Where's all this voltage coming from?

[Terry]

Designing the plate capacitor to be 20Kv rated, I come up with a dielectric of .050" plastic and air gaps of 2/3 cm.

So I tested .0078" aluminum separated with .0625" polyethylene and 1.125" air gaps and came up with about 38 layers to get the desired 20nf capacitance.

However, when I mocked up 3 plates (1.5nf) and hooked it up to the 10Kv rms OBIT, I got arcing through the air gaps. At 30Kv per cm of air, that comes out to 84Kv. WTF?

Not every time. Simply plugging in the OBIT and leaving it seemed ok with just a buzzing noise and no apparent arcing. It was when intermittently powering the OBIT (say 3 times per second) that the problem occurred.

Anybody know the cause and/or how to correct it?

How worried about it should I be if the goal is a good-running SGTC?

[davekni]

Home made HV capacitors are surprisingly difficult.  Creepage distance (distance along an insulating surface) is much larger than air gap distance for a given breakdown voltage.  And 30kV/cm is for perfectly uniform conductors on either side of a gap.  (Think large diameter smooth spheres with small gap distance compared to diameter.)  Other shapes cause field concentration at points and corners and edges.  I've tried making HV capacitors many times.  Only successes are for moderate voltage and limited lifetime use such as ~5kV for an air (nitrogen) laser.

[Twospoons]

And 30kV/cm is for perfectly uniform conductors on either side of a gap. 

And thats for dry air at sea level. When the humidity or altitude get up a bit then you could be as low as 10kV/cm.

[Benbmw]

It was when intermittently powering the OBIT (say 3 times per second) that the problem occurred.

Terry, in addition to what Dave and Twospoons commented, it sounds like you could be getting voltage spikes on the output of the transformer due to turn off transients. Im not sure how you are driving it (directly from mains AC?) or switching it on and off, but presumably when you switch it off it results in a large negative dI/dt across the primary as the switch opens. This quickly changing flux induces a large voltage in the secondary.