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Electrum waveforms, was: Then what's the topload FOR?



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

Peter and all,

Still opening more cans of worms... :^)
Actually, most of the oscillations you're seeing in the lower trace are
actually "ringing" of the topload and the uppermost turns of the
secondary as this mini LC system is shock excited by fast current
transients. These current transients reflect sudden charge transfers
(current pulses) between the top terminal and leader channel(s). These
brief charge transfer events (of perhaps 10-100 nSec duration) can be
observed as superimposed spikes on the peaks of the ringing
oscillations. 

A hot, electrically conductive leader acts much like a charged wire
extending outward from the HV terminal, creating a region of much
increased E-field away from the HV terminal. At the far end of the
leader the character of the discharge changes, fanning out to form a
much larger diffuse region of dimmer, colder discharges (technically
called streamers). It is these streamers which ultimately distribute
charge between the conductive leader and the surrounding non-conducting
region. And, it also turns out that the physics of leader propagation
results in leaders propagating more easily when the HV terminal is
positive than when it's negative. 

The distance that a leader will "jump" is mainly a function of the
reservoir of charge on the top terminal, since once charge begins to
transfer from the terminal to the leader, it also reduces the HV
terminal voltage, starving the E-field necessary to drive further leader
propagation. Since small coils usually have smaller toploads, they have
a smaller reservoir of instantly available charge, resulting in shorter
leader length/jump. If the terminal voltage can be replenished quickly
enough, then another jump can occur... and this process can continue
until peak output voltage is achieved during secondary ringup. So, it's
quite possible that even smaller coils may have a number of leader jumps
along the way to reaching full terminal voltage. 

Of special interest in the discussed waveform: If the topload and top
few turns of the resonator are shock excited so that they ring with
sufficient amplitude, charge flow may actually reverse, pulling charge
back out of the space charge region created from a previous leader jump.
This is apparently what is occurring in the lower waveform for Greg's
system as both positive and negative current transients can be observed 
superimposed upon the ringing displacement current going to/from the
leader channel. 

The overall physics of gaseous discharges and their interactions with
Tesla coils is still far from being fully understood. The above
description hopefully give you a feel for some of the complexities. And,
to my knowledge, there are no scientific groups studying these
fascinating long RF discharges except the few researchers on the Tesla
List...

Best regards,

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com
 
Peter Lawrence wrote:
> 
> Bert,
>      is it believed that the 7 HF oscilations in the 2-us/division B-waveform
> represent streamer "leaders" forming, that the ?-foot arc produced at the
> time this oscillograph was taken was formed in 7 increments.
> 
> Do people think that "leaders" form at a constant length, and that for a
> smaller TC there would only be one leader jump, and that for larger TCs
> generating larger sparks that there could be multiple leader jumps. OR
> is there some reason why leader jumps would be shorter in a small TC and that
> their sparks would also grow in several smaller jumps.
> 
> and why are there these HF oscillations only on the positive(?) excursions
>  of the current into the topload - do TC sparks only break out on one
polarity
>  of the resonant oscillation (that would explain how the coil form gets a
>  net elecrete charge sometimes).
> 
> and why are there hugh negative current swings in this graph, that would
>  indicate that during streamer formation / leader growth that current is
>  oscillating???
> 
> 
> This is why I think that a streak photo of a TC spark formation, especially
> if it could be coordinated with the oscillographs of the type they took of
> Electrum, would be very enlightening to us TC builders/enthusiasts.
> 
> -Peter Lawrence.
> 
> > 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
> >
> >
> > 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.jpgX-Mozilla-Status2: