spark growth issues... waSRe: SSTCs - High Input Power vs. High Input Voltage

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Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

There's a lot of complex processes going on when a spark grows, and I think 
that SSTCs might be able to shed some useful light on what's going on, just 
because they are more "controllable" than a more traditional spark gap coil.

There's a process in growth which occurs very, very fast when the leader 
develops. This occurs on a nanosecond time scale, so I think that the 
inductance and capacitance of the topload and the spark channel that 
already exists is the primary determining thing.

Then, there's the spark channel development process, where the air actually 
gets hot enough to glow and conducts signficant current.  This one might 
take a bit longer, if only because you have to get enough energy into the 
air to heat it up to 7000K or so.  This one probably occurs on a time scale 
comparable to the RF frequency, i.e. 10s of microseconds, as current flows 
in and out of the spark channel/topload/etc and the secondary inductance.

Finally, there's an even slower process where sparks develop in previously 
hot/ionized areas.. this is the "bang rate" influenced one.


So, I think that no matter how fast your RF envelope rises, the leader 
growth is more affected by the Ctop.  Small Ctop >> short leaders

Fast RF envelope rise times probably help most in the second type above.. 
heating the channel and growing the body of the spark.


In any case, the peak powers in spark/disruptive type coils are a lot 
higher than they are in most SSTCs, and that peak power probably has a lot 
to do with spark growth.  There's probably a "growth rate" to "power 
available" relation, and a "growth rate" to "total max spark length" relation.

The field in TC spark situation is so highly non-uniform that I don't think 
topload voltage, per se, has a lot to do with it, other than needed to 
overcome IR and L*di/dt drops in the spark channel to keep the field high 
enough at the end to promote propagation.  At some point, you can probably 
consider the growing spark as a long wire with a sharp point connected to 
the topload C, and the inductance and capacitance of that "wire" is going 
to start to dominate what's going on.

Consider that the capacitance of the spark is around 1pF/cm, so, by the 
time you've got a 24" spark out there (60cm), the capacitance of the spark 
(60 pF) is more than the capacitance of the topload.

>Original poster: "K. C. Herrick
>
>I will repeat here my conjecture as to why rapid voltage rise is
>important:  A spark emanating from the electrode must heat & push aside
>the air in its path.  That air has inertia & thus resists the push over a
>finite amount of time.  If, during that time period, the rate of rise of
>electrode voltage is sufficiently high, that voltage will increase above
>what it would be absent the high rate of rise.  The resultant abnormally
>high voltage on the electrode will then produce the longer spark.
> >
> > I just spoke with Justin over email and he hit it right on the nail
> > on the
> > biggest factor for long arcs in "direct driven" SSTCs.
> >