Re: More on spark delay

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

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Kchdlh-at-aol-dot-com>

Bert (& all)-

Apropos the following--in a typical Tesla coil system, can you say what the
significance is of the (apparent) facts that a) sparks emanate from the toroid
exclusively, absent a near-enough grounded surface or point, and never, say,
from the apparatus of the primary (which is essentially at ground) or from
points on the ground-surface itself beneath the coil apparatus; and b) even
with surrounding grounded surfaces and/or points present, they being far enough
away so as not to seem to emit sparks, the sparks still appear to come "from"
the toroid?

I realize that, in the dark, one can see leaders (if that's the proper term)
apapearing at this object and that--but the big, apparent spark always jumps
off the toroid.

Ken Herrick


I

>
> Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <bert.hickman-at-aquila-dot-net>
>
> Antonio and all,
>
> Interesting observations (and a VERY interesting topic!). However, I
> must disagree...  :^)
>
> The polarity question comes up from time to time on this list, and it is
> certainly an area that's not intuitively obvious. Although electrons are
> the primary current carrier in leaders, the actual polarity of a leader
> can be either negative or positive depending upon the polarity of the
> initiating terminal. And, leaders of either polarity can be generated
> and will propagate under appropriate conditions. For example, most cloud
> to ground (CG) lightning is initiated in the lower (negatively charged)
> section of the cloud, resulting in a negatively charged leader that
> propagates earthward. Since some of the more commonly available texts on
> long sparks deal with lightning, this can lead one to believe that all
> leaders are negatively charged, or that streamers and leaders are
> preferentially generated from negatively charged terminals (lower
> breakdown voltage). This turns out NOT to be the case for nonuniform
> long gaps in air.    
>
> If we specifically look at nonuniform gaps (i.e., where a smaller
> "active" terminal creates an asymmetrical E-field), the initial
> breakdown voltage is always lower when the smaller terminal is of
> positive polarity than when it's negative. And the difference is far
> from trivial - it can be as much as a factor of 2:1 for long rod plane
> gaps! The polarity effect applies to all long non-uniform gaps in
> electronegative gases, and because of the mechanism applies to initial
> breakout, streamer, AND leader propagation. And it should apply to
> spherical toploads, and presumably toroids, atop Tesla Coils. It has, in
> fact, been demonstrated during topload-to-leader current measurements by
> Greg Leyh on Electrum. And, there is ample experimental evidence in the
> literature (a few more readily available modern references are shown
> below). 
>
> Paradoxically, the positive polarity effect actually reverses at low
> pressures - breakdown actually becomes preferential at the cathode
> (Kuffel and Zaengl (3)). This is a consequence of Townsend breakdown
> effects that occur only at low pressures. This may account for Antonio's
> reference to degenerate electron beams. However, this effect cannot be
> extrapolated to nonuniform long gaps in air at STP since space charge
> and avalanche-streamer-leader phenomena dominate Townsend breakdown
> effects at higher pressures and longer distances. Also, I agree with
> Antonio that the positive polarity effect does not apply for uniform
> gaps. However, the uniform field case does NOT apply for the nonuniform
> field case present at the topload of a Tesla Coil.  In fact, the uniform
> field case does not hold in general for most long gaps.   
>
> The fundamental reason for lower nonuniform gap breakout from positive
> terminals has to do with space charge and field distortion effects that
> are a critical part of avalanche and streamer formation. As Raizer (1)
> states (emphasis is Raizer's), 
>
> "The avalanches at the _rod anode_ travel to it from the outside; as
> they come nearer, they enter the region of progressively stronger field.
> This factor _facilitates_ the multiplication of electrons and
> _stimulates the avalanche-streamer transition_. In the case of the _rod
> cathode_, multiplying avalanches move further from the electrode into
> the region of progressively weaker field. The multiplication process is
> therefore _slowed down_ and the _avalanche-streamer transition is
> inhibited_. Moreover, in the case of the positive rod, electrons sink
> into the metal, leaving behind a noncompensated positive space charge,
> which enhances the field at the electrode. In the case of the negative
> rod, however, the field of the corresponding negative space charge is
> somewhat compensated for by the field of positive ions, all of which
> stay in the gas." 
>
> Raizer also presents a typical graph showing the differences in
> breakdown voltage with polarity for identical rod-plane gaps. Since
> streamer formation is facilitated, leaders follow, leading to lower
> breakdown voltages for long gaps from small positive terminals. A
> similar graph appears in reference 2 below. 
>
> References:
> (1) "Gas Discharge Physics", by Yuri P. Raizer, Springer-Verlag, 1991,
> 2nd corrected printing (1997), ISBN 3-540-19462-2, section 12.8.2,
> Effect of Polarity, pages 361-362 
> (2) "Spark-Over Characteristics of Long Gaps", G. Baldo, in "Electrical
> Breakdown and Discharges in Gases" NATO ASI Series, Series B:Physics,
> volume 89a, by Kunhardt and Luessen, Plenum Press, 1983, , ISBN
> 0306411946, pages 291-311. 
> (3) "High Voltage Engineering Fundamentals", by E. Kuffel and W. S.
> Zaengl, Pergamon Press, 1984, ISBN 0-08-024212-X, section 5.12, Polarity
> Effect - Influence of Space Charge", pages 377-383 
>
> Hope this helped!
>
> Best regards,
>
> -- Bert --
> -- 
> Bert Hickman
> Stoneridge Engineering
> Coins Shrunk Electromagnetically!
> http://www.teslamania-dot-com
>
> Tesla list wrote:
> > 
> > Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>
> > 
> > Tesla list wrote:
> > >
> > > Original poster: "Bert Hickman by way of Terry Fritz
> > <twftesla-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>
> > 
> > >...
> > 
> > (I agree with your comments so far)
> > 
> > > The terminal voltage behavior is consistent with rapid charge transfer
> > > during streamer and leader formation during initial breakout. However,
> > > the terminal's polarity is somewhat surprising, since leaders seem to be
> > > preferentially initiated when the topload is positive (i.e., for
> > > "cathode directed" streamers) than when negative.
> > 
> > As far as I know, leaders are packets of negative charges, electrons.
> > Regular sparks, even lightning, always start at the negative side
> > (clouds, in most cases). A spark can be seen as a degenerate electron
> > beam, in a medium that is far from vacuum. For symmetrical conditions,
> > breakout also occurs more easily with negative voltage. The
> > observation that breakout always occur at negative voltage swings
> > agree with this.
> > 
> > Antonio Carlos M. de Queiroz
>
>