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Re: Arc Impedance Study
Tesla List wrote:
>
> Original Poster: Terry Fritz <terryf-at-verinet-dot-com>
>
> At 07:56 PM 10/9/98 -0700, you wrote:
> snip>
> >> It seems to favor the positive peaks somewhat. I will have to look at
this
> >> specifically this weekend to really see if that is true.
> >
> >Mine favors the pos peaks as well, which agrees with
> >the published literature on polarity preferences of
> >sphere-to-plate spark gaps.
> >
> snip>
>
> Hi Greg,
>
> Mine definitely favors positive spikes as well (scope photo is at -
> www.peakpeak-dot-com/~terryf/tesla/misc/spikes.jpg ). They have a sharp rise
> to about twice the normal level and then ramp back more gently. Each one
> (there are usually many in a peak) lasts about 200-300 nS. There are
> negative spikes too but the positive ones are much more prevalent. They
> appear to very much like the ones in your electrum photos both in relative
> scale and timing. Does anyone know why they like to arc on the positive
> peaks??
>
> Terry
>
> >
> >-GL
> >www.lod-dot-org
> >
> >
Terry,
Thanks for the latest round of experimental and theoretical work! Your
latest series of experiments and simulations are helping to remove
another couples layers of mystery! Comparing your latest streamer
current measurements and Greg's earlier measurements is extrememly
interesting - although the scales are different, the behavior is quite
similar! I have little doubt that the current spikes correspond to
consecutive "flashes" associated with positive streamer propagation.
Following is an EXTREMELY condensed, and undoubtedly oversimplified,
attempt to answer your question. For a far better description, see
Raizer, Yuri P., "Gas Discharge Physics", Springer-Verlag, 1991 and
Bazelyan, E.M., Raizer, Y.P., "Spark Discharge", CRC Press, 1997.
There's significant evidence that streamers are preferentially generated
from a positively-charged electrodes. The reasons are complex, and
appear to be due to the way that electron avalanches evolve to become
heavier-conducting and self-propagating streamers in a non-uniform
e-field.
Electron avalanches will propagate more rapidly towards a
positively-charged toroid than away from a negatively charged toroid. As
the initial avalanches grow, they begin to look like dipoles as
ambipolar diffusion separates the more mobile electrons from the slower
ions in the presence of the applied e-field. Once the negative "end" of
an avalanche dipole connects to the positive toroid it disappears,
leaving the opposite positive end as an outward projecting
positively-charged streamer, sort of like a little positive projecting
"stick" extending from the toroid. This outwardly-projecting streamer
forms a localized concentration of the e-field at the streamer's tip.
The tip will rapidly propagate outward as long as the toroid's voltage
remains sufficient or, better yet, is increasing. The latter is the
typical case for a Tesla coil with early breakout followed by + dVout/dt
as the toroid approaches a positive voltage peak.
When the toroid is at a negative potential, avalanche electrons are
repelled away from the toroid towards a weakening e-field. Also, they
are partially shielded by the intervening space-charge of the positive
ions they've left behind. The combination makes initial formation of the
streamer considerably more difficult than for positive streamers.
Furthermore, even once a negative streamer is formed, free electrons
will tend to "run away" from the propagating tip, thus requiring higher
terminal voltages to propagate than for a positive streamer.
>From a practical standpoint, streamers will form at a significantly
lower "breakout" voltage for positive toroid polarities than for
negative. So an optimally-sized toroid will tend to have initial
breakout and subsequent streamer propagation confined only to positive
toroid polarity. BTW, non-uniform long rod-plane gap research has shown
negative breakdown voltages that are more 2-times that for an identical
positively charged rod - the same may hold for RTesla Coil toroids as
well.
One potential (no pun intended...) consequence of this may be that
toroids with too small a Radius of Curvature (ROC) may actually be
generating both positive and negative streamers. This may help explain
why the discharge characteristics are so much different with undersized
ROC toroids (multiple-branches instead of single-rooted longer
streamers).
Hope this helps a bit... and thanks again for some excellent and very
enlightening work!!
-- Bert --