[TCML] MMC dV/dt question

[Jacob Karlström <94jac12@xxxxxxxxx>]

Hello,

I am a first-time TC builder and I am having some problems choosing the
right capacitors for an MMC.
I will be using a 4kV, 50mA NST. The secondary coil will be a 110mm
diameter PVC-pipe, most probably 500mm long, winded with 0.5mm copper wire.
The topload will be a toroid with a minor diameter of 102mm and a
major diameter of about 400mm. This gives the secondary circuit a
theoretical capacitance of about 25pF and  inductance of about 20mH. Which
means a theoretical resonance frequency of 225kHz.

My MMC (50nF, 9.6kV) will comprise three strings of six 100nF, 1.6kV,
1400V/uS polypropylene capacitors (
https://www.elfaelectronics.com/elfa3~ex_en/elfa/init.do?item=65-008-00&toc=18856
).

Knowing the theoretical resonance frequency, I have calculated the minimum
dV/dt for each capacitor to be around 1330V/uS. However, I would guess that
real resonance frequencies tend to be higher than the theoretical, owing to
imperfect coils and toploads. If my resonance frequency is about 240kHz or
higher, each capacitor in the MMC would have a dV/dt larger than 1400V/uS.

So my question is, by how much can I estimate my resonance frequency will
differ from the theoretical value? Should I consider building the MMC
differently?

Regards,
Jacob Karlström
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