Re: the cure for racing sparks

["Tesla list" <tesla-at-pupman-dot-com>]

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

Paul and all,

Thanks for the clarifications on the arc model used in your simulations.
I suspect 50 ohms is much too low, and your estimate of several thousand
ohms is closer to the mark. 

I was reviewing notes I'd taken during a series of grounded arc
experiments many years ago, using a low wattage tungsten light bulb as a
power measurement device and a storage scope. The system used a 32" x 8"
toroid atop a 10.25" diameter resonator having a 31" long winding of
closewound #21 AWG magnet wire. A linear filament halogen lamp was used
as a load to assist in measuring power (and current) of power arcs to a
grounded wire suspended above the toroid. Unfortunately, the connection
to the grounded wire was rather long, through #20 AWG wire back to the
RF ground, so there was likely significant inductance in the ground path
in addition to the arc path. 

During power arcs, the discharges were underdamped, but this may have
been in part an artifact of my poor, inductive ground return. The
envelope of high frequency (10-20 MHz - tough to measure) oscillations
had an estimated time constant of ~300 nSec. Virtually all the topload
energy was depleted in ~ 1.5 uSec. Using this envelope and backfiguring
the estimated RC for this system implies an arc resistance in the range
of 5000 - 10000 ohms. The power arcs were approximately 42" long,
implying an effective arc resistance of about 1500 - 3000 ohms/foot,
with an estimated peak current of several tens of amperes.  

It would be interesting to try a similar experiment with Thor, but
instead using a grounded target and a low inductance ground return
connection. Using a wideband current transformer to measure return
current in the return path, the peak arc current, inductance, discharge
time constant, and arc resistance could be easily determined.

I also agree with your comments regarding influence of topload coupling
to the upper portion of the winding. This also seems to be in agreement
with empirical observations that racing sparks can sometimes be reduced
or eliminated by lowering the toroid vs the secondary. The "racing
sparks" nut is beginning to crack... :^)

Best regards,

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
"Electromagically" Shrunken Coins!
http://www.teslamania-dot-com

Tesla list wrote:
> 
> Original poster: "Paul Nicholson by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>
> 
> Bert Hickman wrote:
> 
> > What model did you use for the arc to ground?
> 
> Just a 50 ohm resistance.  Thus the modeled discharge is an RC
> decay taking around 100nS. This is only a couple of the time steps
> used in the animation (50nS) so essentially it's a step transient.
> 
> In a real coil the collapsing topload charge might oscillate
> back and forth through the arc at some HF or VHF frequency
> determined by the resonance of the top capacitance with the total
> path inductance of the arc, which includes the ground return
> inductance.
> 
> This may have the effect of softening the transient, by stretching
> it out in the time domain, but, on the other hand, if the discharge
> frequency just happened to coincide with a secondary mode...
> 
> > the full pre-arc voltage voltage, plus a bit of VHF oscillatory
> > overshoot, is now present across the top ~3% of the winding
> 
> Yes, although the peak voltage of the transient doesn't seem to
> exceed the topvolts anywhere, it does rise and fall steeply over
> a short coil distance, giving rise to a high vertical surface
> gradient moving right along the coil.  Dispersion seems to deal
> with this after a cycle or two, but as you say, the initial
> transient tries to place the entire top volts across just a few
> turns.  Just how many turns depends upon the steepness of the
> voltage fall, and this in turn depends on the top-C * discharge-L
> time constant.
> 
> This is a case where close proximity of the toroid to the winding
> top helps a lot.  The factor involved here is the toroid-coil
> distributed capacitance.  If this is large enough between the
> toroid and the top few turns, the transient is coupled through this
> C and the turns don't see the full voltage.  You can see this
> happening with frame 6 of thor.anim2.gif, which is the first frame
> after the discharge begins and we see that the top voltage is about
> half way through its RC decay.  Note that the coil voltage is
> immediately depressed for about 20-30% of its length.
> 
> I'm pretty sure the 50 ohms is much too low - Malcolm has described
> the secondary ringing decaying in a cycle or two following a
> topload discharge, so the effective resistance is more likely to
> be many K ohms.  I left it low deliberately so that we could see
> more clearly the post-discharge ringing of the secondary.
> 
> With these comments in mind, it looks as if the modeled transient
> is a worst-case in that the simulated discharge is steeper than
> those we might expect to occur in practice.
> 
> Thus we now have two suggestions why lots of C between toroid and
> coil is a Good Thing(tm).
> a) It increases the Les of the secondary (unfortunately also the
>    Rac too, by the same proportion).
> b) It allows the transient to be spread out to some extent along a
>    good length of the coil.
> 
> Bert, what should we expect from of a typical topload discharge?  Does the R
> dominate to give mainly an RC decay with perhaps a bit of overshoot, or does
> the L make a big appearance to give a sinusoidal ringdown?
> --
> Paul Nicholson
> --