand what is happening here....or they do but they simply don't want people to know what the deal is.

Thanks you both for the tips on the carbon blocks on motor brushes.