So interesting, those HV-diodes they use in plasma-ignition systems in older cars, like in here:

The reason for those huge voltages seems to me explained here;
http://www.energeticforum.com/renewable-energy/3235-gray-tube-replication.html?highlight=water+sparkplug
in message #3
The flowing current when stopped by the rectifier creates this huge reverse voltage.

"The HV potential, which is the voltage gas (it is a gas composed of "particles" smaller than a hydrogen atom - smaller than an electron - very, very small fractional charges - Mendeleev's original periodic table of elements showed some of this) has MOMENTUM.

As it is moving, the HV diode slams shut and the voltage potential with its momentum slams against the diode...compressing it more and more...now that re-compressed potential...which is expanded 90 degrees from the wire has to go somewhere...the only path to ground as of now is the gap on the plug. That un-condensed/expanded potential is what causes the primary effect of the cold plasma.

Then, when voltage on the cathode/output of the HV diode drops below the annode/input of the HV diode, then and only then can LV source move out of it and over the gap.

The lv source is not the cause of the effect, it simply can boost the primary effect - which can be had all by itself with one small cap that is virtually empty when discharged to the primary.

Primary effect is that potential slamming against a diode.

Many think the simple hv spark jumping a gap is enough to ionize the gap to reduce the resistance enough to have the lv source follow it...that is simply a fairytale.

Only the recompressed potential against the diode's abrupt shut off can cause the cold plasma ionization that is necessary to allow the lv source to follow. At this circuit's scale of operation, this is all necessary to make these effects."