Re: the cure for racing sparks

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>

I wrote:

> #8 Discharges to earth from the topload redistributes the coil's
>  remaining stored energy into a 1/2 wave mode with a volts max
>  around a third to half height. (RV) Cure: stay with streamers
>  to air, or use wider toroid.

Antonio wrote:
> The energy trapped in the self-capacitance of the secondary coil
> is really not immediately available, and will usually oscillate
> until dissipated.

Yes, 

> The simple lumped model canīt predict what happens next in this
> case. ...  Maybe those models that we discussed some time ago,
> in a version (surely possible) including the primary circuit.

We just happen to have one of those :)

Modeling this situation, using dimensions for Marco's Thor system

 http://personal.inet.fi/atk/dncmrc/thor.htm

by way of an example, leads to the following time domain response.

 http://www.abelian.demon.co.uk/tssp/tmp/thor.wave.gif

with the animation
  
 http://www.abelian.demon.co.uk/tssp/tmp/thor.anim1.gif

This simulation assumes that there is no breakout until a certain
topvolts threshold is reached.  At this point (40uS after firing)
a discharge arc forms from topload to ground, and the arc is
maintained for the rest of the event.

The resulting negative going transient propagates down the coil,
rebounds from the bottom end, travels back to the top, where it
again rebounds, and so on.   This is shown more clearly in this 
zoomed animation, which begins 100nS before the arc starts and
shows the detail of the transient in slow motion,

 http://www.abelian.demon.co.uk/tssp/tmp/thor.anim2.gif

The transient seems to consist mainly of two modes: the primary
resonance at around 68Khz and the secondary half wave at 226 kHz.
All the other even-wave modes of the coil are also excited, roughly
equally, at about 1/10th the amplitude of the first two.

The size and shape of the transient seems to depend on how much
secondary energy escapes back into the relatively harmless primary
resonance at 68Khz.  Most of the rest goes into the potentially
damaging 1/2 wave component.  Obviously there's a lot more work to
do on this, but there does seem to be the opportunity for primary
tuning and coupling to affect how the coil copes with sudden
topload discharges.  Much may depend on just where wrt the RF phase
the arc discharge begins, which is obviously very sensitive to
tuning and coupling.

Another thing which may affect the transient is how quickly it
disperses, which occurs faster for shorter h/d coils.
--
Paul Nicholson
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