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Re: The Next Coil




From: 	Robert W. Stephens[SMTP:rwstephens-at-headwaters-dot-com]
Reply To: 	rwstephens-at-headwaters-dot-com
Sent: 	Tuesday, July 29, 1997 8:52 PM
To: 	Tesla List
Subject: 	Re: The Next Coil

John Freau comments and asks a question of Greg Leigh after Greg's 
announcement of a magnificent new large TC project:

> Greg,
> 
> This is really great news!!!   Do you still feel that capacitance in the 
> secondary can be substituted for top load capacitance--that they have
> an equivalent effect for the same C value?
> 
> John Freau
 
John, Greg,All,

When I was starting my rapid learning curve about how a Tesla coil 
works about 3 years ago, external information embossed me with the 
notion that secondary coil self-C is a bad property which cannot be avoided in
the real world, and which robs us of power because it has to be charged at the
same time as we are trying to get a peak pulse to occur at the top terminal.

I agree with your recent comment Greg of how stepped leader formation can be
enhanced through support of a large topload C where the energy is available 
without series inductance right there at the base of the ionization 
channel.  I've written a comment about exactly that somewhere in the 
recent past.  I also made a post a while ago rambling about how the local 
air along the streamer which stays ionized after the streamer 
extinguishes, acts more than like a momentarily persistent pathway 
upon which the next TC output pulse will instantly fill and then 
extend through stepped leader action (although I used the term dart 
leader because I am old and confused), but I also went a step further 
and postulated that this locally ionized pathway surrounding the 
extinguished plasma channel might actually be storing energy which 
becomes added to following Tesla coil streamers that go this route 
instants in time later.  The streamers which follow this channel as 
we know go well beyond the distance records made when the streamer 
completely changes direction and starts new pathways.  I received 
absolutely no comments on this idea. 

The energy which is stored amongst the turns of the secondary in the 
self-C is not available to the streamer the same way nor as 
immediately.  The energy stored nearest the topload will be available 
first, but not as soon as that from the topload itself.  Like a 
stretched out pulse, the self-C energy will be communicated from 
secondary coil regions progressively further down this helical delay 
line to the top terminal when a low impedance ionization channel 
calls for it.

The way this slower current helps in streamer growth 
and maintenance is another question.  Perhaps it does not aid in 
streamer growth at all but in fact is responsible solely for channel 
maintenance after it is laid out by the fast currents available from 
the topload. 

Also responsible for channel maintenance is the wave 
which propagates up from the bottom of the secondary upon each firing 
of the break, and successive beat envelopes if any.  This energy from 
the source would be mingled and become one with the discharge pulse of the 
self-C of the secondary coil (delay line), I would think, i.e.,  you would 
not be able to distinguish the two as separate entities in the scoped 
output waveform.

With the hands-on experience I now have  I no 
longer accept the statement handed me initially that self-C robs us of 
output power.  The picture is much more complicated than that and I am still 
trying to piece it all together.  I have not yet had the advantage of 
any form of coiling or CAD program from which to plug in variables 
and watch the theoretical results.  So far, if I wanted to see what 
increased secondary L, or a larger top C would do, I would wind a new 
secondary or build a larger topload.  The nice part about this expensive 
tedium is that I now have a good assortment of equipment which is 
darned handy to have in a Tesla coil laboratory.  

Malcolm recently commented that the self-C energy is entirely 
reactive so there is no power loss.  I hope he's 100% right.

I went to great effort about a year and a half ago to put a 15 inch 
diameter, AWG #8, polythermalese magnet wire wound coil 48 inches long on a 
fiberglass form inside a large 200 gallon steel tank filled with 
transformer oil.  The performance of this coil has yet to be gleaned. 
It just doesn't seem to light up like much smaller coils in air that 
I have built using the same input power.  In 'Fundamentals of Radio' by Terman
he mentions how coils may be installed inside metallic housings and that eddy 
current losses in the surrounding container will drop to an 
acceptable level once the radial distance to the containment vessel from the 
coil matches the radius of the enclosed air core coil.  My enclosed coil follows 
these suggestions exactly.  I haven't had the opportunity to pump 
enough input power into this resonator to overcome thermal inertia and 
feel heat on the steel walls or anything, but I have driven it up to 
about 4 kW CW and 900 VA disruptive style.  In disruptive mode , by 
observing the discharge to a ground wand it is apparent that there is 
*a lot* of C being discharged in the resonator's output.  Streamers, 
although shortened as compared to a similarly powered normal air 
cored Tesla coil are very much louder and visibly brighter indicating 
discharge current from a proportionately large self-C.  I get a 
similar effect when I place an external air-dielectric 100 pF cap 
built from an aluminum spinning suspended within an empty, grounded 55 gallon
steel drum on the output of a regular disruptive Tesla coil and retune to 
compensate for the mucho extra topload C, except in this case I don't appear to
be losing nearly as much voltage at the same time.  We understand well that
topload C foreshortens the 1/4 wave resonator in electrical degrees so that a 
lower Vmax is produced at resonance.

Any comments and further opinions to my ramblings are welcomed.

rwstephens