Re: The Next Coil

From: 	Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
Reply To: 	bert.hickman-at-aquila-dot-com
Sent: 	Friday, August 01, 1997 1:51 AM
To: 	Tesla List
Subject: 	Re: The Next Coil

Tesla List wrote:
> From:   Greg Leyh[SMTP:lod-at-pacbell-dot-net]
> Sent:   Wednesday, July 30, 1997 3:32 PM
> To:     Tesla List
> Subject:        Re: The Next Coil
> Bert Hickman wrote:
> > Best of luck on the new coil! And I thought your current coil was
> > awesome...! I must have missed your experiments with adding 100pf of
> > lumped C to increase streamer propagation  - could you describe them a
> > bit more? And what did you used for a capacitor??
> As part of a quickie experiment, I wanted to add C to the coil without
> affecting the breakout voltage of the toroid.  After rooting around in
> the junk boxes for awhile I found a bag of big doorknob-style caps out of
> a laser power supply.  I stacked twenty of these capacitors, each one
> rated at 2200pF -at-30kV, using threaded 3" standoffs between them.  The
> finished stack, which measured about 10' in length, was then placed inside
> of the secondary along the center axis.
> Each cap had a 10Mohm stick resistor in parallel.
> After retuning the coil (and increasing Cpri by ~10% BTW), the arcs seemed
> to be about 5' longer, but the most noticable difference was the greatly
> increased number of ground strikes.  But how much of this improvement was
> due to the 10% increase in Epri?
> I then ran the coil w/o the sec cap stack but with the same Cpri (and Epri),
> and shunted some of the primary to achieve tuning.  The performance fell
> back to about the original level.
> As a result the next coil will have a much larger Ctop, but only a modest
> increase in the breakout voltage rating, as it appears the the larger C, and
> not the higher breakout voltage, is the secret to the success of bigger toroids.
> -GL


EXCELLENT! This is the first hard evidence that streamer propagation for
HF AC discharges is significantly enhanced by providing a low impedance
source of "instant" charge from a lumped C and not a "higher impedance"
distributed C! Although large toroids have seemingly "worked" better for
most coils, even though they may reduce peak output voltage, until now
there was only hand waving arguments as to possible causes. Although
some of the earlier work done by Meek, and more recently by Gallimberti
["Physical Models of Long Air Gap Breakdown Processes", I. Gallimberti
in "Electrical Breakdown and Discharges in Gases, Volume 89a, E.E.
Kunhardt and L.H. Luessen, Plenum, 1993] suggested that the ability to
supply sharp current peaks assisted in the leader propagation process,
you've confirmed that this also applies to HF partial discharges as

There's just not very much empirical data in this area _except_ from
coilers. Coilers are actually in a rather unique position to rather
easily make visual, recorded, and instrumented measurements of partial
discharge phenomena that would otherwise be VERY difficult to repeatably
create and measure under single-shot (i.e., Marx) conditions. Some of
the arrested rod-plane streamers in the Kunhardt book [see "Spark-over
Characteristics of Long Gaps" by G. Baldo, Fig. 19 for example] JUST
like typical open-air or near-hits to grounded objects from a large
Tesla Coil.  

Greg, yu also made one heck of a nice voltage divider - by any chance
did you happen to measure the resulting voltage across the bottom-most
cap? With this, and the other known input parameters, you'd know your
coil efficiency (perhaps helping to resolves some earlier/animated
discussions of coupling, output voltages, and efficiencies!)

Thank you VERY much for the details on this experiment!

And safe mega-coilin' to you!

-- Bert H --