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Re: the cure for racing sparks
Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>
> ...streamer formation? Maybe as lightning ...about 0.3 m/us?
> ...operating frequency of 100 kHz has 10 us. A streamer could
> then move 3 m in the time of one cycle. The numbers look close
> to what could cause problems, but not quite there yet.
Yes, we're in the same ballpark, but as you say, the numbers don't
quite agree unless we take it that streamer extension only occurs
during quite small portions of the RF cycle, eg the bits marked #
below, and it might even retreat a little during the o portions.
# # # #
# o # o
o o o
o o o
Perhaps we might gain a factor of 5-10 from this, thus forming
circa 30cm per RF cycle? I guess with racing arcs of case #2,
the problem should manifest more with higher frequency coils.
Is there evidence of this I wonder?
Streamers often seem to display a segmented structure - say 2-4
distinct sections, brightest at the base near the topload, weakest
at the extremities, with well defined boundaries between sections.
Perhaps if an N section streamer forms over N half cycles it would
account for this appearance. Does anyone have a good photo of
It'll be real nice to see topvolts scope captures from breaking-out
coils because of lot of this stuff can then be checked out pretty
quickly, I think.
I'll just add another case to the list,
#A Rapid changes to topload boundary conditions during streamering.
Scatters energy into HF modes. (VC)
Here's a little animation which illustrates the idea behind case #A
The time axis should be mS not uS as shown. This model is of a
rather unnatural short discharge from the topload. Not quite case
#8 because the discharge arc is not maintained in this model, but
similar to case #A. We see the transient bounce along the coil a
couple of times, dispersing as it goes, until eventually the coil
is awash with random HF ringing. Along the way, it can produce
some pretty nasty voltages. With case #A you'll have to imagine
lots of small transients instead of the one big one modeled here,
but the effect is to fill the coil with HF noise.
Scope trace capture of the coil base current during streamering
would give enough data to say if case #A is happening or not.
Case #8 might show discharges from near the middle of the coil
occuring only in coincidence with discharges from the topload.
Case #A might show thin weak arcs racing all over the coil, but
only when the topload is issuing streamers.
Does anyone have any examples of these cases?