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Re: Racing Spark Prediction

Original poster: FutureT@xxxxxxx In a message dated 5/21/06 5:27:43 PM Eastern Daylight Time, tesla@xxxxxxxxxx writes:

John wrote:
> There is a TC operation mode that can produce a great number
> of racing sparks all over the secondary.  I don't remember
> anyone mentioning this but they may have.  One time I was
> testing a static series gap under relatively high power.  The
> gap was not cooled so it heated up fairly quickly causing it
> to either power-arc and/or quench poorly.  When the gap was
> still cool, there were no racing sparks.  As soon as the gap
> heated, a great number of racing sparks appeared.

We seem to be hearing stuff that suggests the primary gap
behaviour has something to do with racing arcs.  Couple of
a) Did the racing arcs above cause any damage to the secondary,
    or where they faint, low energy sparks?


The racing arcs were relatively strong and bright.  I don't think
they damaged the secondary.  They weren't as destructive
as the type that occur near the bottom of the secondary due
to overcoupling.

b) Can the phenomena be reproduced on some other coil?

My guess is that the condition would be easy to reproduce.

c) Could the phenomena be made to go away by a small change to
    the primary tapping point?

I didn't try that.  My guess is that it wouldn't help.

> Any thoughts or speculations about why this might happen?

A few.  A tie-up between racing arcs and gap behaviour is a
rich source of possibilities.  We'd better go read Terry's old
paper mentioned in

> http://www.pupman.com/listarchives/1998/July/msg00019.html

Under certain conditions then, is the gap (nonlinearity)
generating lots of unwanted RF at some high frequency?  Is
there a -ve V/I slope involved here?  Driven by power arcing,
is the primary gap trying to be a Poulsen arc?

Interesting about the Poulsen arc idea, etc.  Thanks for those

> I didn't actually scope the firing of this coil.  I think I've
> seen other folks' coils which did the same thing but it seems
> to be something that hasn't been discussed much or at all on
> this list.

John may have given us a handle on the problem here.  Too high k
will surely cause quenching problems, in theory and in practice,
which in turn may cause racing arcs.

Dest wrote:
> what are you talkin about - which other factors?

Factors other than reduction of k.  Reduction of k fails to
significantly reduce the HF modes excited by virtue of the
non-uniform coupling geometry.  I'm pretty sure now that
these 'geometrically' excited overtones are neither strong
enough, nor sharply enough affected by k changes, nor of
high enough frequency, to be responsible for racing arcs.

An experiment I once did tends to confirm the above.  I ran
a coil with a flattish conical primary, and compared it using
a helical primary.  As I tightened the k in both cases, the
racing sparks showed up at the same k value, and were the
same in appearance as far as I could see.

We must look at other things which can generate HF - things
which happen to be alleviated by low k.  Primary overtones
excited by gap non-linearity might be one such 'other factor'.
HF transients from topload discharge might be another, etc.

I'm keen on this 'pathalogical primary behaviour' idea,
associated with quenching difficulty which in turn is brought
on by 'over-coupling'.  It fits - k is not directly the cause.

Peter Terren wrote:
> I can contribute with a picture of racing sparks.
> http://tesladownunder.com/TeslaPhysicsRun5secs.jpg

This looks like a point-to-point breakdown of the secondary,
which I think is probably a different thing.   I'm assuming
that the typical case of racing arcs would be described as
small arcs racing at random all over the secondary, oriented
vertically, short, not very bright, and not pinned to any
particular breakout points on the secondary.  Maybe someone
has a photo of something like that?

If the primary is tuned for a too-high frequency racing sparks may occur
below the top of the secondary winding because the voltage then
peaks there.  If a too high k is the problem, the racing arcs
predominate near the bottom of the secondary and may be very
destructive there.  Some racing sparks may occur higher on
the secondary also.  If gap quenching is failing, and or power arcing,
then the racing sparks will be more evenly spread over the coil.
Using a k setting which does not cause racing sparks, if the power
to the coil is increased, racing sparks may occur due to the
larger bang size.  This is what I've seen on my coils.  Others
may have other comments.

Gerry wrote:
> What measurements do you have in mind and what type and quality
> equipment is needed for the measurement.

Capture of a secondary base current waveform on a digital scope,
transferred to a computer file for analysis.  Only rough amplitude
calibration needed.  Wide bandwidth, though, say up to 30 times the
TC operating frequency.   One sample of the genuine phenomena is
all that's needed, so no long damaging runs required.  We would
analyse the captured waveform to determine what signal components
are present - chances are the offending signal(s) will be quite
prominent.  We would work out their voltage and current
distributions and approximate gradients, and by modelling,
determine which parts of the structure are involved in generating
them.   The cross check at that point would be detection of the
offending resonances during low power sweeps and showing by probing
and loading that the nodes and antinodes are in the predicted
places.  The tricky bit is getting a coil to display classic racing
arcs on demand.  Maybe John has pointed the way there.

Variation on the above:  Just fire the primary under various gap
operating conditions, without the secondary (is that possible
or would that damage something?).  See what frequencies are excited
and at what levels.  Knowing that, we can calculate how the
secondary would cope with them.

Thanks everyone for all the interesting comments in this thread.
Paul Nicholson