Re: Then what's the topload FOR?

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Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>

Hi Peter,

You've really opened a complex can of worms with these questions! There
is a significant body of evidence from research on long spark
propagation which demonstrates that long sparks propagate via a series
of jumps (the "Leader Theory" of spark propagation). Each extension of
the leader (which coilers call "streamers") is accompanied by a brief,
high amplitude current surge from the HV terminal through the root of
the leader, which transfers a slug of charge from the HV terminal to a
region (space charge) near the terminal. Unfortunately, most scientific
research has been confined to high voltage impulses, not Tesla Coils. 

Fortunately, there is some empirical data for Tesla Coils. One of the
most interesting (and impressive!) measurements showing the complexity
and fine structures of streamer currents was performed by Greg Leyh on
his giant 38 foot high system - Electrum - in 1997. Electrum's top
terminal (a 7 foot diameter sphere for artistic reasons) was actually
constructed from a series of stainless steel rings, and it was
physically large enough so that a person could safely be inside the
terminal WHILE the system was running. By taking portable measuring
equipment and wideband current transformers up to the top of the coil,
Greg was actually able to measure currents flowing from the output
terminal to propagating streamers! Some of these measurements can be
seen at Greg's site:
http://www.lod-dot-org/electrum/electrumspecs.html
and a picture of Greg taking these measurements can be seen at:
http://www.lod-dot-org/electrum/Leyh23pwr.jpg

The following URL's show the very fast current surges during streamer
propagation, as well as some associated higher frequency ringing. Both
are superimposed upon the much lower frequency (~38 kHz) waveform of the
resonating secondary-topload system. In particular, look at the 20, 10,
5 and 2 uSec/div traces. These show coil-to-topload current (upper
traces) and topload-to-streamer current (lower traces) - you can very
clearly see the brief, high amplitude spikes which correspond to leader
propagation:
http://www.lod-dot-org/electrum/sphere20us.jpg
http://www.lod-dot-org/electrum/sphere10us.jpg
http://www.lod-dot-org/electrum/sphere05us.jpg
http://www.lod-dot-org/electrum/sphere02us.jpg 

Now to your second question - Tesla Coil spark length is a very complex
function of bang size, operating frequency, topload capacitance, break
rate, coupling coefficient, and quenching. Generally, lower frequency
systems tend to perform better since they typically are physically
larger, tend to use larger bang sizes, larger resonators, and larger
toploads. There's strong theoretical and empirical evidence that larger
toploads tend to produce longer sparks (all other things being equal...
which they never really are). Once initial breakout begins, a leader can
be further extended during subsequent voltage upswings on the current
cycle, a following cycle, or even cycles occurring on the NEXT ringup
(i.e., during 2nd notch quenching). There's some theoretical reason to
believe that the rate of rise of the voltage envelope during ringup may
also be more conducive to leader propagation in lower frequency coils. 

Further complicating this, is that once a hot channel has been
established, the higher temperature (lower air density) region occupied
by an earlier leader channel will have a lower breakdown voltage.
Leaders beginning on the NEXT bang now have an easier time reigniting
and following the path blazed by the earlier leader, allowing the newer
leader to propagate a bit further than the previous one. Leader length
continues to increase from bang to bang until an overall power/loss
balance is achieved. You can actually see leaders growing between
successive bangs, particularly on low bang-rate/SRSG systems. 

Hope this helped...

Best regards,

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com

Tesla list wrote:
> 
> Original poster: "Peter Lawrence by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <Peter.Lawrence-at-Sun-dot-com>
> 
> Bert,
>      do we have physical proof that the spark takes nanoseconds (while the
> resonant F of the secondary is 1000x).
> 
> I've often wondered why my four coils with different wire (hence different
> turns, hence different F-res) but all having the same external dimensions
> behave so differently. Longest sparks from the coil with lowest F-res.
> I've wondered if this has to do with matching F-res to the spark propagation
> time.
> 
> Peter Lawrence.
> 
> >Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <bert.hickman-at-aquila-dot-net>
> >
> >Chris,
> >
> >The topload adds capacitance, lowers the secondary's resonant frequency,
> >and its radius of curvature prevents premature breakout. As the
> >secondary "rings up", energy is transferred from the primary circuit to
> >the secondary, and the amplitude of the voltage oscillations at the
> >topload increase, eventually exceeding the breakdown voltage around the
> >terminal. Once breakout occurs and the spark begins to propagate, events
> >happen very quickly - so quickly that the relatively low frequency
> >oscillations in the secondary "look" much like high voltage DC. This is
> >because the actual current spikes associated with spark propagation are
> >of the order of 10's of nanoseconds while operating frequency of the
> >coil may have a period of the order of 10,000 nanoseconds (1000X
> >greater).
> >
> 
> >
> >Best regards,
> >
> >-- Bert --