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Re: theory(?) for long sparks
From: Jim Lux[SMTP:jimlux-at-earthlink-dot-net]
Sent: Saturday, November 29, 1997 1:46 PM
To: Tesla List
Subject: Re: theory(?) for long sparks
> From: Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
>
> Even though the initial E-field necessary to start the corona/streamer
> process is of the order of 26 - 30 kV/cm, once breakdown has been
> initiated, the voltage breakdown voltage for a long spark gap declines
> significantly, particulrly for non-uniform E-field gaps. Barry's Marx
> generator, for example, breaks down 10 feet of air with 2.2 MV, or about
> 7.2 kV/cm (average). However, generating a 100 meter spark with only a 5
> MV source would imply an average of about 500 Volts/cm (ave.) which
> sounds much too low(!).
Actually, that is just the point that Bazelyan, et.al. make. For long
sparks in gaps with nonuniform field, i.e. just about anything if your
voltage is 2 MV, the average field is quite low. The 100 meter discharge
from 5MV, of which there is a spectacular photograph, is, as you say, only
500 V/cm. The field at the earth's surface prior to lightning strike is
only in the 100 V/cm range. Some sort of leader phenomenon is clearly
necessary for long sparks, so that the small radius leader head, where the
field is 30 kV/cm, can advance.
> The difference between 20" and 63" discharges for the same peak voltage
> is due to the relatively short time between successive bangs (about 2.5
> milliseconds), which permits a streamer from the present "bang" to build
> upon the thermal and ionic remnants of the spark channel blasted by its
> predecessors. This growth can be clearly seen in successive video-tape
> frames of streamers. Large top C and lower Zo for larger coils appears
> to also improve spark length and "heat", probably because of the greater
> current delivery capability during streamer propagation and the fact
> that thermally-generated ions remain longer in hotter channels during
> the time between breaks.
>
As to keeping the ions around between half cycles, I am not too sure. The
recombination from ionization due just to E field takes about a
microsecond. The thermal ionization may hang around a bit longer. I am
doing the calculations now on that. Uman, in his book on lightning,
theorizes that the thermally ionized column, kept hot by the "continuing
current" provides the path for the second and following strokes of a
lightning flash. Here, the interstroke interval is 10's of milliseconds,
even though the lightning channel is only a few cm in diameter, and would
tend to cool quite rapidly thru radiation and convection.
I would also be very suspicious of any measurements or observations
recorded on consumer grade video equipment. Sparks have a very high
contrast ratio, and the possibility of afterimages, etc. is very high. Now,
if you are recording with a film camera, or an electronically shuttered
high speed camera, that is a different story.
Anybody out there that works in the film industry (Bill Wysock?) ever had
your coil photographed at high speed with a Photosonics at several thousand
fps?
> Hopefully, we'll know a bit more about streamer propagation and the
> associated current structures when Greg Leyh takes some actual
> measurements of streamer currents coming off his 130 KVA monster coil...
> from _inside_ his toroid! :^)
>
Especially since resonant frequency is down in the 38 kHz range, giving a
nice long half cycle time. Nice high voltage, a big toroid to provide some
"voltage stiffness", and a long smooth pulse, would seem to be what is
needed for those 100 meter sparks.