Marx generator that imitates a Tesla coil (RF output, 120 Hz repetition)

[Bart M]

Hi! I thought I’d share a Marx generator I built back in 2015-2016, in case it’s of interest. What’s unique about it is that its output imitated that of a spark-gap Tesla coil, with bursts of RF repeating at 120 Hz. To do this, it used oil-immersed inductors between stages for fast charging and a secondary coil with toroid as a load. To minimize losses, I fit the generator inside the secondary coil. Part of my motivation was to see if doing all this would create spark discharge similar to that of a Tesla coil, which worked!

Here’s (a) a side view of this “Marx coil”, (b) its single-ended spark discharge, (c) a view of the top of the generator beneath the toroid, and (d) a side view of the generator with the secondary removed:


Here’s a circuit diagram with component values and closeups of some inductors, a stage capacitor, and some spark gaps:


For full details, here's a paper about it that's free to read:
https://arxiv.org/abs/1808.06706

There’s some additional information in a note on my personal website:
https://bartmcguyer.com/notes/note-7-MarxCoil.html

Last, here’s a video of its spark output:
https://bartmcguyer.com/media/2018_RSI_McGuyer_MarxCoil_Supplement_Video.mov

[davekni]

Hi! I thought I’d share a Marx generator I built back in 2015-2016, in case it’s of interest. What’s unique about it is that its output imitated that of a spark-gap Tesla coil, with bursts of RF repeating at 120 Hz.

Interesting idea.  I learned what Marx generators are shortly after building my first TC.  On the surface they seemed like a simpler way to play with high voltage.  Much more work than I'd thought, at least to hit reasonable repeat frequencies.  Final project has some similarities to yours.  Repetition rate is up to 120Hz, ladder uses inductors, and output load is resonant to avoid excessive dV/dt stress on Marx capacitors.  Difference is that my resonant output inductor is to a separate air capacitor, which then generates one short high-current discharge for every Marx cycle.
    https://highvoltageforum.net/index.php?topic=2755.msg19787#msg19787

it used oil-immersed inductors between stages

Are only the inductors in oil or the entire Marx generator?

[Bart M]

Hi David, thanks for sharing - that's a serious Marx generator! There are definitely similarities. I'd be curious to know if you noticed whether it could generate streamers like a Tesla coil?

About oil, only the inductors were immersed. I placed them in inside clear PVC pipes filled with mineral oil.

[davekni]

I'd be curious to know if you noticed whether it could generate streamers like a Tesla coil?

Yes, it did make small ~15-20cm streamers from the spheres on top.  One time it arced to an adjacent wall (to grounded foil covering the wall) about 60-70cm away.  The spheres are there in order to minimize corona discharge, but they didn't completely.  That created loss, lowering final voltage on air capacitor.  Simulations suggested I'd get 600kV on air capacitor.  Simulation included some losses such as stray capacitance from Marx top.  Air capacitor value was lower than erected Marx capacitance, so should have swung to a higher voltage than the theoretical 500kV Marx output.  I have no way to measure the actual Marx output.  The air capacitor voltage I measured using a capacitive divider (two-sided copper clad adjacent center of grounded air capacitor electrode).  Calibrated divider using directly measured 20kV discharges.  Resulted in 440kV measured across air capacitor.

On your Marx generator, I'm a bit surprised that those blue ceramic capacitors survived 120 Hz discharges without overheating and cracking.  That's why I was wondering if they were in oil too.  Similar capacitors I've purchased have too much dielectric loss.

[Bart M]

Very neat about the streamers!

I didn't notice any issues with those capacitors. It's quite possible the capacitors would've eventually failed the way you suggest if I didn't encounter a problem with the stage inductors first, from the mineral oil becoming contaminated: There was a little air at the top of the pipes, and that oil-air interface burned from the field stress, contaminating the oil and eventually the inductors.

Here's the inferred output voltage from measuring the base current of the secondary:

[davekni]

Here's the inferred output voltage from measuring the base current of the secondary:

Thank you for the interesting data!