Re: FANTC Update to v1.1

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

Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>

 > http://www.classictesla-dot-com/fantc/fantc_1.1.zip

Antonio wrote:

 > I am imagining that it would not be difficult to include a
 > breakdown voltage calculation, by looking at the maximum
 > electric field at the surfaces of the elements.

The program computes the surface charge density distribution
as a step in the calculation of C, and therefore the surface
field strength is immediately available.

Breakout predictions based on these agree with the North report,
as would be expected, and give reasonable (10%) predictions for
small gaps.   However, for longer gaps and isolated electrodes, eg
toploads and breakout terminals, the predictions based solely on
the onset of electron avalanche (30kV/cm) fall well short of the
observed (and measured) actual breakout voltages, sometimes by
a factor of 2 or more.

The reason for this is additional physics taking place between
the initial electron avalanche (first streamer corona) and the
appearance of recognisable visible hot leaders.  Certain conditions
need to be satisfied in the space around the topload for leader
formation to begin.  These conditions involve space charge and the
local field of the leader tip, as well as the background 'geometric'
field.  Leader growth continues as long as these conditions are
satisfied in the locality of the leader tip.

For short gaps, the progression from initial avalanche to gap
closure takes place very rapidly, with little extra gap potential
required for complete breakdown over and above that required for
corona onset.  Thus program estimates for surface corona onset
are valid estimates for gap breakdown.  But to get a worthwhile
value for topload breakout we must take into account the extra bits
of physics.

Fortunately, there is quite a lot in the literature about this,
and a number of numerical software models are described.  These
appear to be quite successful in modelling the time domain
evolution of leaders over large gaps and our hope is to incorporate
some of this into TC calculations.

Unfortunately, existing models seem to apply to slow unipolar pulses,
and leader formation appears to take up to a few hundred microseconds
in gaps of a few metres (thus, several TC RF cycles).  We also have
to deal with memory of previous bangs (low density channels).
The consequence is that we have to launch into the unknown a little
in respect of applying existing research to TC breakout.

The best we can do at present is to offer the corona onset potential,
based on say 26kV/cm at the surface, as a *minimum* potential for
breakout.
--
Paul Nicholson
http://www.abelian.demon.co.uk/tssp/
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