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I'm building my own high voltage capacitors, for science, and I'm running into an issue. I'm 90% sure what I'm looking at is coronal discharge but I wanted to be certain. The caps are two aluminum foil plates with 4 layers of some unknown plastic sheeting between them and then hooked up to my 15kV NST. enter image description here

In this picture you can see the arc that forms after running on AC for a while. The whole thing is much brighter in person but this picture captures the problem I'm having. The thing I'm curious about is that when I hook up diodes to it to make a half wave rectifier this problem does not occur. This only happens on AC and if I reduce the number of layers of dielectric to just 1, it's not so much corona as it is just a constant lighting storm with big arcs like the one seen shooting off all over the place.

So just to confirm, is this coronal discharge? Why does it only happen on AC?

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    \$\begingroup\$ I sure hope you know what you're doing! \$\endgroup\$ – Hearth Oct 14 '18 at 14:10
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    \$\begingroup\$ "Im building my own high voltage capacitors, for science", lol. FOR SCIENCE! \$\endgroup\$ – Harry Svensson Oct 14 '18 at 14:53
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    \$\begingroup\$ Just make the insulators 20 cm wider all-round than the plates. \$\endgroup\$ – Andy aka Oct 14 '18 at 16:28
  • \$\begingroup\$ I mostly know what I'm doing. Enough to not kill myself at least. I have a plan that I hope will fix the problem that doesnt involve larger plastic sheets or smaller plates. I'm just not 100% sure what the problem is. \$\endgroup\$ – Andrew Oct 14 '18 at 20:56
  • \$\begingroup\$ Before silicone or resin potting of assemblies for high voltage withstand it is difficult to test them as air is a weaker dielectric. In these situations various liquid insulators have been used (Freon back in the day) and mineral oils regularly in transformers in service. \$\endgroup\$ – KalleMP Oct 25 '18 at 21:22
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Yes it is corona discharge.

Corona discharges are a particular example of partial discharges : a dielectric breakdown in a part only of the electric insulation. This appears when a sufficient high voltage appears across a gap (most often air gap), which triggers the breakdown, and the corresponding light emission.

You can see this phenomenon occurring on AC, because on AC a capacitive current exists through the entire insulation and is able to periodically charge the equivalent capacitor across the aforementioned airgap. Thus, periodically, the air gap is charged, and then discharged by the breakdown.

In DC, the small capacitor is only charged once, so you won't see light.

To prevent this phenomenon, high voltage devices are impregnated with liquid ("oil") or solid (wax, resins) dielectrics, which have a much higher dielectric strength than gases (air).

Edit: actually, this might not be exactly corona discharges, as these kind of discharges occur with very non homogeneous electric fields (around cables, spikes...). This can perhaps be more directly classified as surface discharges (at the boundary of different insulation materials). Anyway, these are all partial discharges.

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