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Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>
Dave and all,
I still suspect the main reason why leaders follow the previous channel
from bang to bang is lower air density and not the presence of residual
ions. The reason? Although longer lived positive and negative ions can
indeed persist from one bang to the next, their presence does not
directly contribute to the spark breakdown process on the next bang.
Avalanche breakdown initially requires free electrons, accelerated by an
E-field, colliding with air molecules, to create an increasing cascading
shower of free electrons. For the avalanche to proceed, the rate of free
electron creation must exceed the rate of electron loss through
attachment (to oxygen or water vapor molecules to form negative ions).
Electron attachment occurs very quickly in air. Any free electrons
created during the previous bang are long gone by the time of the next
bang, having been captured by electronegative gases in the air since the
average "free" electron life in air at STP is only about 11 nSec.
It is possible to "detach" some free electrons from negative ions via
thermal collisions with other molecules. This process requires fairly
high temperatures (>1,000 K), but is it possible that detachment
provides some free electrons. It's likely more fundamentally that the
spark channel used by the previous leader leaves behind a "tube" of
significantly less dense (lower pressure) air which has not had the
opportunity to dissipate into the cooler surrounding air. Within this
lower density tube, the mean free path of new seed electrons (from
cosmic ray, background radiation, or electron detachment) is longer,
allowing the electrons to gain more kinetic energy from the E-Field
between collisions, increasing the probability of ionizing collisions
and avalanche multiplication. In this manner, the hotter "echo" of the
previous spark channel most likely becomes the preferred region for
avalanche breakdown during the NEXT bang.
The presence of residual ions in the region may be mostly of scientific
interest, since these ions don't appear to have as much of a role in
avalanche breakdown during a subsequent bang as the lower air
-- Bert --
Web Site: http://www.teslamania-dot-com
Tesla list wrote:
> Original poster: "davep by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> Tesla list wrote:
> > Jim, Dave, and all,
> > There's some evidence that the next leader follows the same channel as
> > the previous from bang to bang
> This is well documented in 'natural' lighting. I can't think
> Tesla Coils streamers, with shorter lengths and arguably
> faster 'refresh' would different.
> > due to residual hot air left in the previous leader's channel. While
> > the channel air temperature is much too low, and duration too long,
> > to convincingly explain consecutive breakdown on the next bang from
> > residual ions,
> My understanding of the lighting, and other, data differs.
> Ions can have surprisingly long lifetimes.
> > residual channel heat may simply make the air in the former channel less
> > dense. Since less dense air has a lower breakdown voltage than the
> > surrounding colder air,
> Be it ions or 'temperature':
> > the next leader finds this path the "weakest link", and the next leader
> > preferentially follows the "warm" path blazed by the previous leader.
> Or one near by (eg the invisible ion (or heat) trail gets drifted
> about by normal air movement, and...