Re: negative streamer propagation?

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

Original poster: "Antonio Carlos M. de Queiroz" <acmq-at-compuland-dot-com.br> 

Tesla list wrote:
 >
 > Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net>

 > "Spark Discharge", E. M. Bazalyan, Yu. P. Raizer, CRC Press, 1997, ISBN
 > 0849328683 - See Chapter 6 (The Leader Process) for quantitative discussion
 > of positive leader mechanism, and section 6.11.1 (A Negative Leader), pages
 > 353 - 256 for qualitative discussion of why long gap breakdown threshold
 > for negative discharges is significantly higher than for positive 
discharges.

In static machines it's easy to observe than if the spark gap is made
with balls of different sizes, long sparks are only obtained if the
smaller ball is positive. If the smaller ball is negative, the result
is just corona at the negative terminal.
So, I like to think that the breakdown voltage for -negative- polarity
is that is smaller, but negative breakdown only tends to cause uniform
corona, and not streamers/sparks. The positive terminal must be smaller
to cause a -long- spark to start there, or, more probably, to end there,
allowing it the "capture" a negative leader coming from the negative
terminal.
The reason for the difference is the larger mobility of electrons, but
it's not easy to see exactly how the mechanism operates.
This picture below shows bipolar corona over a film plate:
http://www.coe.ufrj.br/~acmq/turpplat.jpg
The positive is at the left side. Observe that the negative corona
propagated more easily.
Other pictures (Lichtemberg figures in film):
Positive: http://www.coe.ufrj.br/~acmq/gray123.jpg
Negtive: http://www.coe.ufrj.br/~acmq/gray124.jpg

Antonio Carlos M. de Queiroz