How the spark length grows (was Re: Solid State Coil)

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

Original poster: "Marco Denicolai by way of Terry Fritz <twftesla-at-uswest-dot-net>" <marco.denicolai-at-tellabs-dot-com>

tesla-at-pupman-dot-com wrote:
> 
> Original poster: "Gary Johnson by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <gjohnson-at-ksu.edu>
> 

> As voltage is increased, the power into the coil and into the spark will
> also increase, and it is this power that determines the length of the spark.
<SNIP>
>Spark length is determined by the
> peak power applied *before* spark, in the impulse mode. I think this is the
> reason that conventional Tesla coils outperform the solid state coils. 

Hello Gary.

In my understanding the mechanism is much more complex and the key to
compare spark length growth is to first understand how that happens.
When you have sufficient potential, corona will born: sufficient corona
will produce streamers (filaments) that will extend up to a certain
length. In turn, the streamers will generate a new corona and the
process will repeat, extending in length. As nothing comes for free,
this process consumes energy, takes time, and requires energy to be
substained.

So the final length reached depends on:
- how much peak energy you can provide to the toroid
- how fast (how often) you can provide that energy

This because, even if it takes some hundreds of nanoseconds for a new
corona-streamer to take place, it takes a much longer time for the
ionization to disappear. Therefore, you are "allowed" for short pauses
between your energy feeding. This should relate to the BPS (bang rate)
of a disruptive TC and its influence on spark length.

Now, if understand well, your CW coil is on for 10 ms and there is a
time lag of 2 ms before breakout. That can be roughly compared to 5
bangs at 500 BPS of a disruptive coil.

The problem is that how the bang energy and repetition rate influence
the streamer length is still a field where there is no experimental data
(not yet).

>A
> conventional system applies, say, 10 kV to the coil. Buildup occurs over a
> few cycles at most. The lag in spark formation means the toroid reaches a
> much higher voltage than would ever be observed with the application of a
> lower voltage for a longer period of time, and the resulting spark is much
> longer.

The corona-streamer pair takes some hundreds of nanoseconds to start.
Even a disruptive coil couldn't make it to "fool it" and reach a higher
voltage than the required one. You'd need a BPS of some MHz for that.

> 
> Applying power for a longer period of time, say 20 ms rather than 10 ms,
> does not increase the spark length. It will make the spark thicker, however,
> which might be perceived as a longer spark.

You are substaining (more often) the achieved streamer length but
haven't got enough energy to encrease elongation.

Researchers have been modeling the streamer-corona process and have been
able to predict the reached length. Sadly, I didn't find any mention of
the case of REPEATED HV pulses (not a single rising edge only).

Regards


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 Marco Denicolai           Senior Design Engineer 
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