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Re: Slow Wave Helical Resonator Experiment



HI Marco,

> Original Poster: "Marco Denicolai" <Marco.Denicolai-at-tellabs.fi> 
> 
> I am planning to build a big TC with a DC tank supply (20 kV DC 1A max). I
> would like to control both quenching time (and phase) as well as bang
> repetition rate. This becomes complicated with a rotary spark gap.
> 
> Can you please post some more info about your MOSFET switch: schematics,
> max current substained, etc.?

You'll be going into uncharted territory using MOSFETs for a coil 
with those specs. You could look at more modern devices akin to SCRs 
and GTOs in some advanced form. Greg Leyh once spoke about these. He 
also said they cost a fortune. 
    To whit: the experiments were done on a coil with primary 
voltages kept well below 100. At these kinds of voltages, using 
MOSFETs etc was a viable means of obtaining a controllable gap. It 
was the only way open to me at the time but the experiments were 
highly useful in testing some of the ideas out there. The results 
are valid and scalable in principle but won't be so easy in practice.

    My first scheme used paralleled 10A MOSFETs (paralleled to get 
"gap" resistance tolerably low). Note that the primary waveform is a 
log decrementing one because there was no non-linear element in the 
"gap". The second scheme was to use anti-paralleled MOSFETs with 
series source diodes to defeat the substrate zeners. I never 
implemented this one as the first worked just fine due to the 
unipolar charging circuit doing much of the defeating for me. I used 
4x MTP3055E's to obtain a primary Q of 9 (don't remember the L/C 
sorry). Primary energy was measured in mW. I obtained about 1/2" 
sparks under some conditions. However, the object of the exercise was 
to try and induce exotic secondary behaviour and note quenching 
issues when interrupting the "gap" at different parts of the primary 
cycle. I took nearly two rolls of film off the scope.

Malcolm