Re: More on spark delay

["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>

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