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Re: Arc Impedance Study

At 07:32 PM 10/11/98 -0500, you wrote:
snip>
>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 --
>
>

Hi Bert,
	Makes sense!  I have noticed that high voltage power supplies do like to
produce corona and discharges with positive terminal voltages more than
with negative voltages.  This would also suggest that a "rectified"
positive output Tesla coil would give better steamer performance.
Although, in practice, that may not be possible or practical. 
	I doesn't sound like the average coiler can do much to change the plasma
physics to increase performance other than the toroid sizes you mention.  A
different gas (such as argon) would increase streamer length I would guess
- but that would be cheating :-)) 

	Terry
References: